Ecological observations on the mechanisms of dispersal of barnacle larvae during planktonic life and settling

Netherlands Journal of Sea Research 6 (1-2) : 1-129 (1973)

ECOLOGICAL OBSERVATIONS ON THE MECHANISMS OF DISPERSAL OF BARNACLE LARVAE DURING PLANKTONIC LIFE AND SETTLING

by P. D E W O L F *

(Central Laboratory T N O , Delft, The Netherlands)

CONTENTS I. I n t r o d u c t i o n . II. M e t h o d s .

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I I I . Observations on the dispersion of V I t h stage nauplius larvae a n d cyprid larvae in the western part of the W a d d e n Sea . . . . . . . . 1. I n t r o d u c t i o n . . . . . . . . . . . . . . . . 2. Description of the area of investigation . . . . . . . . . 3. M e t h o d s . . . . . . . . . . . . . . . . . 3-1. S a m p l i n g of larvae . . . . . . . . . . . . . 3-2. R e c o g n i t i o n of species . . . . . . . . . . . . 3-3. Chemical a n d physical properties of samples . . . . . . 4. Results . . . . . . . . . . . . . . . . .

4-1. Balanus improvisus 4-2. Balanus erenatus . 4-3. Elrninius modestus

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5. Discussion . . . . . . . . . . . . 5-1. T i d a l variation . . . . . . . . . . 5-2. Short p e r i o d variation . . . . . . . . 5-3. Correlations of n u m b e r s of cyprid larvae a n d pended matter . . . . . . . . . . 5-4. Testing the theory . . . . . . . . . 5-5. Results of other authors . . . . . . . 5-6. Differences in distribution b e t w e e n Balanus improvisus cyprid larvae . . . . . . . . 5-7. O n V I t h stage nauplius larvae . . . . . . 5-8. T h e size a n d form of groups o f c y p r i d larvae . 5-9. Cyprid larvae ofElminius modestus . . . . . IV. L a b o r a t o r y experiments on the sinking, cyprid larvae . . . . . . . . . 1. I n t r o d u c t i o n . . . . . . . . 2. M e t h o d s . . . . . . . . .

7 7 11 13 17 17 21 21 22

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swimming and . . . . . . . . . . . . . . .

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transport of . . . . . . . . .

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* Present address: N e t h e r l a n d s Institute for Sea Research, P.O. Box 59, Texel, T h e Netherlands.

P. D E W O L F 3. R e s u l t s . 4. D i s c u s s i o n V. O b s e r v a t i o n s o n 1. I n t r o d u c t i o n 2. M e t h o d s . 3. R e s u l t s . 4. D i s c u s s i o n

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t h e s e t t l e m e n t o f c y p r i d l a r v a e in r e l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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tidal p h a s e . . . . . . . .

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V I . O b s e r v a t i o n s o n t h e s e t t l e m e n t o f c y p r i d l a r v a e in r e l a t i o n to p l a c e . 1. I n t r o d u c t i o n . . . . . . . . . . . . . . . . 2. M e t h o d s . . . . . . . . . . . . . . . . . 2-1. P a n e l s in a single r o w . . . . . . . . . . . . 2-2. P a n e l s o n a r a f t . . . . . . . . . . . . . . 3. R e s u l t s . . . . . . . . . . . . . . . . . 3-1. P a n e l s in a single r o w . . . . . . . . . . . . 3-2. P a n e l s o n a raft . . . . . . . . . . . . . . 3 - 2 - I . B a l a n u s crenatus . . . . . . . . . . . . 3-2-2. B a l a n u s improvisus . . . . . . . . . . . 4. Discussion . . . . . . . . . . . . . . . . 4-1. L i t e r a t u r e d a t a o n s o m e factors i n f l u e n c i n g t h e s e t t l e m e n t o f barnacles . . . . . . . . . . . . . . . 4-1-1. L i g h t . . . . . . . . . . . . . . 4-1-2. C u r r e n t . . . . . . . . . . . . . . 4- I-3. B e h a v i o u r o f l a r v a e . . . . . . . . . . . 4-2. L i t e r a t u r e d a t a o n e n v i r o n m e n t a l influences o n t h e s e t t l e m e n t of oysters a n d m u s s e l s . . . . . . . . . . . . 4-3. D i s c u s s i o n o f t h e p r e s e n t results . . . . . . . . . 4-4. C r i t i c i s m o f earlier w o r k . . . . . . . . . . .

64 64 65 65 66 68 68 72 72 74 78

VII. Observations on the pattern crenatus o n a s u b s t r a t e . . 1. I n t r o d u c t i o n . . . . 2. M e t h o d s . . . . . 3. R e s u l t s . . . . . 3-1. B a l a n u s amphitrite . 3-2. B a l a n u s crenatus . . 4. D i s c u s s i o n . . . . VIII.

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s e t t l e m e n t of B a l a n u s amphitrite a n d B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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N a t u r a l m o r t a l i t y i n y o u n g settled b a r n a c l e s 1. I n t r o d u c t i o n . . . . . . . . . 2. M e t h o d s . . . . . . . . . . 3. R e s u l t s . . . . . . . . . . 4. D i s c u s s i o n . . . . . . . . .

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78 78 8t 81 83 84 88 91 91 92 93 93 95 95 100 100 102 105 109

I X . A s i m u l a t i o n m o d e l for s e t t l e m e n t d i s p e r s i o n . . . . . . . . 1. I n t r o d u c t i o n . . . . . . . . . . . . . . . . 2. G e n e r a l d e s c r i p t i o n o f t h e m o d e l . . . . . . . . . . 3. T w o e x a m p l e s o f s i m u l a t i o n e x p e r i m e n t s . . . . . . . . X . A p p l i c a t i o n o f results in a n t i f o u l i n g r e s e a r c h . . . . . . . . 1. I n t r o d u c t i o n . . . . . . . . . . . . . . . . 2. O n t h e n u m b e r o f r e p l i c a t e s in testing a n a n t i f o u l i n g p a i n t o n a raft 3. P a t c h i n e s s o f b a r n a c l e s o n a g e d a n t i f o u l i n g p a i n t s . . . . . . 4. A n e w test m e t h o d . . . . . . . . . . . . . .

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XI. Summary

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X I I . References

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I. I N T R O D U C T I O N This paper is concerned with numbers of barnacles, adults as well as some stages of their larvae. It resulted from work on the testing of antifouling paints. During the last 20 years barnacles, settlement of their cyprid larvae, patterns formed by settling of these larvae, and behaviour of free swimming larval stages have received much attention. KNIGHT JONES & STEPHENSON (1950) found that settling cyprids of Elminius modestus Darwin have a gregarious behaviour. Gregariousness of settling larvae had earlier been described for oyster larvae (COLE & KNIGHT JONES, 1949), and has subsequently been described for a number of other species of barnacles and for other species of benthic organisms (KNIGHTJONES & MOYSE, 1961). Moreover several authors found that settling cyprid larvae of barnacles also have a spacing-out mechanism prior to attachment. The result of this searching behaviour is that populations of young barnacles are evenly dispersed on the substrate. It has been observed that cyprid larvae about to attach do so after searching for a place to settle; they leave the substrate again when they have apparently been unable to find a suitable place. In former papers (DE WOLF, 1964, 1966) it was noted that Balanus crenatus Brugi~re showed a regular distribution on non-toxic control substrates, but had a very irregular distribution on aged antifouling paints; it was concluded that the toxic properties of the antifouling paints have an irregular distribution. The purpose of the present paper is to have a closer look into the regular distribution of young adult barnacles on non-toxic substrates, and to study which environmental conditions are a prerequisite for this regular distribution. Regular distributions of organisms are rare. In most animal species the individuals are unevenly or irregularly distributed (ALLEE, 1931) and planktonic animals are no exception (CLUTTER,1969). CASSIE (1957, 1959a, 1959b, 1960, 1962, 1963) has repeatedly found that marine plankton shows groupwise distribution, in inshore waters and the open sea, on any scale studied; more specifically he found that this also applies to nauplius larvae of Elminius modestus. There are indications that cyprid larvae of other species similarly occur in groups (PYEFINCH, 1948a; WEISS, 1947). The question is now whether this groupwise occurrence of cyprids is generally valid, and if so, how they succeed in settling in a regular pattern on a substrate. As to the first part of the question a groupwise dispersion of plankton in coastal or estuarine water seems remarkable; it would be expected

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that tidal currents and accompanying turbulence would disperse existing groups. To explain continuing existence of groups of cyprids in the light of the present knowledge it would be necessary to assume that the larvae can swim very well and have exceptional powers of observation to maintain the group against the dispersing effect of eddy diffusion. This part of the problem will be dealt with in Chapters III and IV; in Chapter III data on the dispersion of V I t h stage nauplius larvae of Balanus crenatus, B. improvisus and Elminius modestus in the western part of the Wadden Sea are given. Anticipating upon the results it can be stated that the distribution of the larvae is found to be groupwise indeed, and a mechanism for the formation of these groups by the tides is postulated. It is thought that the mechanism acts also in the case of other species of larvae, like mussels and oysters. An interesting aspect of the groupwise distribution has been indicated by WI~.BF. & HOLLAND (1968). T h e y found by means of a simulation experiment in a computer that any method of sampling in a groupwise distributed population leads, in general, to an overestimate of the size of the population. The overestimate is larger if the groupwise dispersion is more outspoken. This means that, if groupwise occurrence of animals is generally valid, earlier authors, reporting upon oyster larvae (KoRRINGA, 1941; CAR~K~.R, 1967), mussellarvae (VERwF.Y, 1966), barnacle larvae (BousnELD, 1955) or medusae (VERwEY, 1966) will have overestimated numbers of animals during periods of outspoken group formation. As in the present experiments the degree of groupwise distribution varied with the tide, earlier theories on the use of estuarine circulation for the retention of animals in estuaries will be criticized, as well as m a n y theories on the inducing of swimming or swimming to certain places by the animals, to ensure retention in the estuary (Chapter III). In Chapter IV a study of the swimming of cyprid larvae, in laboratory experiments, will be reported. In Chapter V experiments on the tidal variation of attachment of Balanus crenatus and B. improvisus in the harbour of Den Helder have been described. These experiments were instigated by the large tidal influence on numbers of larvae in the water, as found in Chapter III, and by a few simulation experiments on settlement patterns (Chapter IX). In view of the groupwise distribution of free swimming cyprids-groups for which no clear-cut size could be obtained--settlement pattern experiments have been done on different scales. In one series of experiments for Balanus crenatus and B. improvisus settlements were studied on a lineair series of panels, at an interpanel distance of 5 m, in the entrance to the harbour of Den Helder, and on a series of panels

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that were exposed in a regular three dimensional grid from a raft (Chapter VI). Settlement patterns have also been studied on the much smaller area of a single panel, on a scale of a size order of 10 to 20 cm. These experiments have been done with B. crenatus in the harbour of Den Helder, and with B. amphitrite in the habour of Haifa. In Chapter V I I I the natural mortality of young settled barnacles has been studied, as has been the influence of natural mortality on settlement patterns, for B. amphitrite and B. crenatus. In the settlement experiments it was found that m a n y factors could possibly have an influence on the nature of the settlement patterns; one of these is the age of the cyprid larvae which could not be manipulated in the laboratory nor determined from a field population. To be able to obtain some idea about the importance of these factors a simulation model for settlement patterns for a computer has been devised. Some results obtained in this way have been given in Chapter IX. These results suggested the field experiments reported in Chapter V. Lastly, as the work originated from the testing of antifouling paints, the implications of the results for this testing have been described (Chapter X). ACKNOWLEDGEMENTS

It is a pleasure to convey here m y sincere thanks to m a n y people who, at various stages, worked with me on this investigation. Mrs D. van Drongelen-Sevenhuysen, Miss W. E. Lewis and Mr A. Stare took part in the m a n y countings of barnacles. Special thanks are due to Mrs A. M. Jansen-Douwes, who counted m a n y barnacles, dead, alive or photographed, and all plankton samples. Mr P. Roele, Drs H. J. Wichers and m a n y others took part in the logistics and the execution of the field operations. I am also very grateful to the director of the Netherlands Institute for Sea Research for loan of the R.V. "Ephyra", and to the crew of that ship for dedicated help; to Dr O. H. Oren, Dr B. Kimor and Mr S. Pisanty for help and advice during m y various periods on the Sea Fisheries Research Station in Haifa, Israel; to Drs F. de Heer of the Statistics Department T N O for m a n y discussions and the programming and running of the simulation model; to Mr H. Kauffman who did most of the statistical calculations, and advised on statistical matters; to Dr R. E. Weber, who kept within limits the liberties, I permitted myself with the English language; to Mrs M. J. J. MullerKleywegt for typing various drafts of the manuscript, and last but not least to Prof. Dr G. P. Baerends, Dr H. J. Hueck and Prof. Dr H. Postma for their criticism and advice on the Manuscript.

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P. DE W O L F II. M E T H O D S

The methods used for different parts of the investigation were diverse, and will be described in the appropriate chapters. Here, however, attention is paid to a few statistical matters, as m a n y of the arguments used will be derived from these. CASSlE (1963) has indicated that, in the absence of evidence to the contrary, it is reasonable to assume that individuals in a population are randomly distributed. Provided that the volume (area) of the organisms is small in relation to that of their environment, numbers in samples of equal volume (area) should then be distributed as a Poisson series. It follows, that randomness is a null hypothesis, which can be rejected at any given level of confidence, but cannot be "proved". Departure from randomness can be tested in several ways; e.g. by Fisher's coefficient of dispersion (CF) which is the ratio of variance to the mean, and which for a Poisson series has an expected value of unity. The variance is usually given as 2 n / ( n - - 1 ) ~ (BARNES & MARSHALL, 1951) but GREIGH-SMITH (1964) has indicated that 2/(n--l) is more exact. Departure from Poisson may be positive or negative. In the literature there is considerable confusion in the terminology; usually positive departure (CF > 1) is defined as overdispersion and negative departure (CF < 1) as underdispersion. CASSm (1963) argued that the above definition is logical, if "dispersion", a mathematical term referring to the spread of numbers about their mean, is distinguished from "dispersal", an ecological term referring to the spread of individuals in space. Ecologists however, have not been very strict in their use of these terms, and to prevent confusion I shall use the term "aggregated" or "groupwise" distribution for C~ > 1, and "regular" distributions for CF < 1. CASSm (1963) further remarked that species occurring in small numbers are less likely to show departure from Poisson and that this does not necessarily imply that rarity begets randomness, but that nonrandomness is less readily detected. Any population can be made to appear r a n d o m by making the samples sufficiently small, and when the sample mean is close to or less than unity it should be suspected that the data may be inadequate to detect non-randomness (see also page 16). It has further been shown (GREIOH-SMITH, 1964; PIELOU, 1969) that sample size (area or volume) has an important effect on Fisher's coefficient. This difficulty has been overcome by comparing sample

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series of comparable size only; e.g. for young settled barnacles areas of 5 × 5 mm *, and for plankton cyprids volumes of 200 litres. It is not known to what extent the in situ form of the sample (a sphere in stagnant water as opposed to a horizontal cylinder in flowing water) influences the results, but it is considered (page 49) that this has no adverse effects. To prevent some of the difficulties inherent to CF another measure for dispersion, ¢, has been used also; this coefficient is derived from C--

s~ _ 23

wherein s 2 denotes variance and 2 mean. According to CASSIE (1959) c is not correlated with the mean, and thus has some validity in comparing populations with different means. O n the other hand, the same population will not necessarily have the same c for different patterns of sampling, or for different sizes of samples. As I failed to see the theoretical background for the statement that c is not correlated with the mean I have always tested whether c was indeed independent of the mean; this was usually true. III. OBSERVATIONS ON THE DISPERSION OF VIth STAGE NAUPLIUS LARVAE AND CYPRID LARVAE IN THE WESTERN PART OF THE WADDEN SEA I. I N T R O D U G T I O N

M a n y papers have been published on the occurrence of barnacle larvae in space and time; only a few are of a quantitative nature and allow conclusions to be drawn about the dispersion of the larvae in the water. WEiss (1947) noted that in Biscayne Bay, Florida, at the collecting station, most cyprids of Balanus improvisus, attach at low tide. The water mass, present then, was characterized by a high number of cyprids. PYEFINCH (1949), working with all naupliar as well as cyprid stages of Balanus balanoides, B. crenatus and Verruca stroemia in Millport, Scotland, was the first to note that the numbers of larvae, taken either by net hauls or by plankton pump, could vary to a large extent over very short periods. H e discussed the possibility that the variations were due to sampling error, but he came to the conclusion that sudden variations do occur in the sea. Contrary to WEiss he could not find any correlation between the numbers of larvae and the tidal phase. KNIGHT JONES & STEPHENSON (1950) found that settlement of cyprids of Elmi-

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nius modestus was usually much more intense on collectors placed in areas on the shore with a solid bottom (e.g. shells) than on collectors in an area with a m u d d y bottom. T h e y thought that it was due to gregariousness of cyprids with adult barnacles present on a solid bottom, that higher numbers ofcyprids settled on their collectors. This work was done near Burnham in the estuary of the river Crouch, Essex. As to the large scale dispersion of naupliar stages and cyprids of Balanus balanoides, B. crenatus and B. improvisus, BOUSFI~.L9 (1955) found transport through the Miramichi estuary during larval life; this transport was determined by a particular estuarine circulation, and, according to the author, by the depth at which the larvae swim. From this resulted a definite distribution for each particular larval stage of each species which means that this larval stage forms a patch in the estuary. At the same time, however, the variation of the numbers of larvae of a certain stage, per 125 litre sample, is of the same order of magnitude as found by PYF.FINCrI (BousFIV.L9, 1955:28--31). It should be noted here, that BOUSFIELD considered the number of barnacle larvae per 125 litres to be statistically sufficient; he gives no replicate figures. Spacing between sampling stations in BOUSFIELD'S work is of the order of 1½ mile. Spacing between samples was smaller in studies by CASSlZ (1959a, 1959b, 1960, 1962), on the distribution of Vth and V I t h stage nauplii of Elminius modestus in Wellington Harbour, Port Nicholson, New Zealand. The object of the study was aggregation of plankton; sampling was done continuously from a boat, maintaining a steady speed, by means of a pump and filter. Each sample consisted of a horizontal water mass, 115 m long and 4.7 cm in diameter. It is shown that in Wellington Harbour several different water zones can be distinguished (on the basis of temperature-salinity diagrams), with a varying mean number of nauplii. Within each zone of water the number of nauplii varies again greatly, indicating aggregation within the zone. CASSIE argues, that adults of Elminius are omnipresent in Wellington Harbour, and that the release of first stage nauplii at any one time would vary directly with temperature and inversely with salinity (BARNES, 1953b, 1955). This means that most harbour water will have been in recent contact with adult Elminius, and will have had more or less equal chances, depending upon temperature and salinity of becoming populated with the relatively short lived nauplii. From correlation coefficients it is clear that Elminius nauplii are positively correlated with temperature and negatively with salinity. Summarizing, there is some indication that barnacle larvae do show a patchy distribution; in the case of WF.iss (1947) on a tidal scale, BOUSFIELD (1955) on a scale of 1½ mile, CASSlE (1959b) 115 metres,

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and, if we assume an ebb-current of l knot, in the case of PYEFINCH (1949: 363), on a scale of 120 m. In Chapter I it was seen that most animal species occur patchwise on any scale of sampling (CASSlE, 1963). On the basis of these data it appears probable, therefore, that barnacle larvae in the Wadden Sea are similarly patchy distributed. Moreover, from observations to be described in Chapter VI it will become clear, that settling of barnacles can be patchwise on a much smaller scale than those cited. On the other hand (PoSTMA, 1954: 430) the Wadden Sea can be regarded as a well-mixed estuary, almost without stratification. The high current velocities occurring in such tidal areas are accompanied by strong turbulence (POSTMA, 1967: 162) and eddy diffusivity values up to 500 cm2/sec have been found (GRY, 1942 ; cited by POSTMA, 1967). Now mixing and turbulence are randomizing processes, and it would be expected that any patchy occurrence of organisms should be destroyed soon, resulting in a random dispersion. However, there is an extensive literature, that possibly has implications for the problem of patchy dispersion: viz. the literature on the transport and retention of larvae in estuaries by the tides. Two schools of thought can be distinguished: those who advocate passive transport (KoRRINGA, 1941) and those who think that the larvae further their transport by means of currents, by swimming induced at certain phases of the tides. Both mechanisms, of course, will result in differences in numbers of larvae in the water column at different tidal phases. BOUSFIELD (1955) grouped his data on the vertical distribution of larvae of Balanus improvisus, regardless of date or place of the sample, and found that cyprids were, on the whole nearest the surface during flood. Nauplius larvae were generally nearest the surface at high and low water, but deepest during mid-flood and mid-ebb. Successive stages tended to occur at increasing depth at all stages of the tide, and the highest numbers of successive naupliar stages occurred at the surface at progressively later times during high and low water. BousFIELD concluded that these changes in larval numbers at sampling depths were correlated with current velocities, as vertical mixing was greatest at greatest current velocity. He assumed, that early larval stages were generally stirred downward, while late stages, occurring near the bottom, are stirred upward by increased turbulence. He supposed that the depths of larval stages relative to each other are maintained by swimming throughout the tidal cycle. Many authors have observed tidal variations in numbers of larvae of other species, mainly of molluscs, in the water column. NELSON (1912) suggested that larvae of the American oyster controlled their hori-

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zontal distribution by regulating their depth relative to the direction of tidal currents. Early larval stages of Crasostrea virginica are more or less uniformly distributed in the water column and older stages are found at lower levels of the profile, with a higher proportion of older larvae during flood than during ebb (CARRmER, 1967). In some cases large concentrations of older larvae were found on the bottom during ebb (CARRIKER, 1951). Generally it is assumed, that horizontal transport is controlled by swimming at or to a certain depth, and that swimming is released by some or another environmental factor (VERwEY, 1966). BOUSFIELD (1955) believes to have shown that barnacle larvae swim in response to light intensity, while CARRIKER (1967: 469) notes that in nature light tends to depress vertical distribution during daylight hours, and that turbulence and wave action m a y stimulate the larvae to rise in the watercolumn. For the European oyster, KORRINOA (1941, 1952) found high numbers of larvae in the watercolumn during low tide slack water, and he concluded that currents did not have any influence on the vertical distribution of the larvae, while no influence of light, temperature change, wind or waves could be shown on vertical distribution. CARRIKER (1961) wrote, that even during the night there was a relative scarcity of larvae of Mercenaria mercenaria near the bottom, as was the case for veliger larvae of Crasostrea virginica (Carriker, 1951) except for an occasional concentration of mature larvae. VERW~Y (1966), for Mytilus edulis, reports that young larvae (90 to 185 micron) are uniformly distributed throughout the watercolumn irrespective of the tide. Older larvae (195 to 235 micron) are present near the surface in high numbers during high current velocities of both ebb and flood, and numbers are minimal during both slacks. VERWEY mentions two complications in this rule, firstly that the numbers m a y continue to rise after m a x i m u m current speed has passed. This happened at the falling of the night, and he ascribes it to light influence, following CARRIKER (1961b) and BAYNE (1964). The second complication is, that on the flood during daytime two peaks in numbers of larvae, separated by a low value, were observed. VERWEY cannot give an explanation. It should be mentioned that one sample per hour had been taken, comprising 50 litres of water, pumped in about 5 minutes. In numbers of old larvae, found near the bottom, the maxima and minima coincide more or less with those on the surface. According to VERWEY there is some indication that during the night the distribution of the old bottom larvae corresponds to light intensity rather then to current strength. VERWEY considered that postlarvae ( > 265 micron) occur in the water column as a result of

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LARVAE

11

turbulence; numbers present in the water near the bottom show maxima during periods of fast currents, with the exception that the m a x i m u m during nightly ebb is missing. VERWEY concludes that the post-larvae do not further their transport during darkness. In the same paper VERWEY discussed data on the medusae Rhizostoma pulmo and Chrysaora hysocella. There are large numbers in the water during hours of great current velocities and few around slacks, when they are supposed to rest on the bottom. This distribution is explained in terms of changes in current speed and of light intensity. Lastly, WOOD & HARGIS (1969) reported that larval concentrations of Crasostrea virginica were maximal during the flood in James River, Virginia, but considerably delayed in relation to the m a x i m u m current speed, while larval concentrations during the ebb were minimal. Isopleths strongly indicated vertical movements of larvae, the larvae remaining in suspension long after m a x i m u m flood current velocity. T h e y concluded that the vertical movements are not associated with light, but m a y be released by salinity or pressure changes. Summarizing there is much apparent evidence that numbers of larvae in an estuarine environment do show a tidal variation and a patchwise distribution; no authors, except CASSIE, appear to have taken the scale of the patchiness into consideration. It seems likely, that the tidal variation results from a changing vertical distribution of the larvae during the tidal cycle; the factors that are held responsible vary a great deal. It is remarkable, that although m a n y authors report on the accuracy of their samples, hardly any of them bothers about representativity: samples are usually taken to be representative for what they are expected to represent. This shall be discussed later. In this Chapter some observations on the distribution of V I t h stage nauplii and cyprid larvae of Balanus improvisus and B. crenatus will be given with some remarks on cyprids of Elminius modestus. O n the basis of the results observations on other marine larvae by authors cited above, will be reviewed. 2. D E S C R I P T I O N

OF THE

AREA

OF INVESTIGATION

The area of investigation is the part of the Dutch Wadden Sea, connected with the North Sea through the tidal inlet Marsdiep. O n the south side of the Marsdiep the harbour of Den Helder is situated. Observations on the dispersion of barnacle larvae have been made in the entrance to the harbour, in the Marsdiep, and in the tidal channel Texelstroom, which runs in a northeasterly direction (Fig. 1). The average tidal amplitude at Den Helder is 1.35 m, resulting in tidal currents of considerable velocity in the Marsdiep and in the Texel-

12

P. D E

WOLF

f /

/

5:rsdiep . ~ . . ~ /

•

NORTHSEA

/

t~-)--- i....

2, /

Fig. 1. Map of the area of investigation. The solid circle (O) indicates the raft in the entrance to the harbour of Den Helder; the anchor station on Texelstroorn is given by an asterisk (*) ; and the route followed by the drogue on 30July 1968 by a dashed line, with time indicated along this track.

stroom (up to 2½ m/sec), and in the entrance to the harbour (up to 1 m/sec). In general, currents in the tidal channels are markedly out of phase with respect to tidal level; ebb currents usually occur till about 50 minutes after low water, while flood currents can run till 1½ hours after high water. This, however, is the long term mean (SAGER, 1968), on an individual basis the discrepancies may be large (PoSTMA, 1954), depending mainly upon weather and winds during the preceding days. During westernly storms the Wadden Sea is filled to a higher level than normal, and currents are stronger. In addition, periods of no current during high or low water do not necessarily coincide with predicted or actual vertical tide level, while high or low water, at a given location, does not necessarily imply absence of

DISPERSAL

OF B A R N A C L E

LARVAE

13

current. The hydrography of the Wadden Sea has been described by POSTMA (1954, 1961, 1967). In the entrance of the harbour a raft for fouling studies was anchored and used as a station for sampling water to examine the dispersion of the larvae. Observations in the Marsdiep-Texelstroom area were made from a ship, mostly at anchor, but on one occasion following a freefloating drogue. Stations have been indicated (Fig. 1). 3" M E T H O D S

To study dispersion phenomena one must be relatively sure that collecting and enumeration can be done without introducing extra variation due to sampling, handling, storage, sub-sampling, etc. (CAsSlE, 1958). In plankton studies, not aiming at short distance spatial distribution, heterogeneity is usually considered merely as a source of error; this error is usually so great that accuracy in counting is useless. In the present study heterogeneity itself is studied, and great accuracy in sampling and all further stages, counting included, is essential. Any variance introduced by these successive stages will add to the variance of the population studied, and be indistinguisable from the true population variance in the sea. 3-1. SAMPLING

OF L A R V A E

Larvae were sampled with plankton pumps, which are gasoline driven motor pumps with intake hoses adjustable for depths to 10 metres. The volume of water filtered was measured by filling a 200 litre polyethylene d r u m via a small nylon plankton net with meshes of 90 ~zm. The time necessary to pump 200 litres of seawater through the net was noted, as this time is representative of the water speed at the intake of the pumps. Two different pumps were used; one delivered up to 40 1/min, the other 250 1/min. WIBORG (1948) has indicated that fast moving organisms might be able to escape from the mouth of the suction hose, and he suggests that a pump with a capacity of at least 200 1/min should be used. To control whether cyprid larvae did escape from the suction field, the pumping time and number of cyprids were correlated for both pumps (Table I). Ifcyprids do escape, one would expect low pumping velocities to coincide with relatively low numbers of cyprid larvae and vice versa. The correlation coefficients for the small pump are not significant, indicating that there is no relation between intake velocity and number of cyprids caught. In the case of the large pump the correlation coefficient is probably significant, but negative. It follows that

14

1'. DE WOLF TABLE

I

Correlation between pumping velocity and numbers of cyprid larvae caught by 2 different pumps. Pump

Correlation coefficient r

Significance t

n

small small large

--0.141 0.069 --0.177

1.19 0.97 2.02

60 197 130

p

-4-0.2 ±0.2 4-0.05

at a lower pumping speed a higher number of cyprids would be caught and consequently cyprids would be able to escape at higher water velocities. This seems remarkable at first sight, but the possibility cannot be excluded that the larvae do not sense low water speeds, and react with strong escape movements to strong currents (later it will be shown that this is not so). After pumping special care was taken to hose down the contents of the plankton net. Repeated controls showed that this could be done for cyprid larvae with 100% efficiency; for V I t h stage nauplius larvae 100% could not always be obtained. It seems likely that this is related to the rounded form of cyprids as opposed to the spiny legged nauplius larvae. During the investigation the impression was obtained that some planktonnets, although identically made of the same gauze, gave higher yields in cyprid larvae than others. To check whether this could be true an experiment was carried out on 3 J u l y 1968. In the entrance to the harbour of Den Helder, cyprids were caught, using one plankton pump, with a series of 9 different plankton nets of the same make, numbered 1 through 9. After all nets had been used once, a new series of samples was taken, beginning again with net no. 1. Eight series of 9 samples and one of 8 samples, altogether 80 samples of 200 1., were taken between 14.13 h and 15.30 h on the ebb tide (high water at 12.12 h). The pumping speed was controlled at every ninth sample and found to be very constant. The samples contained numbers of cyprids varying between 1 and 91 (Fig. 2). The impression is obtained that there is in each series of samples, an increase of the number of cyprids with net number. At the beginning of each next series the number of cyprids drops again. This follows also from a calculation of autocorrelations (Table II) : correlation coefficients of 1st, 2nd and following orders are not significant, indicating that samples do contain different numbers of larvae. (Higher order autocorrelations in V I t h stage nauplii were not calculated in view of the high number of samples without any nauplii). However, in the 9th order autocorrelation, the correlation between samples taken with

DISPERSAL

OF

BARNACLE

15

LARVAE

the s a m e net, it a p p e a r s t h a t the c o r r e l a t i o n coefficient is p r o b a b l y significant. T h i s p h e n o m e n o n is ascribed to a n effect o f the nets. T h e effect is not k n o w n w i t h c e r t a i n t y , b u t t h e r e is a possibility t h a t it

8060-

~,~-

~, ~-

.

o 18

2'7

~6

4'5

~'4

63

72

81

90

98

~mp~e number

Fig.2. Numbers of VIth stage nauplius larvae and cyprid larvae per 200 litre, in 9 series of 9 samples each; taken from the raft in the entrance to the harbour of Den I/elder, 3July 1968, 14.13 to 15.30 h. Predicted high water 12.12 h; low water 18.52 h. Samples were thus taken from the ebb. TABLE

II

Autocorrelation between numbers ofcyprid larvae, and between VIth stage nauplius larvae in successive samples taken with different nets 3 July 1968. Larvae

Sample order

Correlation coe~icient

t

p

cyprids

Ist 2nd 3rd 5th 9th

--0.145 0.172 0.170 0.024 0.323

1.29 1.53 1.50 0.21 2.86

>0.05 >0.05 >0.05 >0.05 <0.05 (>0.01)

nauplii

1st 2nd 3rd

--0.180 0.242 0.068

1.64 2.19 0.59

>0.05 0.05 >0.05

is at least p a r t l y d u e to too l a r g e sewing-holes. A l t h o u g h nets are a source o f e x t r a v a r i a n c e t h e y do not a d d significantly to the total variance (Table III). H o w e v e r , in o r d e r to p r e v e n t i n t r o d u c t i o n o f f u r t h e r v a r i a n c e o n l y o n e net was used in the s a m p l i n g series t h a t follow. I m m e d i a t e l y after t r a n s f e r r i n g p l a n k t o n f r o m the nets to collecting j a r s the s a m p l e s w e r e

16

P. DE W O L F TABLE

III

Analysis of variance of net-effect, 3 July 1968.

Source of variation

Degrees of freedom

nets ( = samples) series of 9 samples nets x series -+- residual total

8 8 64 80

Sum of squares 5299 2669 53021 60989

Mean square 662 334 828

fixed with 1% neutralized formaline. This precaution was found to be essential, since initially quite a few of the cyprids were found to settle and metamorphose in the collecting bottles if fixing was postponed. The number of samples determines the degree of precision obtained. For studies of heterogeneity a high degree of precision is necessary, so that the number of samples has to be great; this can be determined beforehand. It was a priori assumed that the larvae are not normally distributed, but do show patchiness. It has been shown that in such cases (BARNES, 1962; CASSIE, 1952) the frequency distribution of numbers of objects can usually be fitted rather good to a negative binomial distribution, and Rojhs (1964) indicated that in such a case the number of samples (N), mean number of items sampled 17), and precision (D) are related by the equation: 1

+-- 1 17 K

N--

D2

where K denotes the dispersion parameter of the negative binomial distribution, K-

J~2

S2 _

and S 2 variance. Hence, an approximate value for J~ and S ~ should be obtained. For the example used earlier (3 July 1968) X = 22.0, 1

- 0.38, and for a precision of 1% (D = 0.01) follows: K 1

- - + 0.38 22 N--

0.0001

-- ~4300

DISPERSAL

OF B A R N A C L E

LARVAE

17

It stands to reason that it is impossible, that such a number of samples could be taken. Conversely it is possible, using the same formula, to calculate the precision obtained for the samples taken: N = 80, D = 0.074. This was obtained at a sampling rate of 1 sample per minute. It will be clear that to study tidal variation such a sampling rate would lead to prohibitively large numbers of samples. On the other hand it was thought worthwhile to sample at a much higher rate than had been done previously by others; sampling schedules in the present investigation were usually dictated by physical possibilities only. As a rule 6 samples, occasionally 12 samples, were taken every hour; while exceptionally samples were taken every minute for limited periods. Counting was done by transferring the contents of the collecting bottle to a large size petri dish under a low power binocular microscope; a hand tally counter was used. Although m a n y reliable methods for sub-sampling are known, (reviews by WmORG, 1961: 74; FROLANDER, 1968: 89) all these will introduce extra variances. It was preferred, therefore, to count whole samples. (In the petri dish it was found that very often dead cyprid larvae clustered together despite thorough mixing). The accuracy of counting proved to be high; cyprids have a characteristic appearance, while V l t h stage nauplii are conspicuous because of the three black eyes. Ten repeated counts of a sample with much organic debris gave a cyprid count of 242, with a standard deviation of 0.82. 3-2. RECOONITION OF SPECIES No efforts have been made to separate V I t h stage nauplii into species; it was only after the experiments had been finished that it was realized that the effort might have been worthwhile. Cyprid larvae have been separated into species: Balanus crenatus, B. improvisus and Elminius modestus. As it is rather difficult to distinguish between the cyprids of these species, this will be discussed at some length. A good deal of information has been collected from literature (Table IV) for the 3 species mentioned and for Balanus balanoides. This species is the only other species present in the area, except Verruca stroemia which is rare. There is considerable variation within each species, as well as overlap of species, as to size and color. PYEFINCH (1948a: 461) compared the cyprids of Balanus balanoides and B. crenatus and noted that "the limits of variation in length of the cyprids of the two species unfortunately are such that the biggest Balanus crenatus are as long as, or slightly longer than, the smallest cyprids of Balanus balanoides".

B. improvisus

E. modestus

B. crenatus

B. balanoides

Species

opaque yellow brown dark brown

0.21 -0.25 0.235 -4- 0.010 0.269

0.5 -0.6 0.523 -4- 0.012

0.58 0.587

-0.6

colorless and of glassy transparency strawcoloured

not brown

yellow brown

variable brown

Colour

0.53

0.6

0.5

0.73 -1.1 0.54 -0.56

0.87 -0.96

0.77

0.07 0.24

0.5

mm

nltn

1.2 0.94 0.9 - I . 1 0.81 -1.22 0.90 -1.2 0.1 0.55

Breadth

Length

eastcoast of England west coast of Sweden east coast of England Baltic Sea east coast of England Zuiderzee Biscayne Bay

Scotland Essex

Bergen, Norway Plymouth Scotland Scotland Baltic Sea Puget Sound California, in culture Scotland, in culture Scotland, in culture

Place

TABLE IV Characteristics from literature of cyprid larvae of different species of barnacles.

GROENEWEGEN, 1922 DOOCmN, 1951

KNIOHTJONES & WAUGH, 1949 BUCHHOLZ, 1951 JONES & CRISP, 1954

TENOSTRAND, 1931

JONES • CRISP, 1954

PYEFINCH, 1949, 1948, see also BARNES& COSTLOW, 1961 : 64 BARNES, 1953 KNIGHTJONES & WAUGH, 1949

PYEFINCH,1949: 921

RUNNSTROM, 1925:10 BASSINDALE,1936 PYEFXNCn, 1948 BARNES, 1953a : 297 BUCHttOLZ, 1951 BOHART, 1929 t-IERz, 1933

Author

CO

DISPERSAL OF BARNACLE LARVAE

19

BARNES (1953a: 297) found that the cyprids of Balanus crenatus, B. balanoides (and Verruca stroemia) varied in length, breadth, and length/ breadth ratio, from place to place, and from moment to moment, as well as at a particular place and moment. The diverse length modes of the cyprids of Balanus crenatus fell between 0.81 and 1.22 mm; with some indication that the mean length became smaller later in the season; this might possibly be related to the temperature at which the embryos developed. BARNES also discussed the length/breadth ratio, which, although varying, is unimodal, which means that no different shapes can be distinguished within each species. Even greater difficulties arise in distinguishing between the cyprids of Elminius modestus and Balanus improvisus. They are approximately of the same size and have no discriminative colour differences (Table IV). JONES & CRISP (1954) found the cyprids of Balanus improvisus to be broader and more oval than those of Elminius. They considered it difficult to distinguish between the species. Separation in time and space makes it possible to recognize the larvae of different species (BARNES& COSTLOW,1961). The settlement seasons of the cyprids of the different species are relatively well known. As the present plankton samples were taken between the end of May and the middle of September, Balanus balanoides larvae were never found. This species settles nearly always before the end of May (Fish, 1925; RUNNSTRSM, 1925; HATTON & FISCHER PIETTE, 1932; GRAVE, 1933; MOORE, 1935; PYEFINCH, 1948; BARNES, 1950; BOUSFIELD,1954; BLOM & NYHOLM, 1961). MEADOWS (1969), however, found settlement during the summer in Scotland. Balanus crenatus spawns more than once a year. PYEFINCH (1948b) found in Millport larvae of this species somewhat earlier in the year than larvae ofB. balanoides. BARNES & COSTLOW (1961) indicate, however, that it is likely that PYEFINCH's material contained larvae of B. balanus as well; the last species settles mainly during the second half of May (CRIsp, 1954). B. balanus occurs usually on rough grounds, in deeper water (BARNES& BARNES, 1954), and does not occur, or is very rare, in the Wadden Sea. Settling orB. crenatus was found by PYEFINCH at the end of March, and then to be of varying magnitude throughout the summer, till mid-October. BARNES (1950) found, in the same area, that settling is stronger during the second half of April. BOUSFIELD (1954) found that B. crenatus spawns in April-June, and again in September-October on the Atlantic coast of Canada. Two different spawning periods were also found by BLOM & NYHOLM (1961) on the west coast of Sweden. There settling occurred mainly during 1 to 2 weeks, between the beginning of May and mid-June; later a few cyprids may be found, but not before September do they settle in

20

P. D E W O L F

numbers again. In Den Helder settlement of B. crenatus is virtually absent during April, begins with a density of 1 barnacle per 10 cm2/day in the beginning of M a y (1969) or at the end of M a y (1968), and lasts till about the end of the third week of June. Settlement occurs again at the end of August (1968) or the beginning of September (1969). In the interval only a limited number ofcyprids is found. Balanus improvisus settled on the west coast of Sweden from the end of J u n e till mid-July (TENGSTRAND, 1931). In Oslo Fjorden, BRocn (1924) found the first settled cyprids at the end of May, and settling continued during the first half of June. in the Baltic Sea, LucKs (i 940) found larvae in the plankton during J u l y and in October. VAN BREEMEN (1934) found settlers in Amsterdam at the end of J u n e and also in August-September. According to BOUSrmLD (1954) B. improvisus spawns on the east coast of Canada in June-July and again in September-October. Settling in Den Helder commenced at the end of J u n e (1969), or the beginning of J u l y (1968); in both years there was a slight overlap with the last settlers orB. crenatus. In both years settling ofB. improvisus continued throughout July, in 1968 till the middle of August, in 1969 till 30 July. Elminius modestus is an extreme eurythermic species, as follows from observations of CRISP & DAVIS (1955). The larvae appear in the plankton about one month after those o f B a l a n u s balanoides, and breeding is intensive during the summer, continuing as long as the temperature does not drop below 6 ° C. Settling commences in Den Helder in M a y ; mainly in the intertidal zone, and continues till late in the year, as has been found by HOUGHTON & STVBmNGS (1963) near Portsmouth on the south coast of England, and KNIGHT JONES & WAUGH (1949) on the east coast. TABLE V

Measurements of barnacle cyprids, in microns. Date

31-5-67 1-7-67 9-9-67

Species

Length and S.D.

Breadth and S.D.

Number measured

B. crenatus E. modestus B. crenatus B. improvisus B. improvisus E. modestus B. crenatus

643 :h 21 444-t- 15 629 -4- 24 514 + 14 511 -4- 15 450-t- 16 595 4- 30

271 -4- 19 216+ 11 272 -4- 17 254 ± 9 240 + 10 217+ II

100 100 40 40 40 40 5

DISPERSAL

OF

BARNACLE

21

LARVAE

Cyprids of Verruca stroemia, a species which is not numerous in the Wadden Sea, were occasionally seen. They are very small (PYEFINCH, 1948) and are readily recognized. Considering the observations and literature data it is clear that only 2 species of cyprids during the end of May and the beginning of June could be present: Balanus crenatus and Elminius modestus. On the basis of length and breadth they could be readily separated (Tables IV and V). In the beginning of July cyprids ofBalanus improvisus appeared in the samples; they could easily be recognized in being considerably smaller than those of B. crenatus, and slightly larger, more rounded and "fatter" than those of Elminius (Table V). Only at a few occasions it proved impossible to distinguish between B. improvisus and Elminius modestus. 3-3.

CHEMICAL

AND

PHYSICAL

PROPERTIES

OF SAMPLES

It was attempted to characterize water masses and samples in which cyprid larvae would or would not be present, by measuring chemical and physical properties of the samples. Initially temperature, chlorinity, total suspended matter, and height of water level were measured; in later series of observations currents were also measured, and the suspended matter was fractionated into two grain size fractions. Temperature was measured in the outflow of the plankton pump. A normal thermometer was used and read to the nearest tenth. Chlorinity was measured in a water sample, taken halfway during the filtering of the 200 litre plankton samples. Total suspended matter was determined by filtering and weighing a one liter sample taken during plankton pumping. Fractionation of total suspended matter in a sand- and a silt fraction was done as described by POSTMA (1954: 412). The water level was read from a tide staff. Current velocities were determined by means of calibrated Savonius type current meters, kindly lent by the Netherlands Institute for Sea Research. 4. RESULTS

Data have been collected during the summers of 1967 to 1969. Usually sampling took place during a considerable part of the tidal cycle; on 3 days 2 consecutive tidal cycles have been sampled. Data are reproduced in Figs. 2 to 11 (the full data may be obtained in mimeographed form from the author). The distribution of cyprids, and sometimes VIth stage nauplii, will be considered on the basis of dispersion coefficients, autocorrelations,

22

p. DE WOLF

and correlations of numbers with chemical and physical properties of the seawater. 4-1. BALANUS IMPROVISUS

Larvae of Balanus improvisus have been sampled in the entrance to the harbour of Den Helder on 27 June 1967, on 3 July 1968, on 1 and 2 J u l y 1969, on 5 and 6 August 1969, on 20 August 1969 and on 9 and 10 September 1969. They were sampled on a fixed station on Texelstroom on 5 July 1968. On 27 June 1967, there was a considerable variation in the number of cyprids of B. improvisus from sample to sample (Fig. 3). Dispersion coefficients CF and c are significantly different from I, respectively 0 (Table VI). 22¢ 2C~ lSC16C140" 12C8100"

u 4c-

u

E~ 1l .l

10

it

12

13

14

15

16 hou s

Fig.3. Total suspended matter (mg/l); chlorinity (%0); temperature (°C); and numbers of cyprid larvae of Balanus improvisus per 200 1 on 27 June 1967; sampled from a raft anchored in the entrance to the harbour of Den Helder. Predicted high water 11.08 h, low water 4.57 h and 17.41 h.

DISPERSAL

OF

BARNACLE

23

LARVAE

In the period from 9 to 14 hours temperature and suspended matter are relatively constant, both being at a low level. After that time both rise to higher levels while they start to fluctuate more than before. T A B L E VI

Dispersion coefficients for cyprids of Balanus improvisus during 3 periods on 27 June 1967. Period h

9.00-16.00 9.00-14.07 14.07-16.00

CF

c

p

35.5 26.3 3.9

0.47 0.28 0.12

(((0.001 (<(0.001 )0.05

The impression is obtained that 2 different water masses have been sampled, and also that the number of cyprids drops to a lower level (Fig. 3). This is substantiated by appreciable differences in the dispersion coefficients in the periods before and after 14 hours (Table VI) : in the first period the cyprids are obviously groupwise distributed; during the second period the distribution cannot be shown to be different from a random dispersion. Autocorrelation coefficients of the 1st, 2nd, 3rd and 4th order, are 0.72, 0.57, 0.46, 0.34; all these are significantly different from zero (p < 0.001); 5th and higher order autocorrelafion coefficients are not significant. This means that the number of cyprids in a certain sample is not independent of the numbers in samples, taken before and after that sample. It suggests, that several waves of cyprids pass the sampling station. It should be remembered, however, that possibly a variability is superimposed on this pattern, depending upon changing physical and chemical properties within the patch (page 28). This is substantiated by a correlation between the numbers of cyprids per sample and its temperature, salinity and suspended matter content (Table VII). From 9.00 to14.07 T A B L E VII

Correlation coefficients for numbers of cyprids of Balanus improvisus with chlorinity, suspended matter, and temperature, respectively, during 2 periods on 97 June 1967 Correlation

- chlorinity suspended matter temperature -

-

9.00-14.07 h

14.07-16.00 h

r

p

r

p

--0.44 0.03 0.40

<0.01 n.s. <0.01

0.07 --0.36 0.08

n.s. >0.05 n.s.

hours, when the larvae are groupwise distributed, high numbers of larvae are associated with relatively low salinities and with relatively high temperatures. There is no correlation with the amount of suspended material in the water. Later, from 14.07 to 16.00 hours, the correlation with temperature and salinity is lost, but a different water

24

P. DE WOLF

mass r e a c h e d the sampling station. H e r e the n u m b e r s o f cyprids are possibly associated with w a t e r with a low c o n t e n t o f suspended matter, a l t h o u g h in the m e a n the n u m b e r s o f cyprids are m u c h lower t h a n before. I t thus appears t h a t in the case shown cyprid larvae o f B. improvisus are sometimes dispersed in groups, a n d sometimes not. W h e n t h e y do o c c u r in groups t h e y are associated with w a t e r t h a t is w a r m a n d relatively fresh. A distribution in groups also follows from the short period o f sampling o n 3 J u l y 1968. E i g h t y samples were t a k e n in 75 minutes (Fig. 2). D u e to the high speed in sampling plankton, no chemical or physical properties could be d e t e r m i n e d . Dispersion coefficients ( T a b l e V I I I ) TABLE

VIII

Dispersion coefficients of numbers of cyprid larvae and VIth stage nauplius larvae of Balanus improvisus, on 3 July 1968. Larvae

Mean

Variance

CF

c

p

nauplii cyprids

1.6 22.0

4.0 208.7

2.5 9.5

0.90 0.38

>0.05 <0.001

show t h a t c y p r i d l a r v a e are clearly distributed in groups, while this c a n n o t be said for V I t h stage nauplius larvae, possibly due to the low m e a n n u m b e r o f nauplii present (cf. CnssiE, 1959b: 402). T w o days later, o n 5 J u l y 1968, on T e x e l s t r o o m , V I t h stage nauplii a n d cyprids are b o t h distributed in groups (Fig. 4); dispersion coefficients CF a n d c are, at all depths, always significantly greater t h a n 1, respectively 0 ( T a b l e I X ) . A l t h o u g h there are some differences beTABLE

IX

Means, variances and dispersion coefficients of Vlth stage nauplii and cyprids of Balanus improvisus on a fixed station on Texelstroom, 5 July 1968. Larvae

Depth

n

Mean

Variance

CF

c

p

nauplii

2 4 6 8 9½ all

27 27 25 27 9 115

31.6 39.2 39.6 41.0 80.7 41.2

432 1218 1274 985 1354 1109

13.7 31.1 32.2 24.0 16.8 27.0

0.40 0.77 0.79 0.95 0.19 0.63

<0.001 <0.001 <0.001 <0.001 <0.001 <0.001

cyprids

2 4 6 8 9½ all

27 27 25 27 9 115

39.3 38.1 37.4 42.4 80.2 42.1

824 924 1399 992 645 1102

21.0 24.3 37.4 23.4 8.0 26.2

0.51 0.61 0.97 0.69 0.09 0.60

<0.001 <0.001 <0.001 <0.001 <0.001 <0.001

D I S P E R S A L OF B A R N A G L E L A R V A E

25

9~.m

¢hrorimty

tempe~ture

lo

12

'

I~ ~rs

Fig.4. Numbers of V I t h stage nauplius larvae and cyprid larvae of Balanus improvisus per 200 1; temperature (°C); chlorinity (~0o) ; and total suspended matter (rag/l) ; in samples taken from an anchored ship at 5 depths, on Texelstroom, 5 July 1968. Predicted high water approximately 14.30 h, low water approximately 8.30 h and 21.30 h.

26

P. D E

WOLf

TABLE

X

Correlation coefficients for V I t h stage nauplii and cyprids of Balanus improvisus with chemical and physical properties of water, Texelstroom, 5 J u l y 1968.

Larvae

Correlation

r

p

nauplii

- temperature - suspended matter chlorinity - temp. and chlor.

0.83 --0.37 --0.82 0.84

((<0.001 (0.001 (<<0.001 <<<0.001

temperature suspended matter - chlorinity temp. and chlor.

0.89 --0.35 --0.87 0.89

(<<0.001 <0.001 <<<0.001 <(<0.001

-

cyprids

-

-

-

tween the distribution of the nauplii and the cyprids, these are not considered to be important. In the sampling period, there is at every depth an increase and subsequent decrease of the content of suspended matter, and a continuous rise of temperature and salinity, while the numbers of larvae of both stages, in spite of heavy fluctuations, continuously decrease. Correlations of numbers of larvae with temperature, salinity and suspended matter content show (Table X) that

5OO

':' 0.3 ~02 0.7 0 .~C~

~

9o

80

5O

E

Q

10

11 12 13 1 Jury1969

14

15

16

17

18

19

20

21

22

23

24

1 2 3 2 July 1969

4

5

(5

7

8

9

0

12 hour5

Fig.5. Numbers of cyprid larvae of Balanus improvisus per 200 1; dispersion coefficient c for each hourly series of samples; m e a n n u m b e r of barnacles settling on 10 panels of 200 cm ~ each; and water level (m) ; on 1 and 2 J u l y 1969, in the entrance to the harbour of Den Helder.

DISPERSAL

OF B A R N A C L E

27

LARVAE

larvae are strongly associated with relatively warm and fresh water, and less strong with water having a low content in suspended matter. From multiple correlations follows (Table X) that the distribution of the larvae can be nearly fully explained by their association with relatively warm and relatively fresh water. Presumably the above mentioned decrease in numbers of larvae is a tidal variation, upon which, as shown earlier, a variation with a much shorter period is superimposed. It follows also that the number of larvae increases slightly with depth (Fig. 4). A tidal variation in numbers of cyprids was observed again, although to a certain extent reversed, on 1 and 2 July 1969 in the entrance to the harbour (Fig. 5). Initially the numbers decrease during ebb; before low water is reached, however, the flood current starts, and the numbers increase sharply, till about mid-tide. Then the numbers drop again during the last part of the flood, high water and most of the ebb. This cycle is repeated during the next tide. (It is rather unfortunate, that only during the last 7 hours cyprids of B.

160 120

~ 31I(3. .11 . 12. 13. 14 . .15 . 1~. I"7 . .le, . 19. 20. 21 . .22 . "23. 24. .1 . 2 . 3. .4 . 5 . 6 . 7. .8 . 9 . 10. 11 . .I ~ 5 AUgUSt 1~59

6 AUght 1 ~ 9

13

14

hour~

Fig.6. N u m b e r s of c y p r i d l a r v a e ofBalanus improvisus p e r 2001; dispersion coefficient c for each h o u r l y series of samples; total n u m b e r of b a r n a c l e s settling on 10 panels of 200 c m ~ e a c h ; a n d w a t e r level (m) ; o n 5 a n d 6 August 1969, in t h e e n t r a n c e to the h a r b o u r of D e n Helder.

28

P. DE WOLF

improvisus and B. crenatus have been distinguished from each other; in this period both species show the p h e n o m e n o n described). Dispersion coefficients c have been calculated for each hourly series of 6 samples (Fig. 5) ; there is some indication, that c varies (independent of the mean!) with the tidal level, being high on the incoming flood, and falling to a m u c h lower value at late flood, high water and ebb. This would m e a n that the groupwise distribution is outspoken during the early flood, that the groups are gradually broken down later, and formed again in the flood. T h e same situation is found again on 5 and 6 August 1969 (Fig. 6) 200 100 50

! ~,o

!o, [__

~ ~IOo

c_Jr-]

I0]

~Q2

11:1 ~3

.

9

.

2O August

10

.

.

11

.

12

.

.

13

.

14

15

16

17

IB

lg

20

hours

lg6g

Fig.7. N u m b e r of cyprid larvae ofBalanus improvisus per 200 1; dispersion coefficient c for each hourly series of samples; m e a n numbers of barnacles settling per hour on 10 panels on each of east and west side of raft; temperature (°C), chlorinity (~o); suspended matter, silt- and sand-fraction, in mg/1; and water level (m) ; on 20 August 1969, in the entrance to the harbour of Den Helder.

DISPERSAL

OF BARNACLE

LARVAE

29

in the entrance to the harbour. Cyprids of B. improvisus show an increase in numbers on the flood, and subsequently a decline on the ebb, on 5 August. During the following ebb in the night the decline, though present, is not as clear as on the day before, and on the following flood the numbers rise again sharply. Also the dispersion coefficient c (independent of the mean) increases steeply, followed by a gradual decrease. This with the exception of a rather high c during the ebb from 2.30 to 3.30 h on 6 August which for the m o m e n t cannot be explained. O n 20 August 1969 (Fig. 7) the same is found: increase of numbers of cyprids and of dispersion coefficient c on the flood, and a gradual decrease of both during the ebb. During this experiment temperature, chlorinity, sand- and silt content were determined. Despite a general increase of chlorinity during the sampling period, and the general decrease of the n u m b e r of cyprids there is no correlation between these two (Table XI). Numbers are negatively correlated with T A B L E XI C o r r e l a t i o n coefficients for n u m b e r s o f c y p r i d s o f Balanus improvisus w i t h p h y s i c a l a n d c h e m i c a l p r o p e r t i e s o f w a t e r i n t h e e n t r a n c e o f t h e h a r b o u r , 20 A u g u s t 1969.

Correlation -

temperature chlorinity silt sand

r --0.31 --0.01 0.53 0.31

p 0.01 n.s. <0.001 0.01

temperature although r is rather low. There is a much better positive correlation with the silt content of the water. The correlation with the sand fraction of the suspended matter is less outspoken. During the last sampling period, on 9 and 10 September 1969 (Fig. 8) the general trend is maintained: a sharp rise of numbers of cyprids of B. improvisus and their dispersion coefficient on the flood, and subsequently general and slow decrease. It should be stated that values of c strictly cannot be compared with those of earlier sampling runs, as now one sample was taken every 5 minutes instead of every 10 minutes. Cyprids of B. improvisus are now associated with high current velocities, with relatively low temperatures and with relatively high salinities (Table X I I ) . Furthermore, there is some indication that they are associated to a certain extent with water with a high silt content, but not with sand content. This seemingly contradictory result (water with high current velocities will usually carry sand and silt) will be discussed later (page 43).

30

P. D E W O L F

i20" ~1o.

wlOO- / ~5oai2o• "~lOg

i: ~

,

aa ~.o.

~

~o

[~

-~°"1

I

I

'

'

~

L--,---L___

~2

9 S~tember 1969

lOSePtembe~ 1969

hours

Fig.8. Numbers of cyprid larvae of Elminius modestus, Balanus improvisus and B. crenatus per 200 1; dispersion coefficients c for each hourly series of samples; mean n u m b e r of settling barnacles (all species) on 20 panels (10 on east side of raft, 10 on west side); current velocity (cm/sec), silt and sand content (mg/l); and water level (m) ; on 9 and 10 September 1969, in the entrance to the harbour of Den Helder.

DISPERSAL

OF BARNACLE

31

LARVAE

T A B L E XII

Correlation coefficients for numbers of cyprids of Balanus improvisus, B. crenatus and Elminius modestus with chemical and physical properties of the water in the entrance of the harbour, 9 and 10 September 1969. Correlation

B. improvisus

r - temperature - chlorinity silt sand total susp. matter - current velocity -

-

-

--0.20 0.21 0.14 --0.13 0.06 0.44

4-2.

B. crenatus

p <0.001 <0.001 <0.01 <0.01 n.s. <<0.001

BALANUS

E. modestus

r

p

r

p

--0.26 0.21 0.20 0.08 0.22 0.18

<0.001 <0.001 <0.001 n.s. <0.001 0.001

--0.15 0.04 --0.04 --0.06 --0.07 0.08

0.01 >0.05 >0.05 >0.05 >0.05 >0.05

CRENATUS

C y p r i d s o f Balanus crenatus h a v e b e e n s a m p l e d on 15 a n d 16 A u g u s t 1967 in the e n t r a n c e o f the h a r b o u r ; on 30 J u l y 1968 while following a free floating d r o g u e o n T e x e l s t r o o m ; on 30 M a y 1969 on a fixed station o n T e x e l s t r o o m ; a n d on 9 a n d 10 S e p t e m b e r 1969 in the e n t r a n c e to the h a r b o u r again. O n 15 a n d 16 A u g u s t 1967 the n u m b e r s at a d e p t h o f 1.50 m w e r e low (Fig. 9); the highest n u m b e r was 17, b u t quite often no l a r v a e at all w e r e found. T h e g e n e r a l t r e n d as seen for cyprids of B. improvisus is f o u n d : r e l a t i v e l y h i g h n u m b e r s o n the flood, g r a d u a l decrease d u r i n g high tide, a n d no l a r v a e p r e s e n t at ebb. A t the b e g i n n i n g o f the n e x t flood n u m b e r s rise again. T h e l a r v a e o c c u r associated with relatively cold w a t e r w i t h a h i g h c o n t e n t in total s u s p e n d e d m a t t e r ( T a b l e X I I I ) . Dispersion coefficients c a l c u l a t e d for several parts o f T A B L E XIII

Correlation coefficients for numbers of cyprids of Balanus crenatus with physical and chemical properties of water, 15 and 16 August 1967, in the entrance to the harbour. Correlation

temperature - chlorinity suspended matter - chlorinity and temperature suspended matter and temp. -

-

-

r

-0.360 0.074 0.405 0.374 0.504

p

((0.001 n.s. (((0.001 0.01--0.001 (((0.001

the tidal cycle ( T a b l e X I V ) g e n e r a l l y i n d i c a t e a g r o u p e d d i s t r i b u t i o n o f the l a r v a e . Since, h o w e v e r , the n u m b e r o f l a r v a e p e r s a m p l e is r a t h e r low the dispersion coefficients lack precision, a n d c o n s e q u e n t l y

32

P. DE WOLF o

~1o4

~z5 J -21z2~

~16eo~6o-

~50~40-

~

20-

~ lO-

O.

~

j J ~ lb

1~

11

1'6

15

1;3

lb

2b

2'1

2'2

2b

F4

15 August 1967

>

~ 10

6. 18.317.917.~ :~ 17.20" "~ 1"Z0060" 50E 40"

0 4:5~3.5" o

~

~

~

~

~

i

~

8

9

lO

11

16August 1967

Fig.9. N u m b e r s of c y p r i d larvae of Balanus crenatus t a k e n with 2 different p l a n k t o n p u m p s ; t e m p e r a t u r e (°C) ; chlorinity (%o) ; total suspended m a t t e r (rag/l) ; a n d water level ( m ) ; on a raft a n c h o r e d in the e n t r a n c e to the h a r b o u r of D e n Helder, on 15 a n d 16 August 1967.

DISPERSAL

OF

BARNACLE

33

LARVAE

T A B L E XIV

Means, variances and dispersion coefficients for cyprids ofBalanus erenatus, on 15 and 16 August 1967, in the entrance to the harbour. Period

12.45-16.00 (HW) 16.00-20.00 (ebb) 20.00-22.50 (LW) 22.50-02.55 (flood) 02.55-04.23 (HW) 04.23-09.06 (ebb) 09.06-I 1.37 ( L W , flood)

Mean

Variance

Ce

c

p

2.4 2.3 1.7 8.0 3.0 1.5 4.2

5.9 2.7 2.5 14.7 3.6 3.2 9.0

2.5 1.2 1.5 1.8 1.2 2.1 2.2

0.61 0.08 0.28 0.10 0.05 0.75 0.27

<0.001 4-0.5 0.1 0.001 0.5 <0.001 <0.001

h a r d l y a n y i n d i c a t i o n is f o u n d o f a relationship of c with the tides, as found for B . improvisus. I n this e x p e r i m e n t 2 different p l a n k t o n p u m p s h a v e been used simultaneously for selected periods, with intake hoses at a distance o f 1 m from each other, b o t h at a d e p t h o f 1.50 m. Correlation b e t w e e n the n u m b e r s o f cyprids c a u g h t b y the 2 p u m p s gave r = 0.60 (p ( ( ( 0.001). This correlation coefficient, t h o u g h highly significant, indicates t h a t there can be considerable difference b e t w e e n the 2 p u m p s which p r o b a b l y reflects differences at the 2 sampling points. O n 30 J u l y 1968 an a t t e m p t has been m a d e to avoid the effect of position in the tidal current, b y sampling at a d r o g u e floating with the tidal c u r r e n t . T h e d r o g u e followed the c u r r e n t at a d e p t h o f 3 m, a n d also sampling was d o n e at t h a t d e p t h , 10 to 35 m e t e r f r o m the drogue. T h e position o f the ship was plotted at 30 min intervals (Fig. 1). Results (Fig. 10) show the tidal v a r i a t i o n to be still present in this e x p e r i m e n t . T h e n u m b e r o f cyprids increases on the flood, followed b y a decrease on the last p a r t o f the flood, high w a t e r slack, and on the first p a r t o f the ebb. H o w e v e r , opposite to the results o b t a i n e d in the e n t r a n c e to the h a r b o u r , the n u m b e r s o f larvae increase again on the T A B L E XV

Means, variances and dispersion coefficients for Vlth stage nauplii and cyprids ot Balanus erenatus, during 3 periods while following a drogue on Texelstroom on 30July 1968. Larvae

Period

Mean

Variance

nauplii

09.20-11.18 11.18-14.15 14.15-16.20 09.20-16.20

22.3 10.4 13.5 16.2

118.4 21.6 72.7 102.4

cyprids

09.20-11.18 11.18-14.15 14.15-16.20 09.20-16.20

66.3 21.9 56.8 52.0

1908 151 1452 1605

CF

c

p

5.3 2.1 5.4 6.3

0.19 0.10 0.32 0.33

<<0.001 0.001 <<0.001 <<0.001

28.8 6.9 25.6 30.9

0.42 0.27 0.43 0.57

<<0.001 <<0.001 <<0.001 <<0.001

34

P. DE W O L F

ebb, while at a later stage of the ebb a decrease sets in. On the basis of numbers of cyprids (and VIth stage nauplii) 3 periods can be distinguished, for which dispersion coefficients have been calculated (Table XV). It is seen that both larval stages occur markedly grouped during flood, and less so during high water. During the ebb the groupwise dispersion is restored again to the same extent as existed on the flood. The degree of over-dispersion is higher in cyprids than it is in VIth stage nauplii. Temperature, chlorinity and suspended matter changed at the drogue (Fig. 10) thus showing that a considerable vertical and lateral mixing of water took place. Correlations of numbers of cyprids with water characteristics (Table XVI) show over the whole period an

2C~ 16¢

0-

i

16

~164

:!1

i7 eo-

e~

i . 0 9

1~)

11

~2

1~3

1;;

1'5

1~

hours

Fig. 10. Numbers of cyprid larvae of Balanus crenatus per 200 1; numbers of V I t h stage nauplius larvae per 200 1; chlorinity (~oo); temperature (°C); suspended matter (mg/1) ; sampled while following a drogue on Texelstroom, 30 J u l y 1968. Predicted high water D e n Helder 10.56 h, low water 17.33 h.

DISPERSAL

OF

BARNACLE

LARVAE

35

T A B L E XVI

Correlation coefficientsfor numbers of cyprids and Vlth stage nauplii of Balanus crenatus with physical and chemical properties of water, during 3 periods while following a drogue on Texelstroom on 30 July 1968; ++ means p < 0.01, + means p < 0.05. Larvae

cyprids

Correlation

09.20ll.18h

11.1814.15h

14.1516.20 h

09.2016.20h

temperature - chlorinity suspended matter

--0.36 ++ --0.11 --0.06

--0.20 0.08 0.23

--0.16 0.12 --0.22

--0.17+ 0.08 0.19

temperature - chlorinity suspended matter

--0.38 ++ 0.13 0.14

--0.45++ 0.22 0.12

--0.38++ 0.47++ 0.53++

--0.47++ 0.42++ 0.39++

-

-

nauplii

Period

-

-

association with cold water, rich in suspended matter; for the different periods distinguished this association is much less clear. Nauplius larvae are, generally, associated with cold and salt water rich in suspended matter. On the 30 M a y 1969 cyprids and V I t h stage nauplii have been sampled from an anchor station on Texelstroom (Fig. 1), from 09.44 to 21.00 h. The general trend for the numbers of cyprids and nauplii (Fig. 11) is an increase on the ebb till the time of highest current speeds and after that a general decrease; this decrease continued during the flood, high tide, and the beginning of the next ebb. The slight difference from earlier and later observations in that the sudden increase in numbers on the flood is lacking, will be discussed later (page 46). Dispersion coefficients c are always significantly greater than zero, indicating for all periods an outspoken groupwise distribution of the larvae (Table X V I I ) . Correlation coefficients for numbers and chemical and physical properties show that the larvae in this May experiment are associated with relatively warm, fresh water, that is rich in suspended matter (Table X V I I I ) . On 9 and 10 September 1969, in the entrance to the harbour the total number of cyprid larvae of B . crenatus was not high, but 2 short periods with high numbers can be distinguished (Fig. 8). These occur both on the floods; inbetween these periods the numbers generally drop to low values, or even zero. There is some indication that the dispersion coefficient c has a high value during flood, (at least during the flood on 9 September) and is generally much lower during slacks and ebb. From correlations it follows (Table X I I ) that the cyprids are associated, in this September experiment, with water that is cold,

36

P. DE W O L F

o-

chloe nily

\

,~,q//"

,,

I

!\1

t emp ~ t . r e

~Bp?r#ed m a t t e r

k3]l

.

I

.

fin~ fPGction

.

.

~. ;? _ _ j l _ ~ l

II

s u s ; ~ d ~ 3 ~ , ; t t s - . co~r~# fr¢ctio~

~-1 :i g--T-

,i~, ~ .................. <1

--i'2

tg

~7~r~

"

6

7,-

B

~b

2b

2~

Fig.11. Numbers of V I t h stage nauplius larvae; and numbers of cyprid larvae of Balanus crenatus per 200 1; chlorinity (%o) ; temperature (°C) ; suspended silt and sand

(mg/1) ; on an anchor station on Texelstroom at 2 depths, on 30 May 1969. Predicted high water at Den Helder 6.39 h and 19.04 h, low water 12.57 h.

DISPERSAL

OF

BARNACLE

TABL

TM

37

LARVAE

XVII

Dispersion coefficients for cyprids and VIth stage nauplii of Balanus erenatus on 30 May 1969, for 4 periods, and different depths, on a fixed station on Texelstroom. Larvae

Depth

Period

CF

c

t

p

3 3 3 3

09.44-14.10 14.10-14.50 14.50-18.00 18.00-21.00

64.0 7.2 61.7 12.2

0.31 1.17 0.39 0.22

227 7.5 166 31.7

<0.001 <0.001 <0.001 <0.001

6 6 6 6

0 9 A l 14.10 14.10-14.50 14.50-18.00 18.00--21.00

41.4 89.6 94.0 16.6

0.09 0.30 0.38 0.23

114 109 271 44.1

<0.001 <0.001 <0.001 <0.001

3 3 3 3

09.44-14.10 14.10-14.50 14.50-18.00 18.00-21.00

19.8 5.5 29.3 18.6

0.14 0.29 0.20 0.47

60.9 5.5 76.0 50.0

<0.001 <0.001 <0.001 <0.001

6 6 6 6

09.44-14.10 14.10-14.50 14.50-18.00 18.00-21.00

24.7 58.2 38.t 14.5

0.09 0.29 0.17 0.28

m

cyprids

nauplii

58 70 108 38

<0.001 <0.001 <0.001 <0.001

salt, a n d t h a t h a s a h i g h silt c o n t e n t , a n d w i t h h i g h c u r r e n t v e l o c i t i e s . Although significant, correlation coefficients are rather low.

TABLE

XVIII

Correlation coefficients for numbers of cyprid larvae and VIth stage nauplii of Balanus crenatus, at 3 m and at 6 m depth and physical and chemical properties of water, on 30 M a y 1969, on a fixed station on Texelstroom. Correlation

chlorinity --temperature cyprids (3 m ) - - t e m p e r a t u r e cyprids (6 m ) - - t e m p e r a t u r e nauplii (3 m ) - - t e m p e r a t u r e nauplii (6 m ) - - t e m p e r a t u r e cyprids (3 m)---chlorinity cyprids (6 m)--chlorinity nauplii (3 m)--chlorinity nauplii (6 m)--chlorinity cyprids --suspended matter nauplii --suspended matter

r

p

--0.97 0.37 0.76 0.37 0.80 --0.39 -- 0.53 -- 0.37 -- 0.54 + 0.64 +0.54

(((0.001 (0.001 ((0.001 (0.001 (((0.001 (0.001 (0.001 (0.001 (0.001 (0.001 (0.001

38

P. DE WOLF 4-3.

ELMINIUS MODESTUS

Cyprids of Elminius were not sampled from the beginning, as in the W a d d e n Sea adults do not to a large extent settle below the intertidal zone. It was realized only towards the end of the experiments, that these cyprid larvae are equally interesting, and can perhaps just as well be used to study group formation of larvae in the Wadden Sea. The occurrence of the adults, nearly exclusively in the intertidal zone, is difficult to explain in view of the fact that the larvae are mixed through the W a d d e n Sea. Cyprids have been sampled on 9 and I0 September 1969 in the entrance to the harbour at a depth of 1.50 m (Fig. 8). It appears that they behave quite different from those of the other 2 species; they show a considerable rise and fall, but apparently independent of the phase of the tide. Further, the dispersion coefficient c is generally higher than for the other 2 species, and cannot be shown to have any relation to the tide. Moreover there are no significant correlations with current velocity, chlorinity, silt or sand content, nor with total suspended matter (Table X I I ) ; there is a slight negative correlation of numbers of cyprid larvae with temperature. 5" D I S C U S S I O N 5-I. TIDAL VARIATION

Summarizing results it is clear, that at the sampling depths, the numbers of cyprids of Balanus crenatus and B. improvisus do show a variation related to the tides. For the cyprids ofElminius modestus such a variation can not be shown from the limited data. It appears further that the tidal variation for the 2 Balanus species is markedly different in the 2 general sampling areas, Texelstroom, and at the entrance to the harbour of Den Helder. O n Texelstroom the number of cyprids of both species, at any depth studied, usually increases sharply after low water, or during early or late flood, and subsequently decreases slowly during high water; at some time during ebb the number increases again, and subsequently decreases. In the entrance to the harbour the increase in numbers during ebb is usually not found; numbers are high on the early flood only. On Texelstroom, due to the horizontal tide, the gross movement of the larvae will be an oscillation to and fro. They disappear from the watercolumn with decreased current velocities. In the entrance to the harbour the same general picture is apparent, except that many larvae are transported to the harbour, and a much smaller number, or none at all, return on the outgoing ebb.

DISPERSAL

OF BARNACLE

LARVAE

39

The horizontal, oscillatingmovement with the tidal current could be responsiblefor the presence of the larvae; it could be imagined that a water mass containing larvae passes the sampling station at every flood, and returns on the next ebb. However, this cannot explain the sudden increase and the slow decrease in numbers. Neither can it explain the fact that virtually no larvae return from the harbour, nor the data from 30 July 1968, when increase and decrease were found when sampling approximately the same water mass throughout the tide. The horizontal current of the tides thus cannot explain the tidal variation in numbers of cyprid larvae; with BOUSrlELD (1956) it will be presumed that vertical mixing, due to tidal current, is instrumental in the vertical dispersal of the larvae. When vertical mixing is presumed, one needs something to be mixed, and when cyprid larvae disappear from the water column, they have to go somewhere; either to the bottom, or to the surface. BOUSFIELD (1955: 40) supposed that nauplii and cyprids maintained their positions in the vertical to some degree throughout the tidal cycle, by swimming to an average depth, chiefly in response to light intensity. In shellfish work m a n y authors believe to have shown that larvae control their position in the vertical, by swimming, under the influence of turbulence (CARRIKER, 1967). Light is considered to have a depressing effect on larvae of Mytilus edulis (VERwE7, 1966). WOOD & HARGIS (1969) think that salinity or pressure changes control swimming in Crasostrea virginica. KORRI•OA (1942, 1952) denies that currents, light or any other factor tested influences the vertical distribution of the larvae of Ostrea edulis. The crucial point appears to be whether larvae do swim, and can swim strongly enough, to reach a certain position in the vertical and maintain that position, despite displacement by vertical mixing. Continuous swimming would be necessary, as vertical mixing is not limited to periods of high current velocities, but goes on continuously to a certain extent. Further it has been stated by the authors cited that changes in the factors mentioned induce swimming behaviour, with the apparent implication that the larvae did not swim before the change. If larvae do not swim, they either sink or float, unless they are of exactly the same specific weight as the surrounding water. Thus, not swimming would tend to bring the larvae either on the bottom or at the surface, but it would be difficult for them to prevent being mixed vertically. Usually sand and silt from the bottom are transported to quite a large extent by the tides, and it would be difficult to see how floating larvae could prevent being mixed; for sinking larvae an anchoring

40

P. D E W O L F

mechanism on a soft bottom should be postulated. Even this appears unlikely to be of much help in the Wadden Sea where large amounts of sand and silt are transported on every tide. Mechanisms allowing for anchoring in a soft bottom have never been indicated in shellfish larvae or barnacle larvae. Returning to the data in this paper it should be remarked that no influence of light on the vertical distribution ofcyprid larvae of Balanus crenatus or B. improvisus could be shown; further, changes in turbulence, salinity, pressure, and temperature are generally associated with tidal currents. Thus, it will be supposed that cyprid larvae are transported by tidal currents (Texelstroom area), that they sink to the bottom in periods of low current velocity, and are redispersed in the water column by increasing current velocities (experiments concerning sinking and redispersion will be described in Chapter IV). If this supposition is right, it can also be explained why no larvae, or at least much less than on the flood, are transported on the ebb in the entrance of the harbour. It is known that the tide in the entrance is asymmetrical (ebb currents running during a longer time than flood currents) resulting in lower current speeds during ebb (Fig. 8). Hence vertical mixing into the ebb current will be less thorough than on the flood, and part of the larvae remain in the harbour. Secondly, sinking brings m a n y larvae on the anaerobic bottom of the harbonr, and finally, larvae carried into the harbour by the flood have a bigger chance to meet some solid substrate to settle upon than they have on Texelstroom. This model for tidal transport of larvae is simple; it does not need reactions of larvae to changing environmental conditions. At a later stage it will be considered to what extent transport of oyster and mussel larvae, as found by other authors, cited above, can be explained by this model. 5°~2. S H O R T

PERIOD

VARIATION

As has been seen, on the long term tidal variation a second variation is superimposed, resulting in large differences in numbers of larvae in successive samples. This variation is thought to be the result of a patchwise distribution of the larvae, and is characterized by the dispersion coefficient c. As c is independent of the mean number of larvae present - - b y definition (CASSlE, 1959), and as calculated in the present d a t a - it follows that the groupwise distribution of the cyprids of Balanus crenatus and B. improvisus is changing with the tide. The dispersion coefficient c is high during periods of high current velocities, implying that groups of cyprids are present, while c becomes smaller later, indi-

DISPERSAL

OF

BARNACLE

LARVAE

41

caring that the existing groups are gradually broken down, until the groupwise occurrence cannot be distinguished from a random distribution (c = o). Examples for thls phenomenon have been shown in Figs 5, 6, 7 and 8. While the gradual breakdown of groups of larvae can be attributed to a random process like eddy diffusion (STo~MEL, 1949) it is not clear at first sight how groups could be Formed in water with high current velocities. CLUTTER(1969) has listed a number of mechanisms of group formation such as retention in wind generated convective cells (LANOMUIR, 1938, STO~MEL, 1949), reaction to or association with temperature, salinity and density gradients or discontinuities (CASSlE, 1959; HARDER, 1957), exclusion of animals by certain species of phytoplankton (BAINBRIDGE, 1953), group Forming of predators or uneven attack by predators (CUSHINC, 1955), assOciation of predators with prey (ToNOLLI, 1958) or aggregation for breeding (ALLE~, 1935). Most of these have been described for the open sea, or lakes, or parts of the sea without strong currents, and it would appear that active grouping needs strong swimming capacities on the part of VIth stage nauplii and cyprids in the Wadden Sea. Although it has been shown that nauplius larvae of barnacles can, in an experimental situation, swim up to 15 m/h (HARDY & BAINBRIDCE, 1954) it is to be doubted whether even this is strong enough to overcome the random dispersion due to eddy diffusion. Figures on swimming by cyprids are not available in the literature, but will be given in Chapter IV. For these reasons the active formation of groups through swimming by the animals is considered unlikely. Apart from their ability to swim towards one another in currents with a great eddy diffusion, it would call for sensitive observation of either each other at the time when the water is most turbid, or of a certain environmental condition when properties like salinity and temperature vary rapidly. Further, it cannot be understood why this group forming behaviour would fail in more quiet circumstances, during high or low water. Parallel to the group formation the increase in total numbers occurs which suggests that cyprids behave much like dead suspended material. They would be eroded away from the bottom by the increased tidal currents, just like "clouds" of sediment, while spatial and temporal variations in the current velocity are responsible for the "clouds". When current velocities decrease the cyprids start sinking to the bottom; while they do so the groups are gradually dispersed. This model is fully mechanical, except that instead of sinking it would be possible that the larvae swim downward or swim upward with a resulting downward displacement. For the moment it is impossible to choose between these alternatives.

42

P. DE WOLF

Sustaining evidence for this model is largely circumstantial, and can be found in the fact that on a small scale water masses are hardly ever homogeneous. LIEBERMANN (1951) has shown that, even in open ocean, temperature differences of 0.1 ° C occur regularly over distances of 0.60 m; CASSIE (1957) showed that in a tidal current in an inshore area temperature changes of 1.5 ° C occurred in h a l f a minute. Changes in temperature, salinity and the content of suspended matter within short periods are apparent from the present data (Figs 3, 4, and 7 to 11), they are largest during high current velocities. When the current velocities subside, mixing becomes gradually more complete, and the variation obviates to an extent. 5-3" CORRELATIONS

OF NUMBERS

AMOUNTS

OF CYPRID

OF SUSPENDED

LARVAE

AND

MATTER

Since the transport of cyprids, according to the theory presented, resembles the transport of bottom material as suspended matter by tidal currents, it is interesting to calculate their correlation. This is done for each species separately. Correlation coefficients for numbers of cyprids and amounts of suspended matter during the present series vary from --0.36 to + 0 . 5 3 for Balanus improvisus, and from --0.22 to ÷ 0.40 for B. crenatus. These correlation coefficients cannot be compared, since they are concerned with varying parts of the tidal cycle. For observations during a full tidal period or longer, correlations are positive, and usually significant; 0.14 to 0.53 for Balanus improvisus, and 0.20 to 0.40 for B. crenatus. Negative correlations for both species resulted from short period observations, e.g. for B. improvisus in the entrance of the harbour on 27 J u n e 1967 on the ebb current. The latter observation gave rise to the idea that the cyprid larvae do not return from the harbour because they settle there. A second example is B. improvisus on Texelstroom with the flood. If B. crenatus cyprids are negatively correlated with suspended matter this happens usually on the ebb current. Thus although there is some correlation of numbers ofcyprid larvae with total amount of suspended matter, the correlation coefficients are rather low; separation of suspended matter into 2 fractions does not help much (Table X I I ) . However, cyprid larvae are only about 0.5 mm long, and assumed they have a density slightly higher than the surrounding water, they will arrive at the bottom after all other suspended matter has settled, and thus the cyprid larvae will lay on top of the bottom material. Now increasing water currents will first pick up tile cyprids, and when all have been carried away the bottom will still be undisturbed; later silt,

DISPERSAL

OF BARNACLE

LARVAE

43

and perhaps sand, will be carried by the current. Thus poor correlations may be ascribed to the nature of the bottom, while further complications may arise from short period temporal and spatial variations in current velocity and different sinking rates for silt, sand and cyprid larvae. 5-4.

TESTING

THE

THEORY

A direct test of the proposed theory is obtained in the laboratory by experiments on the sinking of larvae, in stagnant water, and on the influence of currents on cyprids resting on the bottom (Chapter IV). A rough idea on the sinking rate of cyprids can be obtained from field data, assuming the model to be right. JOSEeH (1954) discussed the non-stationary vertical distribution of a suspension during a tidal cycle. Boundary conditions are, that the grain size of the suspended material is uniform and that the distribution in a horizontal direction, in the direction of flow, is sufficiently uniform. The degree of stratification of the water mass should be small and finally the vertical distribution of suspended matter should be quasi stationary. If so, (in a slightly diverging notation): A =

bT 4 - - ~ " v / ( b ~ - - s~)

where A denotes the eddy diffusivity; b the vertical phase velocity, more specifically the velocity of upward movement of a maximum of suspension concentration in the water column; s the sinking rate; and T the tidal period. Concerning the boundary conditions, the size of cyprids is very uniform; to fulfill the condition of horizontal uniformity the observations obtained while following a drogue will be used; stratification in the Wadden Sea is usually negligible. Taking the observations of 30July 1968 (Fig. 10) it is seen, that on the ebb a maximum ofcyprids reaches sampling depth (3 m) from the bottom (17 m) in about 52 minutes. From this follows b = 1400/3120 = 0.45 cm/sec. Assuming A = 500 cm2/sec (at a current velocity of 1.6 m/see), T = 6h 25 rain, it follows that s = 0.44 cm/sec or 28.4 cm/min. The apparent sinking velocity (sinking and own movement of larvae) is thus 28.4 cm/min, a value which fits reasonably in the range of experimental observations in the laboratory (page 52). Assuming the model to be right, it would be expected that, despite a lower current velocity at the entrance of the harbour, the rise in numbers on the flood would arrive at an earlier moment after pre-

44

e. D E

WOLF

dicted low water, than on Texelstroom, as the depth of sampling in relation to total depth was much smaller in the first case (I m against 2 m) than in the second (3 m against 17 to 22 m). Inspection of the Figs 2 to 11 shows this to be true; increase in numbers of larvae at sampling depth in the harbour starts around low water, or at most with a delay of 1½ h, while at a greater depth on Texelstroom the increase in numbers starts usually only 3 h after low water. This, of course, may also be explained by swimming upward of the larvae over a greater distance, but it is not at variance with passive upward transport. Such passive transport would lead to retention of larvae in estuaries in the way described for bottom material of different grain sizes by POSTMA (1957) and VAN STRAATEN • KUENEN (1957, 1958). 5-5" R E S U L T S

OF OTHER

AUTHORS

Assuming the theory of the foregoing paragraph to be right, many results of other authors can be explained in a simpler way than has been done by them. KORRINOA (1941: 107, 108, l l8) explained time-variations in numbers of larvae of Ostrea edulis on the tide by differences in horizontal distribution only. Although this effect is undoubtedly true, his data can at least partly be explained by the mechanism proposed here. On his station Yersche Bank, maxima for small larvae appear shortly after low water, while on the station Kattendijke, on much deeper water, these maxima m a y occur 3 hours after low water. Further, there is in both instances some indication of an influence of inequality of the tides on the larvae. It seems that transport of larvae occurs only on the flood, although there is one indication of transport of larvae on the ebb on the station Ycrschc Bank. It is difficult to evaluate KORRINCA'S data, as one sample has been taken every l½ hours, and if the mechanism as proposed here is right, the representativity of his samples must be poor. If on the othcr hand the mechanism proposed here is not correct, it cannot be understood why, at all stations, his larvae are transported mainly in flood direction. BOUSFIELD'S (1955) data on Balanus improvisus can equally be explained by the model, and this explanation is even more likely in view of the great variation in numbers of larvae in his samples. Also CARRIKER'S (1967) review on Crasostrea virginica, with early larval stages more or less uniformly distributed in the water column, while older stages are found at lower levels of the profile, and an increasing proportion of older larvae on the flood over that on the ebb, can be explained from an asymmetrical tide, and from concentrations of older larvae on the bottom.

DISPERSAL

OF BARNACLE

LARVAE

45

V E R W E Y (1966) found for older larvae of Mytilus edulis,that numbers present at the surface are high at moments of great current velocity. His observations, that the numbers continue to rise at dusk need not be explained by light influence on swimming, but can equally well be caused by a phase difference of eddy diffusivity and current (Cf. POSTMA, 1965). VERWEY'S conclusion that old larvae occur in the water column as a result of turbulence fits with the present model, but the observation that the m a x i m u m on the nightly ebb is missing can equally well be ascribed, instead of to the absence of light, to the fact that only one sample per hour was taken. If the same amount of short period variation as found in cyprid larvae is present in older Mytilus larvae, the representativity of one sample per hour is very limited. VERWEY'S (1966) conclusions on Inedusae are not very convincing either; he explained the pattern found by changes in current speed and changes in light intensity. However, it seems possible that here, in view of the fact that medusae are strong swimmers, the currents and accompanying strong turbulence, destroy to a certain extent the pattern as proposed by VERWEY. He gives 3 reasons for believing that vertical displacement of the medusae is not the direct mechanical result of turbulence: (1) the distribution of the animals in the water during ebb and flood is different from a distribution caused by turbulence; (2) a medusa in open water is always pulsating, and the pulsations in principle determine its direction; and (3) these medusae prefer more superficial water at night and succeed in reaching such levels in strong currents. Now the first argument is wrong, if it is assumed that the density of the medusae is high. This is probably true, since during night and day the medusae maxima disappear rapidly from the upper water layers. With regard to the second argument, a medusa can determine its own direction only if the swimming movement is stronger than the water velocities in its environment. The third argument can be reversed: medusae do not "prefer superficial water during the night", but cannot help being transported into the surface water during the day. All this is not meant to show that Rhizostoma or Chrysaorado not react to light, but only to indicate that such a phenomenon cannot be proved in a region with strong tidal currents. Lastly, WOOD & HARGIS (1969) found a marked difference in transport between coal particles and oyster larvae in the James River estuary: oyster larvae were transported on the flood, but not on the ebb current, while coal particles were found on the flood as well as on the ebb. A full manuscript is not available at the time of writing, but it seems that small differences in density of the coal particles and the larvae, together with an asymmetrical tide could explain their data just

46

P. D E

WOLF

as well. The transport of coal particles on both ebb and flood indicates that there is considerable vertical mixing, and thus a rather large eddy diffusivity. It is not unlikely that flood currents are so strong that coal particles and oyster larvae are transported, while ebb currents are less strong, and have a lower eddy diffusivity. T h e n it follows from JosEpH (1954), that a turbidity cloud of oyster larvae will never reach the surface, no equilibrium between sinking and upward phase velocity will be obtained, and no groups of oyster larvae will be recognizable in a series of samples. Further, sampling in a groupwise distributed population, no matter how it is done, will always over-estimate the true population, when compared with an equally dense normal population not showing grouped distribution (WIEBE & HOLLAND, 1968). Again, a full manuscript is not available, but the impression is obtained that WOOD & HARGIS did not show that swimming on the part of the larvae explains the transport found in the estuary. The conclusion is that it will require a programme of very intensive sampling to show, in the field, that larvae further their transport by swimming.

5-6.

DIFFERENCES CRENATUS

IN DISTRIBUTION

AND

B. I M P R O V I S U S

BETWEEN CYPRID

BALANUS

LARVAE

From correlations of numbers of cyprid larvae with temperatures and salinities it follows that cyprids ofBalanus crenatus are usually associated with relatively cold and salt water. To this general rule there is the exception of 30 M a y 1969, when they occur in relatively warm and fresh water. This last observation can probably be explained by the fact that although early spring in 1969 was cold, May was a warm month. The shallow Wadden Sea was at the end of May warmer than normal by about 11° C, while the deeper North Sea was less warmed. Sampling on 30 M a y started on the ebb current with relatively warm and fresh water coming from the W a d d e n Sea; the ebb current ran for 2 h after the predicted time for low water. On the following flood, first relatively w a r m and fresh water came back, cold and salt water not arriving at the sampling station till after high water. At that time this cold, salt water already contained relatively low numbers of cyprids. Cyprids of Balanus improvisus during late J u n e and early J u l y are associated with relatively warm and fresh water. Later, when the shallow Wadden Sea is cooling faster than the North Sea they do occur in relatively cold water (20 August 1967; 9 and 10 September 1969).

DISPERSAL

OF BARNACLE

47

LARVAE

The distribution of the cyprid larvae reflects the distribution of the adults; Balanus crenatus occurs in greater numbers in the outer parts of the W a d d e n Sea; in the more inward parts B. improvisus is more numerous. It is rather amazing that correlations with temperature and salinity can still be shown after a rather long period of larval life, despite mixing; but on the other hand correlation coefficients are not high. Further, there is some indication (Tables X and X V to X V I I I ) that correlation coefficients become lower during larval life which means that larvae are gradually better mixed through the Wadden Sea, irrespective temperature and salinity. This is an indication that the larvae do not succeed in selecting patches of water of a certain quality, and argues once more for passive transport by tidal currents. 5-7.

ON

vlth

STAGE N A U P L I U S L A R V A E

The same picture as obtained for cyprids, is seen, when considering V I t h stage nauplii, from much less data. Again tidal variation of the numbers, groupwise occurrence and association with cold and salt, or warm and fresh water, are present. It should be noted however, that, as a rule, all these phenomena are less clear and less outspoken. Tidal variation is not nearly as marked as for cyprid larvae (Fig. 10) ; the same holds for the groupwise occurrence (Fig. 2). Group formation is again associated with the horizontal tide, and groups are lost subsequently. The correlation coefficients for V I t h stage nauplii with temperature and salinity are nearly always somewhat lower than those for cyprids. All this can be explained by assuming that the downward movement of the larvae, as a result of sinking and swimming, is not as fast as that of cyprid larvae. Over a tidal period this results in a less deep occurrence in the water column, as has been found by BOUSFIELD(1955) for Balanus improvisus. Any sinking would lead to a certain degree of concentration of the larvae at greater depth; after which they can be redispersed by the increasing current. A less defined horizontal concentration of larvae than the well defined stratum of cyprids laying on the bottom, would lead to less well defined groups. It will be clear, however, that this calls for confirmation. 5-8. THE

SIZE AND

FORM

OF GROUPS

OF CYPRID

LARVAE

In Chapter VI it is presumed in order to explain settlements, that groups of cyprid larvae have an elongated form; here it will be attempted to evaluate the basis for such a supposition.

48

P. D E W O L F

Actually, as executed, the sampling scheme does not permit to see groups as they are. Supposing that groups of larvae have a definite form at the m o m e n t of sampling it is not possible to k n o w whether the whole sample is taken from inside a group, from outside a group, from partly in and partly out of a group, or whether several small groups

are collected. CASSm (1962) has listed a number of frequency distributions fitting groupwise distribution, among which the N e y m a n and Thomas series. These allow, when it is assumed that they are applicable in a certain case, for an estimate of numbers of organisms per group and numbers of groups per volume. PIELOU (1955) has indicated, however, that sample sizes greatly influence the value of the parameters tbund. This can also be derived from comparing data published by BARNES & MARSHALL (1951) and by CASSIE (1960). For this reason it will not be attempted here to calculate sizes of groups of larvae based upon the assumption of one or another frequency model. To try to evaluate the form of the groups it will be assumed that a group has at first the form of a sphere with no cyprid larvae outside this sphere. The size of the sphere is limited by the depth of the water. The sphere moves with the tidal current. SELIGMAN (1956) and BOWLES et al. (1958) have shown that the eddy diffusivity increases with increasing tidal current, and that the eddy diffusivity is greater in the direction of the current than at right angles with it. This means, that any sphere will become larger and elongated in the tidal current. Afterwards the current decreases and the now elongated groups of cyprids sink to the bottom; even if they are collected into the form of a sphere again, which appears unlikely, it is elongated again by the next tide. It is interesting to consider the influence of the sampling regime on this phenomenon: sampling in slack water will mean collecting, more or less, spheres of water, while sampling in a tidal current means collecting of a horizontal core of water with dimensions depending upon the water velocity. In the present case 200 1 per 50 sec corresponds, in a tidal current of ½ m/see, to a cylinder of 25 m long. WIEBE ~; HOLLAND (1968) have shown, in a simulation model, that in a patchy plankton population, size and distribution of the patches significantly affect the a b u n d a n c y estimates. The largest plankton net gave the most representative results, while WIEBE (1968) found that the longest tow and greatest diameter of net of a Longhurst plankton sampler gave in the field greatest representativity, although the longest tow is the most important of these two. This would mean that the present samples taken on a current would have a greater representativity than those taken during slack waters,

DISPERSAL

OF BARNACLE

LARVAE

49

and although the statistic c is much higher in flowing water it is probably too low when compared with c found during slack periods. Thus, current is instrumental in reducing the comparability of samples; this holds equally for samples taken at a drogue, as lateral and vertical vectors of current are still present, although the vector in general current direction has been removed. This, incidentally, might be part of the reason why CONOVER (1968) found that, in this respect, a drogue is not entirely successfull in marking a certain watermass. In view of these difficulties it is understandable that no definite figure can be given for the size of the groups of larvae, although it may be significant that the limits given above (60 cm, the diameter of a sphere of 200 1, to 25 m) are in the same range as those given by WIEBE (1970) : 1 to 15 m for oceanic zooplankton, WmSOR & CLARKE (1940), and BEm'qHARD & RAMPI (1965) who also found the same range. McALICE (1970) identified tentatively phytoplankton patches in an estuarine environment of a size varying from < 1 to 12 m. WIE~E (1970) discussed the possibility that in all these cases the causal mechanisms may be fundamentally physical, although the possibility of reacting of organisms to small scale inhomogeneities in physical or chemical factors has not been ruled out. WIEBE then notes that it is difficult to see how such small scale inhomogeneities of favourable or unfavourable conditions are maintained. It seems likely, however, that as in the present investigation, also in WIEBE'S work, the patchiness is not a given situation, but much more a process of continuous change. As to the form of the groups of larvae there is only theoretical evidence that they have an elongated form in flowing water. Later (Chapter VI) it will be shown that this evidence fits with results obtained in experiments on the settlement of the larvae. 5-9.

CYPRID

LARVAE

OF ELMINIUS

MODESTUS

It has been seen that cyprids of Elminius modestus show a groupwise occurrence at sampling depth, and that groups are not associated with the tides. Further there is only slight evidence that the cyprids are associated with warm water. Although the amount of data is limited it will be postulated here that the cyprids of Elminius modestus have a density that is slightly lower than the density of the water in which they live. For this, no proof can be given. The postulate, however, can explain, that these larvae are brought downwards by currents, but are in their group forming behaviour relatively independent of currents, as they are slowly rising to the water surface, when not mixed. The postulate can further explain why settlement of the species

50

P. D E W O L F

occurs in the W a d d e n Sea almost exclusively in the intertidal zone, and why the species did show a rapid spread along the coasts of Western Europe (DEN HARTOC, 1953). It is realized that this is very speculative; but it tallies with data on cyprids of Balanus balanoides (BRocH, 1924; RUNNSTROM, 1925). IV. L A B O R A T O R Y E X P E R I M E N T S ON THE SINKING, S W I M M I N G AND T R A N S P O R T OF CYPRID LARVAE I. INTRODUCTION

In Chapter I I I the hypothesis was put forward that the result of swimming and sinking in cyprid larvae of barnacles would be a downward movement; by this the larvae would reach the bottom of the estuary during periods of low current velocities. From the bottom they would be eroded away, like sediments, by current, in periods of high current velocities. Data on swimming and sinking of cyprids are not known in the literature; on the influence of current some data are available. M a n y authors have mentioned that cyprids are not able to settle in currents greater than 50 cm/sec (1 knot); all these can be brought back to experiments of SMITH (1951). He used a spinning circular disk in a natural environment and measured the distance from the centre of the disk at which larvae of Balanus amphitrite, B. eburneus and B. improvisus could successfully settle, in relation to circumferential speed. Although the method is open to criticism, his results show that currents preventing attachment are 0.5 to 0.9 knots for B. amphitrite, 0.4 to 0.7 knots for B. eburneus, and greater than 1.1 knots for B. improvisus. Conversely, this should mean that at greater velocities cyprid larvae are transported over or eroded from the substrate (supposing that under these conditions they ever reached the substrate). Two different experiments will be described here: (1) on the effect of swimming and sinking on the vertical position in the water column, (2) on the effect of water currents on transport of the larvae. 2. M E T H O D S

For the first experiment a glass tube of 20 cm diameter and 1.25 m length was set up vertically, and filled with filtered seawater of appropriate salinity and temperature. Cyprids of Balanus crenatus, one in each experiment, were introduced at the top, in the center of the tube, and their behaviour observed. Collection of cyprids for the experiments was given special care; larvae were pumped from a depth of 1½ m, from the Marsdiep area,

D I S P E R S A L OF B A R N A C L E L A R V A E

51

daily at 9.00 h. Experiments were done over a period of 14 days (from 6 to 20 J u n e 1969) so that larvae of every phase of the tide have been used in the experiments. (It will be clear from the results of Chapter III, that over a large part of the tidal cycle larvae have been collected that did not yet sink!) Immediately after pumping the larvae were brought to the laboratory and put into a shallow dish. It appeared then that 2 groups of cyprids could be distinguished: those crawling over the bottom of the dish, and those swimming in the 2 cm of water. To prevent bias 3 crawling and 3 swimming larvae were used for the experiments every day. As it proved difficult to follow the larvae in the vertical tube, only a limited number of experiments (75) have been carried out. No attempts have been made to control light, or other factors not mentioned, in the experiments. Although experiments have been done in light intensities from full sunlight to such an intensity that the larvae could only just be seen, no differences in behaviour were noted. The second type of experiment has been done with an even smaller number of larvae, namely 10 cyprids of Balanus improvisus. These larvae were brought into an apparatus in which circular currents could be produced, as described by CREUTZBERG (1961: 299). It consists of a circular channel in which currents with different speeds can be generated by means of a motor driven rotating shaft and paddle boards. The cyprids were allowed to sink to the bottom in still water, after which at 5 minute intervals current speeds were increased and the behaviour of the larvae observed. 3" R E S U L T S

Results of the swimming and sinking experiment are given as sinking velocities (Fig. 12). Three different kinds of behaviour could be distinguished:

Fig. 12. Frequency distribution of the sinking velocity for cyprid larvae of Balanus cT6natu$.

52

P. D E

WOLF

(a) A small number of larvae were active swimmers; although they paused swimming every now and then, in i hour of observation they never, after introduction at the top, reached a depth of more than 20 c m and on the other hand never reached the surface. Half of them were collected as crawlers, the other half as swimmers. (b) Nearly all other larvae showed the same swimming-upwards sinking-downwards behaviour, however, with much longer sinking periods and much less upward swimming, resulting in a downward movement. D o w n w a r d speed varied from 10 to 32 cm/min. N o larvae swam upward after having reached the bottom. Swimming in horizontal directions was hardly ever observed; no larvae ever reached the wall of the tube. (c) One larva sank, without swimming, straight to the bottom, with a speed of 45 cm/min; after collection from the tube this larva was active, and settled successfully. The conclusion from these experiments is that the mean rate of downward movement is 19.5 cm/min (standard deviation 8.0 cm/min), while there is some suggestion that some larvae are stronger swimmers than the majority. In the second type of experiment larvae ofBalanus improvisus were allowed to sink to the perspex bottom of the circular channel, in still water. It has been tried to use either sand or silt bottoms. Because of lack of contrast, however, the larvae could not be seen on the sand, while in the case of silt, increasing currents stirred up silt and the larvae disappeared from view. Also the reverse experiment with larvae in the water at diminishing current velocities, proved to be possible. In a typical experiment the larvae would sink to the bottom, while swimming every now and then, and after arrival crawl around on the bottom. No larvae have been seen swimming up at low current speeds. At a current speed of about 42 cm/sec (range 35 to 57 cm/sec), the larvae would start to roll over the bottom, while at a speed of 45 cm/sec (range 35 to 67.5 cm/sec) the larvae would be taken from the bottom by the stream and carried away. It is thought that the high parts of the ranges, necessary either for rolling or for transport in the water, might be due to attempts of the larvae to attach to the bottom; at high speeds some larvae were straight away transported into the water, without initial rolling. It appears, therefore, that the mean values for rolling over the bottom and transport are the most likely values. It should be kept in mind that these figures have no absolute value, as the velocity profile in the experimental set up is not known. The figures found indicate the current velocity at a height of 30 cm above the bottom; current velocities in the 1 m m water layer on the bottom, containing the cyprid larvae, are certainly lower.

DISPERSAL OF BARNACLE LARVAE

53

4. DISCUSSION

Results obtained on the behaviour of cyprids in a vertical water column in the absence of currents fit in reasonably well with the model proposed for transport of larvae in an estuary. A mean downward movement of 19.5 cm/min, as found, which, for reasons of selection of cyprids in sampling, is probably on the low side, would bring larvae to the bottom, on most of the sampling stations in less than one hour, and in 1½ to 2 hours everywhere in the Wadden Sea. Further, it appears that the speed of downward movement as found in the laboratory experiment does not differ too much from the speed as calculated from field data, under rather liberal assumptions (page 43). Swimming appears to be rather erratic in cyprid larvae; it helps them to keep in suspension. From the data presented here it cannot be analyzed whether gradual changes of environmental factors have influence on the amount of swimming. Three apparently different kinds of behaviour have been noted, and it is tempting to interprete these as developmental stages. Strong swimmers then would be young cyprids, while later in life the amount of energy spent to swimming would decrease with time. This hypothesis cannot be substantiated, however, although PYEFINCH (1948: 469) supposed that physiological changes do take place during the free-swimming life of the cyprids. A proof for this hypothesis will be difficult as in culture vessels cyprids usually settle soon; the distance to be covered to reach the wall or bottom is short, after which they start to attach. It has been seen, that swimming in horizontal directions hardly occurs which means that in the W a d d e n Sea, with a limited amount of suitable solid substrate like mussel banks or stones, cyprids are dependent upon currents to reach such a substrate. As the chance to reach a substrate is remote the mean duration of the cyprid life will be much longer than is usually indicated in the literature (Table X I X ) . Lifetimes in literature are usually minima; derived from the first appearance of settlers on a substrate after the first appearance ofcyprids in the plankton, or from cultures. The latter are subject to the faults indicated above, when compared with the estuary. Reconsidering again the adequacy of the method of sampling of the larvae used in these experiments: the larva could have had rather different ages, but it is difficult to see how a differentiated method for collection can be obtained. The results of the second experiment are also in agreement with the model for transport of the larvae as postulated in Chapter III, be it

54

P. DE

WOLF

T A B L E XIX

Durations of development of nauplius stages (all together) and cyprid larvae, as given in literature. Species B. balanoides

B. crenatus B. improvisus

B. eburneus B. amphitrite B. trigonus E. modestus Chthamalus stellatus

Temp. °C

Field or lab.

Nauplii days

Cyprids days

4-5 5-6 3 20 4- 3 cool 4-5 15 21 16 14--16 20 15-20 21 21 21 20 4- 3

field field field lab. lab. field lab. lab. ? field field field lab. lab. lab. lab.

34 25 29 10 21-25 30 16 8-14 > 14 21 14

3-14

20 -4- 3 cool

lab. lab.

10 13-25

MOYSE, 1963 BASSINDALE, 1936

cool

lab.

22-27

BASSINDALE,1936

7-12 I0-14 9 6

3 4-6

7-10 2-5 1-4 2-4

Author

PYEFINCH,1948 PYEFINCH,1948 BOUSFXELD,1954 MoYsE, 1963 BASmNDALE,1936 PYEFINCH,1948 PYEFINCH, 1948 FREmERGER,1965 GRAHAM, 1945 BOUSFIELD,1954 BOUSFmLD,1954 GRAVE,1933 FREmEROER,1965 FREIBERGER, 1965 FREIBERGER,1965 MOYSE, 1963

Verruca

stroemia

t h a t the c u r r e n t velocities o b t a i n e d , h a v e no absolute value. I t is interesting to c o m p a r e the values o b t a i n e d (35 to 67.5 cm/sec) with those f o u n d b y SMITH ( p a g e 50). I t t h e n a p p e a r s t h a t t h e r e is a fair agreement. T h u s the results o f l a b o r a t o r y e x p e r i m e n t s do s u p p o r t the hypothesis p u t f o r w a r d to e x p l a i n the d i s t r i b u t i o n o f c y p r i d l a r v a e as f o u n d in the W a d d e n Sea. V. O B S E R V A T I O N S O N T H E S E T T L E M E N T OF C Y P R I D LARVAE IN RELATION TO TIDAL PHASE I. I N T R O D U C T I O N

I n view o f the l a r g e tidal v a r i a t i o n in the n u m b e r o f cyprids o f Balanus crenatus a n d B. improvisus in the w a t e r it w o u l d be r e a s o n a b l e to expect a v a r i a t i o n w i t h t i m e in the s e t t l e m e n t o f these species. H i g h n u m b e r s in the w a t e r do o c c u r at the t i m e o f strong c u r r e n t s ; o n the o t h e r h a n d , it has b e e n s h o w n t h a t a b o v e a c e r t a i n c u r r e n t speed the l a r v a e c a n n o t

DISPERSAL

OF

BARNACLE

LARVAE

55

settle (Chapter IV). Therefore, it seems that these 2 factors interact. In this chapter observations on the relation between the phase of the tide and number of settlers will be described; further the number of settlers will be related to the number of cyprids found in the water. In the literature such observations are rare, on barnacle larvae as well as on other species. W~.ISS (1947: 248) found that in Biscayne Bay, Miami Beach, Florida, cyprids of Balanus improvisus attached mainly during low water, irrespective of the time of the day. He related this to the occurrence of the larvae in the water mass occupying the site of exposure at low water. For oyster larvae KORRINGA (1952 : 305) remarked that most of the settling was concentrated at the periods of slack water. 2. M E T H O D S

The tidal variation of the settlement of Balanus crenatus and B. irnprovisus, was studied by exposing from a raft anchored in the entrance to the harbour of Den Helder (page 12), red PVC panels of 20 × 30 cm as a substrate, at a depth of 1.50 m. Water depth at the sampling station at mid-tide was about 2½ m. The experiments were done in conjunction with the collection of cyprids in the plankton as described in Chapter III. The number of panels used in different experiments varied; barnacle spat (i.e. young settled barnacles, and attached cyprids not washed off by removing the panel from the water) were counted on an area of 20 × 10 cm in the centre of the panel (to avoid edge effects). Counting was done every hour, after which spat was cleaned off by means of cotton wool and sea water. T h e n the panels were exposed again. This technique can be criticized, because KNIGHTJONES (1953: 597) has shown for several species of barnacles that settling usually occurred in the neighbourhood of previously settled barnacles of the same species, or their fragments. He found that this effect could not be avoided by washing the substrate with water. For reasons that will become clear later (page 72), for the densities of settled spat and cyprid larvae occurring in the present experiments, it is not likely that this could have been an important effect. Experiments have been done from 1 J u l y 1969, 10.00 h till 2 July 1969, 11.30 h (settlement of B. crenatus and B. improvisus); from 5 August 1969, 9.30 h till 6 August 1969, 14.40 h. (B. improvisus) ; on 20 August 1969 from 8.30 h to 20.30 h (B. improvisus); and from 9 September 1969, 10.40 h till 10 September 1969, 10.40 h. (B. crenatus, B. improvisus and perhaps also Elminius modestus). No attempts have been made to determine the species of the at most one hour old settlers.

56

P. DE W O L F

3. RESULTS

The results will be treated as follows: (a) dispersion coefficients and their significance have been calculated, and given in tables; (b) mean, or total, settlements on the panels will be compared with the stage of the tide; (c) numbers of settlers on the panels will be correlated with the numbers of eyprid larvae in plankton samples. On 1 and 2 J u l y 1969, 12 panels have been exposed from the raft; 10 of these were facing east, at interpanel distances of 1 m, one was facing south (at a distance of about 1 m from the nearest east facing panel), and one was facing west (at a distance of about 4 m from the nearest south facing panel) (Fig. 13). Numbers of settlers per hour s~

view

f r o n t vie~¢

trame r~rml~r$/

i

L .......

J

~1 ~ _ I

|

Fig. 13. Raft for settlement experiments (on scale 1 : 300) ; distance between frames 30 cm; distance between panels on a frame in horizontal direction 5 cm, in vertical direction 20 cm. Positions of panels on the raft during experiments on short period settlement, on 1-2 July, 5-6 August, 20 August, and 9-10 September 1969.

per 200 cm ~ (Fig. 5, Table XX) varied between panels and from hour to hour on the east facing panels. Dispersion coefficients c were usually significantly greater than 0, except during the nightly ebb (from 23.15 to 3.15 h). At all other hours the numbers of cyprid larvae settling could not be considered to belong to a random distributed population. The numbers of settlers on the south facing panel usually fall within the range of the numbers on the east facing panels. The number of settlers on the west facing panel, however, is rather often larger than the number on each of the other panels: this occurred from 18.15 to

DISPERSAL

OF BARNACLE

57

LARVAE

T A B L E XX

Numbers of barnacle spat settling per hour on 12 panels, on 1 and 2 July 1969. Panels 1 to I0 facing east, no. 1 north end of row, no. 10 south end of row; no. 11 facing south; no. 12 facing west. Hourly time periods indicated by the starting time of the periods. Period

Panel number 1

10.12 11.15 12.18 13.15 14.15 15.15 16.15 17.15 18.15 19.15 20.15 21.15 22.15 23.15 00.15 01.15 02.15 03.15 04.15 05.15 06.15 07.15 08.15 09.15 10.15

2

2 9 5 11 7 10 5 22 20 41 107 232 8 10 0 3 0 4 14 6 16 21 8 10 2 0 1 1 2 3 4 1 14 16 55 135 3 23 9 4 6 11 19 10 17 14 24 6 2 8

3

4

5

11 9 27 25 46 145 7 3 1 5 8 3 0 1 3 4 20 161 25 2 6 16 5 16 3

7 6 13 17 26 125 9 4 6 8 11 4 0 3 2 6 14 167 26 3 12 9 2 3 9

4 4 3 9 14 131 13 3 3 7 13 3 2 3 4 2 6 108 12 1 7 7 13 11 4

6

7

9 6 5 3 3 2 15 3 11 14 88 71 10 2 2 4 25 13 I1 14 20 24 2 2 0 1 0 2 1 1 1 4 9 4 70 37 17 13 5 3 21 18 21 7 7 4 6 4 8 5

Panels 1-10 8

9

10 11

11 5 23 9 7 0 6 7 14 1 5 6 0 1 3 8 9 7 11 12 39 29 46 28 5 3 68 28 5 2 19 7 21 9 46 12 23 11 41 21 30 7 43 29 5 4 18 5 0 2 2 3 2 6 2 6 5 2 3 2 5 4 1 3 3 4 3 2 34 29 53 19 12 4 35 42 0 0 2 9 20 16 27 10 5 14 23 29 14 7 18 16 20 12 23 11 3 8 12 7

I2

17 2 2 2 1 77 49 54 23 45 33 26 3 2 0 0 9 107 47 35 30 51 26 21 6

c

p

0.33 <0.001 0.00 )0.05 0.76 <0.001 0.55 <0.001 0.42 <0.001 0.35 <<<0.001 1.99 <<<0.001 1.15 <0.001 1.18 <<0.001 0.53 <<0.001 0.27 <0.001 0.54 <0.001 0.10 )0.05 0.15 )0.05 0.15 )0.05 0.01 )0.05 0.34 <0.001 0.38 <<<0.001 0.30 <0.001 0.51 !0.01 0.18 <0.001 0.16 <0.01 0.22 ::k0.001 0.32 <0.001 0.11 )0.05

22.15 h, a n d f r o m 05.15 to 09.15 h, b o t h t i m e s o n t h e l a t e f l o o d a n d during high water. W h e n t h e m e a n s e t t l e m e n t o n t h e e a s t f a c i n g p a n e l s is c o m p a r e d w i t h t h e s t a t e o f t h e t i d e ( F i g . 5) i t is i m m e d i a t e l y c l e a r t h a t m a j o r settlement took place during late ebb and the beginning of the flood, d u r i n g t h e n i g h t as w e l l as d u r i n g t h e d a y ; this s e t t l e m e n t w i t h i n o n e h o u r , is n e a r l y j u s t as h i g h as t h e t o t a l s e t t l e m e n t d u r i n g t h e r e s t o f t h e tidal period. Correlations have been calculated (Spearmans rank correlation coefficient) b e t w e e n t h e n u m b e r s o f s e t t l e r s o n t h e 2 p a n e l s ( n u m b e r s 9 a n d 10) a d j a c e n t to t h e s u c t i o n h o s e o f t h e p l a n k t o n p u m p , a n d t h e m e a n n u m b e r o f p l a n k t o n i c c y p r i d s i n t h e first 6, 5, 4, 3 t e n minutes' samples, respectively, taken during the hour of exposure of the

58

P. Dig WOLF

panels. T h e same procedure was carried out for the m e a n numbers of settlers on 3, 4, 5, 6 a n d 10 panels nearest to the suction hose (Table X X I ) . T h e critical correlation coefficient for p = 0.05 and n = 27 is T A B L E XXI

Correlation coefficients for numbers of barnacle settlers and numbers of planktonic cyprid larvae per 200 l on 1 to 2 July 1969. Average settlement during 1 hour on those 2 to l0 panels on a row which were nearest to the auction hose of the plankton pump. The cyprlds in 6 to 3 plankton samples, taken during subsequent periods of l0 rain starting at the same time as panel exposure. Number of plankton samples considered

Number of panels considered 2

3

4

5

6

!0

6 5 4 3

0.24 0.18 0.16 0.23

0.14 0.09 0.09 0.16

0.05 0.01 0.00 0.07

--0.02 --0.19 --0.19 --0.11

--0.08 --0.12 --0.12 --0.07

--0.19 --0.22 --0.20 --0.20

r = 0.40; there is no correlation between numbers of planktonic cyprids and numbers of settlers. For the short period from 5.00 to 11.00 h on 2 J u l y 1969, for which period total barnacle larvae were separated into species, correlation coefficients for numbers of planktonic cyprids and numbers of settlers are always negative, and never significant. O n 5 a n d 6 August 1969, m a i n l y cyprids of Balanus irnprovisus were found in the plankton samples, and it is assumed t h a t the majority of settlers belonged to this species. T e n panels were exposed to collect settlers, all facing east (Fig. 13). T h e variation in the n u m b e r of settlers between panels was not nearly as large as on 1 a n d 2 J u l y ; a n d although the dispersion coefficient c was nearly always greater t h a n 0, this was generally not significant (Table X X I I ) . T h e dispersion coefficient of the numbers of settlers was lowest d u r i n g m i d n i g h t high water slack, indicating a r a n d o m distribution on the series of panels. M e a n settlement on the 10 panels was high d u r i n g the last part of low water and the first h a l f of the flood d u r i n g the day, as h a d been found before. D u r i n g the night m a n y larvae settled d u r i n g the ebb as well (Fig. 6). As before, no significant correlations could be shown between numbers of settlers on panels (mean of 2, 3, 4, 5, 6 or 10 panels) and numbers of planktonic cypris larvae (mean of 6, 5, 4 or 3 samples). O n 20 August 1969 again m a i n l y Balanus improvisus cyprids were present in the plankton samples. T h e numbers were m u c h lower t h a n on 1 a n d 2 J u l y , b u t at about the same level as on 5 and 6 August.

DISPERSAL

OF BARNACLE

59

LARVAE

T A B L E XXII

Numbers of barnacle spat (probably Balanus improvisus) settling on 10 east facing panels on 5 and 6 August; panel 1 north end of row, panel I0 south end of row. Hourly time periods indicated by the starting time of the periods.

Period

09.35 10.35 11.35 12.35 13.35 14.35 15.35 16.35 17.35 18.35 19.35 20.35 21.35 22.35 23.35 00,35 01,35 02.35 03.35 04,35 05.35 06.35 07.35 08.35 09.35 10.35 11.35 12.35 13.35

Panel number 1

2

3

4

5

6

7

8

9

10

c

p

2 0 1 0 0 0 3 0 1 7 3 7 2 0 2 2 1 0 5 15 4 18 39 5 0 0 0 0 0

2 1 1 1 1 8 2 2 2 10 8 11 3 1 2 2 1 0 6 14 6 13 21 6 0 5 2 2 0

5 1 0 0 0 1 0 2 1 6 10 12 9 6 4 1 1 1 23 41 8 19 23 2 2 2 0 3 1

1 0 0 0 1 3 3 0 7 19 3 6 7 5 0 2 2 0 14 27 6 18 36 2 1 0 0 0 2

9 4 0 0 0 1 6 2 3 10 9 4 2 3 0 0 1 0 14 9 6 1 5 5 1 4 0 0 2

2 1 2 0 0 4 1 0 0 8 12 3 0 4 1 0 2 2 9 8 6 3 5 4 1 2 0 1 1

3 2 2 8 0 4 3 1 1 11 6 7 3 3 1 1 2 5 13 12 1 0 1 4 2 0 1 0 0

2 1 2 2 0 3 5 0 5 8 5 6 2 4 3 0 0 2 8 5 1 7 8 9 0 1 0 0 0

1 0 3 1 1 2 5 0 6 17 3 4 4 0 2 1 7 0 12 7 2 0 6 2 0 1 0 1 3

3 5 0 1 2 3 2 0 1 5 3 5 2 1 1 0 2 5 11 2 3 0 8 12 8 2 0 2 1

0.31 0.65 0.09 2.78 0.02 0.25 0.06 0.41 0.45 0.11 0.13 0.05 0.32 0.24 --0.01 --0.17 0.48 1.13 0.11 0.63 0.09 0.96 0.75 0.21 1.94 0.41 1.65 0.38 0.10

>0.05 >0.05 >0.05 <<0.001 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 <0.01 >0.05 <<0.001 >0.05 <<0.001 <<0.001 >0.05 <0.001 >0.05 >0.05 >0.05 >0.05

S e t t l e m e n t w a s c o u n t e d i n t h e s a m e w a y as b e f o r e , 19 p a n e l s w e r e u s e d , 10 f a c i n g east, 9 f a c i n g w e s t ( F i g . 13). T h e n u m b e r s o f s e t t l e r s were lower than on both earlier occasions (Table XXIII). T h e r e is a c o n s i d e r a b l e d i f f e r e n c e i n s e t t l e m e n t b e t w e e n e a s t a n d west facing panels. Dispersion coefficients show that in each row n u m b e r s o f s e t t l e r s a r e r a t h e r c o n s i s t e n t ; h o w e v e r , i n t h o s e cases w h e r e t h e d i s t r i b u t i o n o f t h e s e t t l e r s o v e r t h e series o f p a n e l s is s i g n i ficantly different from a random distribution, this can usually be a t t r i b u t e d t o o n e o r a few p a n e l s c a r r y i n g a h i g h n u m b e r o f s e t t l e r s (e.g. p a n e l 6 f r o m 10.20 to 11.20 h, p a n e l 2 a n d 3 f r o m 16.20 to 17.20 h, a n d p a n e l 1, 2 a n d 6 f r o m 18.20 to 19.20 h ) .

60

P. DE

WOLF

T A B L E , XXIII

Numbers of barnacle spat, settling on 19 panels, per hour, on 20 August 1969; panels 1 to 10 facing east, panels 11 to 19 facing west; panels ! and 19 north end of rows, panels 10 and 11 south end of rows. Hourly time periods indicated by the starting time of the periods.

Period

08.20 09.20 10.20 11.20 12.20 13.20 14.20 15.20 16.20 17.20 18.20 19.20

Panel number 1

2

3

4

5

6

7

8

9

4 3 1 2 3 3 4 5 6 5 22 3

5 1 1 1 1 1 6 4 12 2 26 3

6 1 2 2 2 5 0 3 15 4 14 1

12 4 4 3 1 5 1 2 2 3 11 6

9 4 5 1 1 4 5 2 3 2 6 9

14 3 17 1 3 8 7 8 2 5 21 2

8 3 1 1 2 1 2 5 1 2 7 2

7 7 2 0 6 2 0 7 4 2 9 5

7 1 2 0 4 2 0 8 2 4 2 8

Period

08.20 09.20 10.20 11.20 12.20 13.20 14.20 15.20 16.20 17.20 18.20 19.20

10

c

p

6 8 8 2 1 2 2 7 1 3 5 6

0.03 0.19 1.11 --0.24 0.06 0.15 0.58 0.01 0.82 --0.16 0.36 0.14

>.0.05 >0.05 <0.001 >0.05 >0.05 >0.05 <0.01 >0.05 <0.001 >0.05 <0.001 >0.05

Panel number 11

12

13

14

15

16

17

18

19

c

p

36 33 24 13 6 7 5 3 5 8 8 20

15 27 15 12 6 5 4 9 ! 2 6 14

25 17 26 11 8 3 4 6 3 1 3 7

19 15 9 9 7 8 6 0 4 6 4 33

31 12 25 37 7 12 3 0 3 2 14 24

8 19 14 26 11 2 4 1 2 3 12 25

23 11 7 10 12 2 2 3 5 1 3 9

16 14 13 14 8 2 0 1 6 3 7 11

-19 21 23 7 4 2 2 4 11 24 9

0.13 0.13 0.11 0.24 --0.05 0.27 --0.01 0.80 --0.09 0.47 0.46 0.23

<0.001 <0.01 <0.01 <0.001 >0.05 4-0.01 >0.05 <0.01 >0.05 <0.01 <0.001 <0.001

A l t h o u g h t h e r e a r e m a r k e d d i f f e r e n c e s b e t w e e n t h e east a n d west f a c i n g r o w i n n u m b e r s o f settlers, m e a n v a l u e s (Fig. 7) a g a i n s h o w t h a t t h e m o s t i m p o r t a n t p e r i o d for s e t t l i n g is t h e b e g i n n i n g o f t h e flood. O n 9 a n d 10 S e p t e m b e r 1969 3 species o f c y p r i d s w e r e p r e s e n t i n t h e p l a n k t o n s a m p l e s : Elminius modestus a n d Balanus improvisus b o t h i n l a r g e n u m b e r s , B. crenatus i n s m a l l n u m b e r s . T h e o b s e r v a t i o n s w e r e m a d e i n e x a c t l y t h e s a m e w a y as o n 20 A u g u s t 1969 (Fig. 13). S e t t l e m e n t o n east a n d west f a c i n g p a n e l s is u s u a l l y n o t s i g n i f i c a n t l y d i f f e r e n t , a l t h o u g h o f t e n t h e n u m b e r o f settlers o n west f a c i n g p a n e l s is h i g h e r t h a n o n east f a c i n g p a n e l s ( T a b l e X X I V ) .

DISPERSAL

OF B A R N A C L E

LARVAE

61

From the dispersion coefficient it follows that the distribution of settlers on the east facing panels is much more even than on the west facing panels. O n the east facing panels the uneven distribution of settlers occurs mainly at low water and during the flood, while on the west facing panels the distribution is less uneven during ebb only. Again when the distribution is uneven, this is largely due to one or a few panels with extremely high (or occasionally low) numbers of settlers in an otherwise fairly uniform series (e.g.panel 1 from 10.40 to 11.40 h, panels 4 and 9 from 4.40 to 5.40 h, panels 6, 7 and 10 from 5.40 to 6.40h). The impression obtained that some panels (facing east, at the ends of the row) collect more settlers than others, is shown to be not significant (variance ratio F ----- 1,9 at 9 degrees of freedom for the greater estimate, and 207 degrees of freedom for the lesser estimate: p > 0.05). Comparison of the amount of settlers with the stage of the tide (Fig. 8) shows a slight difference from earlier observations: high numbers coincide again with low water and the beginning of the flood, but a second heavy settlement is observed just before high water on both tides. Correlations between numbers of planktonic cyprids and numbers of settlers on panels, in 42 combinations of mean settlement on 2, 4, 6, 8, 9, 10, 19 panels and mean number of cyprids in 2, 4, 6, 8, 10 or 12 plankton samples are never significant. 4" D I S C U S S I O N

During the flood there are m a n y larvae in the water column, and in the entrance of the harbour mainly so at the beginning of the flood (page 38); at that time the groupwise dispersion is accentuated (page 39). O n the other hand, it follows from SMITH (1946), and from Chapter IV (page 52) that if current velocity exceeds a certain value, no settlement of barnacle spat can take place. The panel experiments show settlement to occur throughout the tidal cycle in low numbers, though a large proportion of the total settlement takes place in the relatively short period (1 or 2 hours) of early flood. U p to 50% of all settlement in a tidal period coincides with the increase of the number of larvae with the flood. As a rule settlement decreases when the current reaches m a x i m u m velocity but to this there are a few exceptions: west facing panels, south end of row. When flood-current velocity dccreases again, only low numbers of larvae are available in the water column for attachment on the panels as before. It appears likely, that the same situation holds for the ebb current, however, as a rule this current does not carry so m a n y larvae (page 33). Nevertheless it has been found once that an important settlement took place on the ebb current (page 58).

62

P. DE W O L F

W i t h c u r r e n t as a n a d d i t i o n a l v a r i a b l e i n t e r f e r i n g i n t h e r e l a t i o n b e t w e e n n u m b e r s o f l a r v a e i n t h e w a t e r a n d n u m b e r s o f settlers, it is not amazing that no positive correlations between these two have been found. A further reason might be that the mean values for numbers of cyprids in the water and numbers of settlers are not very precise. F o l l o w i n g t h e s a m e l i n e o f r e a s o n i n g as b e f o r e ( p a g e 17) i t c a n b e s h o w n , t h a t t h e p r e c i s i o n o f t h e m e a n v a l u e s is n e v e r b e t t e r t h a n a p p r o x i m a t e l y 2 0 % a n d o f t e n m u c h w o r s e . T h i s is p a r t i c u l a r l y so when the mean number of settlers on 2 panels, adjacent to the suction h o s e ( w i t h r e g a r d to p l a c e t h o u g h t t o b e t h e b e s t r e p r e s e n t a t i v e s ) a r e correlated with the number of plankton cyprids. A n o t h e r o b s e r v a t i o n c a l l s for c o m m e n t : o f t e n t h e n u m b e r o f s e t t l e r s o n a series o f p a n e l s w a s r e l a t i v e l y u n i f o r m , w h i l e o n e o r a few p a n e l s c o l l e c t e d m u c h h i g h e r n u m b e r s o f settlers. T h i s o c c u r r e d a t t i m e s o n all p a n e l s , i r r e s p e c t i v e o f t h e i r p l a c e . I t s u g g e s t s t h a t a g r o u p o f c y p r i d s p a s s e d s u c h a p a n e l , a n d t h a t p a r t o f t h e g r o u p a t t a c h e d to t h e p a n e l . TABLE

XXIV

Numbers of barnacle spat, settling on 19 panels, per hour, on 9 and I0 September 1969; panels 1 to 10 facing east, panels 11 to 19 facing west, panels i and 19 north end of rows, panels 10 and 11 south end of rows. Period

10.40 11.40 12.40 13.40 14.40 15.40 16.40 17.40 18.40 19.40 20.40 21.40 22.40 30.40 20.40 01.40 02.40 03.40 04.40 05.40 06.40 07.40 08.40 09.40

Panel number 1

2

3

4

5

6

7

8

9

10

c

p

12 12 32 9 8 5 19 8 6 1 0 5 3 5 29 2 4 4 12 12 6 1 3 0

1 7 9 4 4 2 15 3 4 3 1 0 2 7 19 5 5 3 4 19 2 3 2 1

6 7 22 5 4 9 16 4 10 1 1 2 5 8 32 6 5 2 11 6 11 1 4 1

3 2 17 6 5 4 20 7 3 3 1 1 1 5 24 6 6 3 31 16 6 1 6 2

4 2 6 11 3 1 18 3 5 0 1 0 0 2 17 5 0 2 14 11 3 0 1 3

5 7 11 10 11 12 18 5 7 2 1 0 2 4 20 6 1 8 8 42 5 6 5 4

2 3 15 7 8 8 16 2 6 1 0 0 0 2 18 3 5 2 13 48 8 3 1 3

5 2 14 5 8 4 17 0 15 1 0 1 2 1 11 3 4 9 15 21 2 4 3 0

8 5 9 3 9 9 13 5 7 3 0 0 0 2 15 5 2 7 28 19 6 5 2 2

2 4 21 9 16 17 25 2 6 4 0 0 1 1 19 1 3 6 9 37 1 7 3 2

0.26 0.20 0.18 0.01 0.13 0.34 --0.02 0.13 0.10 --0.07 --0.89 2.03 0.35 0.19 0.05 --0.05 0.03 0.12 0.28 0.34 0.18 0.27 --0.04 --0.02

<0.05 >0.05 <0.01 >0.05 0.05 <0.001 >0.05 >0.05 >0.05 >0.05 >0.05 <0.01 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 <<0.001 <<0.001 >0.05 >0.05 >0.05 >0.05

DISPERSAL

OF BARNACLE

63

LARVAE

I f t h i s is c o r r e c t , t h e d i s t a n c e b e t w e e n p a n e l s a l l o w s for a n e s t i m a t e o f t h e size o f g r o u p s o f c y p r i d s i n t h e s e a ; i t w o u l d a p p e a r t h a t s u c h g r o u p s h a v e a l i n e a r d i m e n s i o n o f 1 to 3 m . M o r e d a t a w o u l d b e needed, however, before a definite conclusion can be reached. O n 9 a n d 10 S e p t e m b e r c u r r e n t v e l o c i t i e s h a v e b e e n m e a s u r e d ( F i g . 8), a n d i t h a s b e e n a t t e m p t e d t o c o r r e l a t e m e a n c u r r e n t v e l o c i t i e s for e v e r y h o u r w i t h m e a n s e t t l e m e n t . A l t h o u g h t h e d a t a for s e t t l e m e n t are not very precise, it could nevertheless be shown that a quadratic r e g r e s s i o n fits t h e d a t a m u c h b e t t e r t h a n a l i n e a r o n e ( F i g . 14). T h i s s u p p o r t s t h e i d e a t h a t s l o w o r fast c u r r e n t s a r e w i t h o u t s e t t l e m e n t while medium current speeds are favourable. Since no cyprid larvae settle at very low current speeds, swimming d o e s n o t s e e m to p l a y a n i m p o r t a n t r o l e to r e a c h a s u b s t r a t e i n t h e sea. T h i s fits t h e l a b o r a t o r y o b s e r v a t i o n s , w h e r e h o r i z o n t a l s w i m m i n g w a s h a r d l y e v e r o b s e r v e d ( p a g e 52). T h e o b v i o u s c o n c l u s i o n is t h a t t h e larvae arrive on a substrate carried by currents; and that the numbers arriving at any particular area can be highly variable. If swimming of t h e c y p r i d l a r v a e w o u l d p l a y a r o l e i n r e a c h i n g a s u b s t r a t e it w i l l b e TABLE xxIv (continued) Period

10.40 11.40 12.40 13.40 14.40 15.40 16.40 17.40 18.40 19.40 20.40 21.40 22.40 23.40 00.40 01.40 02.40 03.40 04.40 05.40 06.40 07.40 08.40 09.40

Panel number 11

12

13

14

15

16

17

18

19

c

p

4 2 12 3 24 10 33 9 5 1 0 0 0 1 11 2 19 21 15 4 7 0 5 0

2 1 14 7 10 6 6 16 7 0 0 0 0 1 6 1 9 5 23 11 8 2 1 2

1 2 9 7 11 3 I 6 0 3 0 0 0 0 5 1 4 11 9 26 3 1 1 1

8 9 33 9 29 27 26 28 7 4 1 0 3 3 23 3 22 7 19 31 10 7 6 1

6 5 21 6 39 18 19 25 5 2 1 1 0 4 15 1 14 45 37 45 13 11 2 3

4 3 24 4 17 17 12 12 3 6 2 2 1 4 19 0 5 12 18 36 5 1 1 5

3 6 18 4 7 6 5 7 2 0 0 1 0 1 14 0 4 7 6 21 3 1 2 0

7 5 54 19 30 10 14 20 2 1 3 1 0 15 35 4 8 18 28 54 9 2 4 6

8 17 43 21 13 20 12 15 6 1 0 0 0 12 18 5 10 6 20 53 12 9 5 3

0.08 0.61 0.32 0.44 0.25 0.29 0.46 0.19 0.12 0.50 0.69 --0.09 2.95 1.23 0.27 0.34 0.29 0.67 0.18 0.29 0.09 0.91 0.I 1 0.40

>0.05 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 >0.05 >0.05 0.05 >0.05 >0.05 <0.001 .<0.001 >0.005 -<0.001 <0.001 <0.001 <0.001 >0.05 <0.001 > 0.05 > 0.05

64

P. DE WOLF

difficult to prove it beyond doubt. It is realized that this conclusion upsets the opinion of m a n y authors regarding the choice of substrate and the influence of light on settlement, because these opinions are based on the assumption that larvae compare substrates by swimming from one substrate to another. Most of their observations can, however, be explained in terms of current; the larvae then are brought to the substrate by current, test the substrate, accept it and settle, or reject it (cf. TI-IORSON, 1966: 275). See also Chapter VI (page 87). 22-

20-

18"

~4

10

-I

.•

Fig. 14. Relation between mean current velocity and mean settlement of barnacles on 9-10 September 1969. From the present data an influence of light on settlement cannot be shown• There is only some indication that on the night of 1 to 2 J u l y 1969 the highest settlement is somewhat lower than the highest settlement during the day before, but as indicated earlier, the precision of the mean values is insufficient to allow for such a conclusion• The result of these series of panel exposure is that a large part of all settlement on a certain panel occurs within a short period• This is important in relation to settlement patterns (Chapter V I I , page 97). VI. OBSERVATIONS ON THE S E T T L E M E N T OF CYPRID LARVAE IN R E L A T I O N TO PLACE I. I N T R O D U C T I O N In the foregoing chapters it was seen that cyprid larvae show groupwise distributions in the sea. Large numbers of cyprids do occur on

DISPERSAL OF BARNACLE LARVAE

65

the flood in the entrance to the harbour. Cyprids do not swim in horizontal directions, and the result of active upward swimming and passive sinking is a downward movement. It was further observed that the numbers of larvae, settling during short periods, could vary to a large extent from one place to another, even within one metre. Such differences in numbers of settlers on places near each other have been observed before, in barnacles as well as in oysters and mussels. T h e y are usually explained in terms of differences in the environment, such as light (many authors; for a review see THORSON, 1964) and current (PYErINCH, 1948), or by gregarious and territorial behaviour of larvae (KNIGHT JONES & STEPHENSON, 1950; SCHAFER, 1952; ROOTH, 1952; KNIGHT JONES, 1953; KNIGHT JONES & CRISP, 1953, ROSKELL, 1960; KNIGHT JONES & MOYSE, 1961). The relative importance of these factors in the settlement of barnacles will be discussed later (page 84) ; much less importance will be allotted to them than is usually done. It is to be expected that the groupwise occurrence of cyprids in the water influences the differences in settlement. In this chapter 2 series of experiments on settlement of barnacles will be described; the first employs a number of panels exposed in a single row between the raft and the shore, while the second series employs panels exposed in a regular pattern from a raft, with as variables: depth, north and south facing, and position on the raft. 2. METHODS 2-1. P A N E L S

IN A SINGLE

ROW

Twenty panels, as described earlier (page 55), were exposed in the entrance of the harbour of Den Helder, from floats, between the raft and the dike, in a direction perpendicular to general current. The panels were suspended at a depth of 1½ m, weighted down to prevent swinging in the current. Interpanel distance was 5 m, to prevent exchange of larvae between panels. Numbers of settled spat were counted at intervals. Counting was done in different ways. In some experiments all spat on an area of 70 cm ~ in the middle of the panel were counted. Varying densities of settlement have been found; it will be clear, in view of ROjAS' formula (page 16), that the low densities are much less representative than the high densities. Therefore, in later experiments a procedure was followed whereby on each panel barnacles were counted in randomly chosen 4 cm * squares, until either 200 barnacles were counted, or 200 cm * had been observed. It has been shown that, if the density of settlement is higher than 1 barnacle

66

P. DE WOLF

per cm ~, there is 95% chance that the mean found in this way is within 20% of the true mean of the panel (IwAo, 1968; I w n o & KUNO, 1968). At lower densities representativity is lower. Intervals between counts varied and in some cases barnacle spat were allowed to accumulate on the panels over varying periods, on other occasions spat were cleaned off the panels after each count. The bottom of the area was covered with mud, except near the shore, where stones of the dike foot extend under water. Observations have been made in periods when only one species settled: Balanus crenatus was counted from 20 to 26 J u n e 1968, and from 10 to 16 J u n e 1969; and B. improvisus from 12 to 31 J u l y 1968. 2-2.

PANELS

ON A RAFT

Settlement of Balanus crenatus on the raft was studied during J u n e 1966 and J u n e 1968 and o f B . improvisus during August 1967 and J u l y 1968. The raft (Fig. 13) was moored in a fixed position in the entrance to the harbour of Den Helder, the length axis of the raft orientated in the north-south direction (Fig. 1). The raft floats on 2 rows of glass fiber reinforced polyester tanks, one row on each side. They reach a depth in the water of 30 to 60 cm, depending upon the load on the raft. Between the floats 28 frames (Fig. 13) can be lowered. Each frame can carry 21 panels, 7 at each of the depths: 0.50 m, 1.00 m, and 1.50 m. Frames are numbered from 1 to 28 from north to south; the panels of one frame at each depth from a to g from west to east. The distance between the panels on adjacent frames is 0.30 m; the horizontal distance between adjacent panels on a frame is 0.05 m, the vertical distance 0.20 m. Thus also when all positions are occupied by panels the raft is rather open to currents. In the experiments different numbers of panels for settlements have been used. In 1966 72 panels were exposed; 12 on each of the frames 1, 7, 13, 19, 24 and 28, on the positions a, c, e and g at each of the 3 depths. In 1967 panels were exposed on frames 2, 8, 11, 16, 17 and 26; but otherwise in the same order as the year before. In 1968 63 panels were exposed on frames 21, 23 and 25, at 3 depths and at 7 positions in west-east direction. In most experiments the numbers of barnacles on both faces of each panel were counted. Counting was done as described for the single row of panels. Sometimes, when settlement was dense, counting was done on an area of 75 × 93 mm 2, from photographs on the same scale, taken with a Polaroid Land Close U p Camera. Frequency of counting varied during these experiments; in 1966 counts were done at approximately 5-days intervals; in 1967 and 1968 at 1, 2, 3 or 4-days intervals.

DISPERSAL

OF BARNACLE

67

LARVAE

After counting the panels were either cleaned, and used again, or the b a r n a c l e p o p u l a t i o n w a s a l l o w e d to a c c u m u l a t e . I t is a s s u m e d ( p a g e 55), t h a t i n s p a r s e p o p u l a t i o n s t h e g r e g a r i o u s n e s s w i t h e a r l i e r settlers, o r p a r t s r e m a i n i n g f r o m e a r l i e r settlers after c l e a n i n g , is n o t i m p o r t a n t ; i n m o r e d e n s e p o p u l a t i o n s t h i s c o u l d h a v e p l a y e d a role. I n a n a l y z i n g t h e results, a n a l y s i s o f v a r i a n c e was u s e d ; as t h e freq u e n c y d i s t r i b u t i o n o f n u m b e r s o f b a r n a c l e s o n p a n e l s is far f r o m n o r m a l , a n a l y s i s o f v a r i a n c e was d o n e o n r a w d a t a a n d after t r a n s f o r m a t i o n o f r a w d a t a a c c o r d i n g to x -+ log (x -k 1) a n d x -+ ~ / x -t- ~ / ( x + 1) (BARNES, 1952). H o w e v e r , n o e s s e n t i a l d i f f e r e n c e s w e r e f o u n d . T A B L E XXV

Numbers of spat of Balanus crenatus per 70 cm ~, settling on panels in a row in the entrance to the harbour of Den Helder, on days indicated; start of experiment 20 June 1968. Panel 1 near the raft, and 20 near the dike.

Panel 21/6 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 S~ CF e t* r1 t p

5

East side of panels 24/6 25[6 2

5

26/6

21/6

3

18

13 19 20 20 2 11 11 5 4 5 3 2 7 6 7 7 7 11 11 14 8 4 5 6 6 10 8 12 3 5 5 7 12 14 14 15 9 16 10 10 10 16 15 16 8 11 11 12 15 20 23 31 10 19 31 20 24 37 41 38 6 7 18 10 10 27 32 30 32 51 53 61 10 26 33 45 10.1 15.4 17.8 18.2 50.0 151.3 187.7 240.3 5.0 9.8 10.5 13.2 0.39 0.57 0.54 0.67 11.5 25.8 28.2 35.8 --0.004 +0.33 +0.64 +0.83 1.43 3.41 6.08 0.2 0.004 <0.001

West side of panels 24/6 25/6 9

6

26]6 6

12 13 14 6 3 7 0 5 6 8 3 5 9 6 3 4 13 4 1 3 5 13 9 14 8 9 5 10 5 6 4 5 14 16 13 17 7 17 14 19 8 18 9 11 6 13 12 12 8 12 17 18 17 12 27 17 18 37 37 39 12 41 39 59 19 53 38 76 25 46 47 42 33 83 63 97 12.3 21.2 17.6 23.3 57.5 416.2 309.8 691.7 4.7 19.6 17.6 29.7 0.30 0.88 0.94 1.23 10.8 54.9 48.6 84.2 +0.75 +0.81 +0.87 +0.71 4.66 5.63 7.28 4.16 <0.001 <0.001 <0.001 <0.001

* p 0.05 ~ t = 2.101,p 0.01 ~ t = 2.878, p 0.001 ~ t = 3.922, at 18 degrees of freedom.

68

P. D E W O L F

3. RESULTS 3-1.

PANELS

IN A SINGLE

ROW

As results obtained for Balanus crenatus (Tables XXV, X X V I and XXVII) do not differ from those obtained for B. improvisus (Tables X X V I I I , X X I X and XXX) they will be treated together. The mean 10000"

J

|= el

100

•

2" 10

'

'

~

'

4'0

'

~

'

~o

"

~o'

~o

'

~,io

'

,~o

'

~o'

~3o

numbers

Fig. 15. R e l a t i o n b e t w e e n m e a n n u m b e r o f b a r n a c l e s a n d v a r i a n c e , for b o t h Balanus crenatus a n d B. improvisus. CF

o

60.

50"

o .

o

40' o

o 30 o

20

0

a

,

A

,

20

t,

o

,

,

40

,

.

60

.

.

80

.

.

.

100

.

.

.

120

.

.

.

140 number

.

.

160

.

180

200

Fig. 16. R e l a t i o n b e t w e e n m e a n n u m b e r ofBalanus crenatus (/k) a n d B. improvisus (V]) p e r 4 e m 2 a n d F i s h e r s d i s p e r s i o n coefficient, o n a r o w o f p a n e l s a t a n i n t e r - p a n e l distance of 5 m.

number of spat present per panel is highly variable. On panels, whereon spat was allowed to accumulate during longer periods, the

DISPERSAL

OF BARNACLE

69

LARVAE

mean number generally increases with time. The rate of increase, however, is different for different panels, and for the same panel, as can clearly be seen in Table X X X . There are exceptions to these rules: sometimes a decrease in numbers on a panel takes place, indicating that new settlement was lower than natural mortality. Superimposed upon the general increase in time is a variation in numbers on individual panels. The variance of the numbers of spat has a complicated relation with the mean (Fig. 15); at low densities the variance increases with increasing density, until a m a x i m u m is reached; at high settlement densities, when the panels are nearly fully covered with barnacles, the variance decreases again. This is shown also by the coefficient of Fisher CF which increases with increasing population density, until a m a x i m u m is reached at 25 barT A B L E XXVI

Numbers of spat of Balanus erer~tusper 4 cm 2, settling on the east side of panels in a row in the entrance to the harbour of Den Helder, on days indicated; start of experiment 9 J u n e 1969. Panel 1 near the dike, and 20 near the raft. Panels were cleaned from spat after each count.

Panel 10/6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 S2 CF c t p

0.78 1.18 2.78 8.96 2.10 1.94 3.00 1.70 1.00 1.44 2.62 1.90 1.30 0.92 1.32 1.28 2.48 2.08 0.64 1.32 2.04 3.11 1.52 0.26 1.63 0.14

11/6 0.30 0.40 1.02 29.75 0.80 0.72 0.58 0.42 0.42 0.52 1.80 1.16 0.46 1.06 1.22 0.48 1.12 1.26 0.18 0.52 2.21 42.19 19.09 8.19 55.8 <0.001

12/6 0.42 0.44 2.86 14.44 1.20 0.38 0.54 1.18 0.94 0.56 1.80 0.80 0.90 0.82 1.40 0.94 2.36 2.18 0.52 0.54 1.76 9.40 5.34 2.47 14.0 <~0.001

13[6

14/6

15/6

16/6

0.40 0.58 4.34 56.50 1.66 0.70 0.24 1.64 1.04 1.26 1.88 1.64 0.58 0.68 1.52 1.90 2.82 3.52 0.70 0.68 4.21 152.59 36.24 8.37 108.6 <0.001

1.14 2.94 8.56 17.58 2.72 1.18 0.30 2.50 2.10 2.34 4.50 3.54 2.06 1.18 3.56 2.56 5.38 5.92 1.96 2.66 3.73 14.26 3.82 0.76 8.74 -<0.001

4.16 2.76 21.70 28.86 4.92 3.46 1.02 9.59 2.94 3.52 14.29 6.96 4.42 2.88 9.27 4.34 13.00 21.90 6.88 5.34 8.61 58.20 6.76 0.67 17.8 <0.001

19.91 25.00 40.60 115.50 29.86 22.78 4.88 52.00 29.29 22.70 18.73 29.13 18.45 11.00 25.56 20.10 25.25 31.14 17.00 17.08 28.80 517.95 17.98 0.59 52.4 <0.001

70

P. DE WOLF

nacles per cm 2. With larger population densities CF decreases again (Fig. 16). Furthermore CF is always significantly larger than 1, indicating that the barnacles are groupwise distributed over the series of panels. Dispersion coefficient c gives the same indication; c being always significantly greater than 0. Autocorrelations of the first order show that: (1) correlation coefficients for the numbers of spat on the west sides of the panels are nearly always higher than those for the east sides of the panels; (2) correlation coefficients are in less than half of the observations (days) probably significant (p _< 0.05), even when including the observations of 20 to 26 June 1968 for Balanus crenatus (Table X X V ) where the correlation coefficient is in all but one case highly significant; (3) correlation coefficients, for populations with a low mean density TABLE

XKVII

N u m b e r s of spat of Balanus crenatus per 4 cm 2, settling on the west side of panels in a row in the entrance to the harbour of D e n Helder, on days indicated; start of experiment 9 J u n e 1969. Panel 1 near the dike, and 20 near the raft. Panels were cleaned from spat after each count. Panel

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 S~ CF c t p

10[6

11/6

12/6

13/6

14/6

15/6

16/6

1.32 1.22 2.64 3.62 2.34 2.82 2.86 1.70 2.06 2.26 3.22 2.60 1.40 2.70 2.60 2.58 2.60 1.16 0.92 2.08 2.24 0.54 0.24 --0.34 2.35 0.035

0.48 0.22 0.96 2.06 0.84 0.96 0.42 0.32 0.78 0.52 1.78 2.14 0.64 0.94 1.12 0.70 0.88 1.50 0.44 0.46 0.91 0.31 0.34 --0.72 2.02 0.06

0.74 0.46 1.44 1.78 1.70 1.56 0.64 0.58 0.78 0.94 1.28 0.50 0.78 0.86 1.18 1.64 1.48 -0.66 0.38 1.02 0.22 0.22 --0.77 2.42 0.03

0.62 0.58 2.08 1.66 1.26 1.44 0.78 0.62 0.76 1.04 1.06 0.96 0.48 1.06 0.90 1.08 0.56 1.92 0.24 0.54 0.98 0.24 0.24 --0.77 2.33 0.035

0.90 1.90 3.28 2.92 1.86 4.14 0.96 0.86 1.12 2.46 2.44 2.26 1.84 1.86 2.04 3.44 1.20 3.22 0.70 0.62 2.00 1.04 0.52 --0.24 1.48 0.16

2.08 2.32 6.82 3.38 4.32 6.36 2.06 1.62 3.82 3.90 4.16 5.02 3.66 4.66 5.36 6.96 1.94 6.48 0.98 0.82 3.84 3.75 0.98 --0.006 0.07 >0.50

10.36 15.57 23.44 23.22 17.25 34.67 18.36 14.64 16.62 18.17 19.82 19.55 12.06 19.27 21.70 20.20 7.94 17.25 3.44 2.22 16.79 53.02 3.16 0.13 6.73 <0.001

D I S P E R S A L OF B A R N A C L E show

a much

greater

variation

than

71

LARVAE

those for a population

with

a

higher mean density of spat over the series of panels; (4) correlation coefficients for populations with a high mean density for the series are low. It follows, that on neighbouring panels the numbers of spat present, may be more or less equal or contrary very different. As a rule adjacent panels bear markedly different numbers; suggesting that groups of cyprids in the water are in an across-current direction smaller than 5m. TABLE

XXVIII

N u m b e r s of spat of Balanus improvisus per 4 cm ~, settling on panels in a row in the entrance of the harbour of Den Helder on days indicated; start of experiment 8 J u l y 1968. Interpanel distance 5 m. Panel 1 near the raft, and 20 near the dike. After each count panels were cleaned.

Panel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

S~ CF c t* r1 t p

East side of panels

West side of panels

1517

17/7

19/7

22/7

15/7

88 17 12 13 21 6 20 15 14 21 8 19 6 13 2 18 15 7 35 40 15.5 82.8 5.3 0.28 12.7 0.007 0.029 0.8

4 6 4 8 14 14 13 20 13 12 11 16 2 8 1 8 12 7 31 29 11.6 61.0 5.3 0.37 12.6 0.44 2.01 0.06

9 6 12 12 16 7 13 9 13 10 22 7 6 4 6 9 8 3 18 6 9.8 22.2 2.3 0.11 3.2 --0.06

16 20 47 54 20 17 21 19 30 21 21 33 35 5 8 18 30 9 52 31 25.3 181.3 7.1 0.24 17.9 0.11 0.46 0.62

4 12 4 6 28 32 26 9 29 8 10 7 12 6 7 3 40 21 51 58 18.6 258.6 13.9 0.69 37.7 0.48 2.33 0.03

1717

19]7

22/7

2 2 2 3 4 17 18 15 10 8 13 8 6 6 4 8 11 4 28 21 9.5 49.0 5.1 0.44 12.2 0.46 2.19 0.04

3 14 3 8 17 20 19 17 11 12 15 6 21 3 4 7 11 6 15 25 11.8 44.0 3.7 0.23 7.9 0.14 0.60 0.55

21 22 24 24 24 12 24 75 28 21 11 26 28 12 4 12 15 11 34 54 24.1 245.5 10.2 0.38 26.9 0.24 1.05 0.3

* p 0.05 ~ t = 2.101,p 0.01 -+ t = 2.878, p 0.001 -+ t = 3.922, at 18 degrees of freedom.

72

P. D E

WOLF

Despite this, rank correlation shows that there is in general some similarity in numbers for certain panels at successive counts in one series; some panels bearing more often high numbers of spat, other panels having usually low numbers. Ranking of all data gives a coefficient of concordance W = 0.23 (F = 7.47: p < 0.001): panels can be ranked according to the mean number of spat. Comparing this general rank with the panel numbers from the dike to the raft, it can be shown that there is a decrease of numbers of spat in that direction; its significance is low, however (p = 0.1).

3-2. P A N E L S

ON

A RAFT

Since full tabulation of results would require too m u c h space, only a few examples of the figures obtained will be given here, while the others will be described in more general terms.

3-2-1. B A L A N U S C R E N A T U S

An example of the kind of data obtained has been given in Table X X X I , while results of the analysis of variance have been given in Tables X X X I I and X X X I I I , as ratios of actual F values over theoretical F values, based upon the nullhypothesis that no significant differences between settlements on different panels exist. In accordance with literature data (PYEFINCH, 1948; BOUSFIELD, 1954) it has been found that numbers of settlers increase with depth, numbers of Balanus crenatus settling at a depth of 0.50 m are generally negligible. As an exception to this general rule they settle in appreciable numbers at a depth of 0.50 m on the northern-most frame, while at depths of 1.00 m and 1.50 m settlement on the panels on the southern-most frame (No. 28) is generally much lower than average. Significant and appreciable differences have been found between the numbers settling on north- and south faces of panels, on all frames, at all depths, and at all positions on the frames. During 1966 usually higher numbers settled on the north faces of the panels, while during 1968 usually the number on the south faces of the panels was the higher one; for both, however, exceptions can be found. Numbers of Balanus crenatus settling on panels in different positions on frames were usually not significantly different in J u n e 1966, but in J u n e 1968 such differences did occur occasionally; then the numbers settling on the panels at the east end of a frame were higher than those on the west end. About the same situation existed in J u n e 1968.

DISPERSAL

OF BARNACLE

73

LARVAE

T A B L E XXIX

Number of spat of Balanus improvisus, per 4 cm ~ (in last 2 columns per 1 cm 2, and multiplied by 4), settling on the east side of panels in a row in the entrance of the harbour of Den Helder, on days indicated; start of experiment 21 June 1968. Panel 1 near the raft, and 20 near the dike. Interpanel distance 5 m. Panels were not cleaned between counts.

Panel 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 S~ CF c t rx t p

22/7 16

20 47 54 20 17 21 19 30 21 21 33 35 5 8 18 30 9 52 31 25.3 181.3 7.15 0.24 18.1 0.11 0.46 0.62

23/7

24[7

26/7

29]7

9

12 35 77 37 85 84 125 48 95 121 111 38 19 10 12 24 159 4 25 23 57.2 2132.0 37.3 0.63 106.7 0.16 0.67 0.50

16 84 164 104 184 172 172 108 176 192 224 184 32 72 28 84 236 8 64 48 117.6 5329.0 45.3 0.38 130.2 0.21 0.89 0.38

84 244 188 196 268 188 304 252 216 188 152 176 152 196 172 104 156 68 152 168 181.2 3422.0 18.9 0.10 52.6 0.29 1.25 0.24

21 61 39 57 67 88 44 91 103 84 67 12 12 8 21 95 5 21 25 46.5 1120.0 24.1 0.50 67.9 0.38 1.69 0.10

N u m b e r s o f Balanus crenatus settling o n t h e d i f f e r e n t f r a m e s w e r e u s u a l l y s i g n i f i c a n t l y d i f f e r e n t in 1966, o n t h e f r a m e s 1, 7, 13, 19, 24 a n d 28. As a r u l e n u m b e r s o n t h e n o r t h e r n - m o s t f r a m e (No. 1) w e r e h i g h e s t , t h e o t h e r s c a r r y i n g , in t h e o r d e r i n d i c a t e d , d e c r e a s i n g n u m bers o f settlers. I n J u n e 1968, n o s i g n i f i c a n t differences b e t w e e n t h e f r a m e s 21, 23 a n d 25 c o u l d be s h o w n , a l t h o u g h o n o n e o c c a s i o n (at 1.50 m) t h e n u m b e r o f settlers o n f r a m e 21 was s i g n i f i c a n t l y low, t h e n u m b e r o n f r a m e 25 s i g n i f i c a n t l y h i g h . I n t e r a c t i o n s o f t h e first o r d e r in t h e analysis o f v a r i a n c e w e r e s o m e t i m e s significant, b u t this c o u l d a l w a y s b e e x p l a i n e d f r o m effects o f t h e m o s t n o r t h e r n a n d s o u t h e r n

74

P. DE W O L F TABLE XXX

Numbers of spat of Balanus improvisus, per 4 cm * (in last 2 columns per 1 cm *, and multiplied by 4), settling on the west side of panels in a row in the entrance of the harbour of Den Helder, on days indicated; start of experiment 21 June 1968. Panel 1 near the raft, and 20 near the dike. Interpanel distance 5 m. Panels were not cleaned between counts.

Panel 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 S2 Cr c t r1 t p

22/7

23/7

24/7

26/7

29/7

22 22 24 24 24 12 24 75 28 21 11 26 28 12 4 12 15 11 34 54 24.1 247.6 10.3 0.38 27.3 0.24 1.05 0.3

12 4 19 15 22 58 107 71 108 81 12 53 15 8 2 2 6 1 58 66 36.0 1282.0 35.6 0.96 101.7 0.61 3.27 <0.01

7 27 18 30 26 67 171 63 119 92 18 76 9 4 3 3 5 1 50 67 42.8 2084.0 48.7 1.11 140.2 0.42 1.95 0.07

24 24 76 76 92 140 232 128 256 224 60 164 36 32 16 4 40 0 140 124 94.4 6217.0 65.9 0.69 190.8 0.48 2.33 0.03

132 228 152 224 220 232 280 196 t68 216 220 244 140 156 148 132 236 172 248 224 198.4 1971.0 9.9 0.04 26.1 0.059 0.25 0.8

frames; interactions of the second order were never significant. One further observation should be mentioned: the upper half of the 5 × 10 g r i d o f 4 c m 2 s q u a r e s o n t h e p a n e l s a l w a y s c o n t a i n e d a g r e a t e r n u m b e r o f settlers t h a n t h e l o w e r half. T h i s will b e d i s c u s s e d l a t e r ( p a g e 88). 3-2-2. BALANUS IMPROVISUS R e s u l t s o f a n a l y s i s o f v a r i a n c e for d a t a c o l l e c t e d for Balanus improvisus h a v e b e e n g i v e n for A u g u s t 1967 ( T a b l e X X X I V ) , a n d J u l y 1968

D I S P E R S A L OF B A R N A C L E L A R V A E TABLE

75

XXXI

Mean number of Balanus crenatus (settlers and young barnacles) per 4 cm 2, during the period of 24 J u n e to 1 July 1966, on north- and south side of panels mounted in frames on a raft, every time in 4 positions on each of 3 depths. Frame Depth 0.50 m 1.00 m 1.50 m a c e g a c e g a c e g 1 north 6 11 7 8 21 26 22 26 40 42 41 37 south 3 4 4 2 12 18 14 7 34 40 33 42 7 north 3 4 2 3 18 17 16 24 40 37 41 36 south 0 0 0 0 9 7 4 10 32 31 22 27 13 north 0 0 0 0 6 15 8 I0 37 27 24 22 south 0 0 0 0 16 13 7 6 36 31 24 24 19 north 0 0 0 0 18 8 8 11 31 32 24 24 south 0 0 0 0 7 7 3 3 29 24 21 20 24 north 0 0 0 0 7 9 9 7 33 26 28 25 south 0 0 0 0 9 7 4 5 24 21 18 18 28 north 0 0 0 0 11 17 18 20 29 26 21 26 south 0 0 0 0 0 0 0 0 12 11 13 5 TABLE

XXXII

Ratio between actual variance ratio and theoretical variance ratio, based upon the null-hypothesis that homogeneous settlement occurred (values > l indicate significance at 99% level), for data on settlement of Balanus crenatus on panels on a raft, during dates in J u n e 1966. Source o f 1/6 7/6 17[6 20/6 24/6 1/7 ~)aria~ frame 2.3 62.1 3.4 5.3 1.10 18.5 position 0.004 0.2 0.3 0.29 0.01 1.7 depth 13.2 79.3 48.8 38.7 15.5 236.4 side 1.0 2.9 0.46 0.48 18.3 TABLE

XXXIII

Ratio between actual variance ratio, and theoretical variance ratio, based upon the null-hypothesis that settlement was homogeneous (values > 1 indicate significance at 99 % level), for data on settlement of Balanus crenatus on panels on a raft, J u n e 1968. Source o f 19/6 20]6 21/6 24/6 26/6 1/7 variance at 0.50 m frame 0.06 0.07 0.07 0,22 0.03 0.23 position 0.31 0.07 0.05 0,14 0.01 0.29 side 0.25 0.05 0.01 0.20 O. 10 2.16 at 1.00 m frame 0.28 0.33 0.38 0.29 0.48 0.39 position 0.56 0.34 0.52 0.31 0.07 1.62 side 2.66 0.77 1.77 1.01 1.55 2.02 at 1.50 m frame 0.07 0.29 O. 11 0.47 0,34 0.66 position 0.59 0.35 0.38 0.98 0.76 1.04 side 9.76 2.04 1.62 3.84 2.76 4.29

76

e. DE WOLF T A B L E XXXIV

Ratio of actual variance ratio and theoretical variance ratio based upon the nullhypothesis that settlement was homogeneous (values > 1 indicate significance at 99% level), for data on settlement of Balanus improvisuson panels on a raft, a during dates in August 1967. Sou~g o f

variance

9/8

10]8

11/8

14/8

1818 2 2 [ 8 2 4 ] 8 25/8

~ame position depth side

8.0 8.1 108.6 0.2

3.6 1.9 27.4 1.1

4.1 3.9 36.7 1.6

5.6 3.2 34.3 1.5

6.7 3.6 48.5 2.6

1.4 3.9 35.9 4.2

2.3 4.2 58.4 1.2

1.5 2.5 25.8 0.7

Table X X X V ) . As it had been found in 1966, for Balanus crenatus, that frames No. 1 and No. 28 collected always high, respectively low numbers of settlers, these 2 frames were not used in experiments during 1967; it was thought that due to their outer position they might give anomalous results. Therefore the frames 1 and 28 were fitted with a full number of panels, which were cleaned regularly, but not counted in the experiment; only panels on frames 2, 8, 11, 16, 17 and 96 were used. As in Balanus crenatus, the n u m b e r of settlers of B. improvisus on the different frames is always significantly different (Table X X X I V ) . Similarly, the number of barnacles on panels at different positions in the east-west direction on each frame varies greatly, while the influence of depth is again outspoken. Again the numbers settling on north- and south faces of panels is usually significantly different. Looking into the numbers of Balanus improvisus settling on different frames it follows that generally the numbers on the northern half of the raft are highest, and those on the southern half are lowest. The n u m b e r of settlers on frame 2 is in 6 out of 8 days the highest of all, the n u m b e r of frame 26 is in 6 out of 8 days the lowest of all. On the other frames no definite order can be indicated though numbers on even adjacent frames are often significantly different. The differences in settlement on panels in positions in east-west direction in general show highest numbers in the g-position (east end of the frames) lower numbers going west, with lowest numbers in the c-position, and again higher numbers in the a position (west end of the frames). The influence of depth on settlement is again: increasing numbers with increasing depth. Numbers of settlers on north faces of panels furthermore are generally somewhat higher than on the south faces; although the difference is significant and consistent, the absolute value of the difference is not great. It further appears (Tables X X X I V and X X X V ) that there is no

DISPERSAL

OF BARNACLE

77

LARVAE

relation between the length of the period during which the settlers were allowed to accumulate and the significance of the differences in numbers of settlers due to the sources of variance noted (frame, position, depth, and face). During J u l y 1968 a procedure, slightly different from those followed hitherto, was used: panels (on frames 21, 23, 25) were exposed at all 7 positions, but, while the settlers were allowed to accumulate on positions a, c, e and g, those on positions b, d and f were cleaned at each count. It was hoped that in this w a y the numbers settling on these panels could be used as a control for the numbers of arrivals on the panels at positions a, c, e and g. It was found, however, that irrespective of the numbers of barnacles present on the not-cleaned panels, the number of settlers on cleaned panels was usually higher; this holds on all faces, positions, frames and depths. It is further relevant that the variance of the mean number of settlers is higher on cleaned panels than it is in accumulating settlements. It follows that it is hardly possible to have any means of control of the amount of settlement, resulting from phenomena as gregariousness and the spacing out mechanism. Panels that were cleaned at each count will not be considered further. Frames are a significant factor in differences in numbers of settlers at a depth of 0.50 m (Table X X X V ) ; this is mainly due to high numTABLE

XXXV

Ratio of actual variance ratio and theoretical variance ratio based upon the nullhypothesis that settlement was homogeneous (values > 1 indicate significance at 99% level), for data on settlement of Balanus improvisus, July 1968. Date

Depth 0.50 m

July 2 3 4 8 9 11 15 17 19 23 25 29

1.00 m

Frame Position

Side

0.09 0.00 0.33 0.09 3.87 0.04 0.46

0.16 0.09 0.35 1.05 0.01 1.03 0.11

0.02 0.14 0.13 0.15 0.19 0.30 0.37

Frame Position

2.90 0.19 0.03 0.26 0.55 1.37 0.66 0.01 0.26 0.18 0.04 0.21

0.01 0.07 0.08 0.49 1.50 0.44 0.73 0.93 0.58 0.17 0.20 1.00

1.50 m Side

0.18 0.02 0.02 0.01 0.27 0.18 0.02 0.50 2.19 0.21 0.13 0.56

Frame Position

0.04 0.22 0.07 0.51 1.72 0.01 0.15 0.29 0.05 0.06 0.41 0.12

0.13 0.21 0.28 0.77 0.86 0.03 0.22 0.11 0.31 0.04 0.09 0.03

Side 0.05 0.10 0.18 2.66 0.11 0.23 0.14 0.24 0.93 0.00 0.08 0.03

78

P. D E

WOLF

bers of settlers at frame 21. In some isolated cases the facing of the panels is a significant source of variance; it is difficult to indicate what actually happens in these cases. Sometimes the number of settlers is significantly high on the north side of the panels (at 0.50 m depth on 8 and 11 July, and at a depth of 1.00 m on 19 July), on other occasions the number of settlers on the south side of the panels is significantly high (at a depth of 1.00 m on 17 and 29 July, at 1.50 m on 8 July). In the east-west direction (positions) significant high settlements occur on the east side of the raft, mainly at a depth of 1.00 m. In general depth is a highly significant factor in determining the numbers of settlers. 4. D I S C U S S I O N

From the results given it seems that there are no differences in settlement between the 2 species, Balanus crenatus and B. improvisus, other than the difference in season. It may be that differences are present, but due to the large intra-specific variations they cannot be shown. In the following therefore both species will be treated together. As stated earlier, differences in numbers of settlers have hitherto been explained in terms of light, current and behaviour of larvae with respect to each other. Before discussing the relevance of the present data it appears useful to review these earlier explanations for barnacles (page 78) and for oysters and mussels (page 83). Then the present data will be discussed (page 84) and subsequently earlier explanations will be sujected to a critical examination (page 88). 4-I.

LITERATURE THE

DATA

ON SOME FACTORS

SETTLEMENT

INFLUENCING

OF BARNACLES

4-i-i. LIGHT VISSCHER (1928) showed for what he described as cyprids of Balanus improvisus, but which were most probably cyprids of B. amphitrite (McDouGALL, 1943) and for cyprids of Chthamalus fragilis, in small crystallizing dishes, that "these larvae are usually positive in their reactions to light, but in the later stages they are very erratic and may not show either positive or negative reactions for a considerable period of time". However, at the time of a t t a c h m e n t . . . "they become decidedly negative and move away from the source of stimulation". No duration has been recorded for these experiments. VISSCHER & LUCE (1928) found in the laboratory again that B. improvisus (?) and B.

DISPERSAL

OF BARNACLE

LARVAE

79

amphitritecyprids react at firstphotopositive; after a few hours, they do not react any longer, and after arrival on a substrate they react photonegatively. VISSCHER ~ LUCE used in their experiments different regions of the spectrum, and found green light of 530 to 545 n m wave length to be most effective as a stimulus. N E U (1933) measured the settlement of B. fmprovisuscyprids on standard colours, and presents results similar to those of VISSCHER & LUCE, during short exposures. However, in a longer lasting exposure of paints of different colour (red versus green) no difference was apparent (NEu, 1933: 240). Observations made by POMERAT & REINER (1942) on the settlement of Balanus eburneus in Pensacola, Florida, on the north coast of the Gulf, fall into 2 categories. T h e y used clear glass panels exposed at different angles with the vertical; further black, opal and clear glass panels were used to analyse the influence of the photic factor. All experiments were done at 2 stations; one at about 1 foot above the bottom in a mean depth of 2 feet, the other at 2 feet above the bottom in a mean depth of 11 feet. The shallow water station was characterized by "mild current and generally slight wave action", while on the deep station there was "an active current and wave action". Numbers of barnacles on panels exposed at various angles after varying periods at the shallow station were as follows: on horizontal panels highest numbers settled on the under surface, while practically no barnacles settled on the upper surface. Hardly any barnacles settled on vertical panels, while the numbers settling on panels at an angle of 45 degrees were intermediate between those on the under surfaces and vertical surfaces. Numbers settling at the deep station were always much higher, even during shorter periods. Differences between numbers on under- and upperfaces of horizontal panels were sometimes considerable, but sometimes the numbers on the upperface were higher. Numbers settling on vertical panels were relatively high when compared with the shallow water station, sometimes even higher than those on horizontal underfaces, and numbers of settlers on 45 ° panels (upper and under) were usually intermediate. In another experiment at the deep station it was found that settlement on horizontal panels, counted every 6 hours over a 24 hour period, increased regularly with time on the under surface, while the increase on the upper surface was irregular. Total counts, over a period of 24 hours, were lower on the upper surface. Unfortunately only means for 4 panels are given in the paper. In the experiments on settlements on black, opal, and clear glass panels no significant differences could be found for under- and upperfaces of horizontal panels. The number of settlers on black glass was always higher than on clear or opal glass. Nevertheless considerable

80

P. D E W O L F

numbers settled on the last2 materials. W h e n exposed during the night only, the 3 materials collected about equal numbers of settlers.POMERAT & REINER suggest that a photic factor is of primary importance in the settlement of Balanus eburneu~. McDoUGALL (1943) in Beaufort, North Carolina, also working with B. eburneus, found that on panels, painted half black half white, black was preferred to white in a ratio of 1.5 to 1 ; this was more marked in a sunny place (3.3 to 1) than in the shade (1.4 to 1) and was not evident at night. During the night much less barnacles settled than during the day, and on one of the 3 nights the experiment lasted, no barnacles settled. Panels exposed for a week collected almost equal populations on black and white. McDoUGALL (1943) noted further that Chthamalusfragilis, an intertidal species, was more numerous, and extended to a higher level, on shaded places. For Balanus amphitrite it is noted that they settled (or survived?) at higher levels in winter than during summer. EDMONDSON & INGRAM (1939) note for B. amphitrite, at Hawaii, as a common observation, that the under surface of a horizontal panel on the surface or just below it will collect large numbers of barnacles while the upper surface may be entirely free of them. Further, observations on black and white coloured panels, support the observations by earlier authors. Observations on the influence of spectral colours on the amounts of settlement do not generally agree with findings by earlier authors: generally less organisms settled on white panels than on green ones, while green collected less than either red or blue; occasionally, however, green panels attracted more cyprids than any of the others (1939: 278). Similarly, the influence of night and day was varying, and different from those reported earlier: sometimes considerable numbers of organisms attached during the day but none at night, whereas only one week later "considerable numbers of oysters, bryozoans, ascidians and both cyprids and young barnacles were affixed to black and white panels" during the night. EDMONDSON & INGRAM (1939: 289) concluded that "Greater fouling on shaded or dark surfaces seems to indicate that negative phototropism on the part of the larvae obtains at the time of attachment. However, settling of organisms occurs as freely at night as during daylight hours, an observation which challenges the view that colour of surface is an important factor in fouling. White surfaces have an antifouling advantage over darker ones, for short periods of time. Sometimes green panels repel organisms to a greater degree than red or blue ones if all are non-toxic; often the reverse is true. Spectral colours apparently have little differential value after periods of one or two months."

DISPERSAL

OF BARNACLE

LARVAE

81

Observations and experiments on the settlement of Balanus eburneus, by GREQG (1945), confirmed observations by POMERAT & REINER (1942) at the same station at Pensacola, Florida while further a definite relation was found to exist between the frequency of attachment and a decrease in the intensity of general illumination. WEiss (1947) showed that cyprid larvae of Balanus improvisus react to light; numbers settling were higher during the day than during the night. On the other hand no consistent pattern could be found in the vertical distribution of setters. K N I G H T JONES & MOYSE (1961) mention for Balanus balanoides cyprids that "they remain positive to light until the time of attachment, with no more than brief photonegative excursions, which may help them to find dark rocks." Lastly, CALLA~tE(1962) experimented in a harbour with transparent and non-transparent plates and an underwater lamp. These experiments were done in a situation where tides and currents were absent, and daylight could play only a very limited role. CALLAMEconcluded for Balanus amphitrite that light prevents settlement to a large extent, and that the light gradient is much more important than the absolute value of the light intensity. Summarizing it can be stated that most authors are of opinion that light plays an important role in determining settlement of barnacle cyprids; often, however, erratic results have been obtained. 4-I-2. C U R R E N T

PYEFINCH (1948: 474) advocated that current could be at least as important as light in determining the numbers of settlers of Balanus balanoides at a certain place. He supposed that different numbers of settlers arrived on different places on his raft by slightly varying orientations of the panels relative to general current direction, and by slight variations in current speed from place to place. It should be noted that he counted the numbers of settlers after an exposure period of one month, when a mean number of about 100 individuals was present per square inch. 4-1-3. BEHAVIOUR

OF L A R V A E

Differences in numbers of settlers on identical substrates have also been described by KNIGHT JONES & STEPHENSON (1950: 281) for Elminius modestus. They explained these differences in numbers by the observation that cyprid larvae tend to settle near to previously settled individuals of the same species; they named this behaviour gregarious.

82

P. D E W O L F

It appears, however, that by this tcrm they describe 2 different phenomena. Firstly they observed that settlement on an empty slate is much higher when exposed in an environment with adult Elminius on a bottom covered with shells, than when exposed over a m u d d y bottom, without adult Elminius present. The authors suggest that, due to gregariousness, cyprids trying to settle arc much more numerous on a shelly bottom, because of the presence of adult Elminius. This needs not necessarily to be true when the larvae would not be able to maintain themselves in a current on a m u d d y bottom, while they probably could do so on a shell-bottom. The contribution of grcgarious behaviour can, however, not be excluded. Secondly, the authors observed that on a substrate where barnacles had already settled previously, the increase in numbers was faster than on an substrate devoid of the species. At first sight there secms to bc no difference with the above phenomenon; however, there is a difference of scale. Although it is possible, of course, that currents do bring higher numbers of larvae to one place than to another (larvae do not swim horizontally!), it is likely that the gregarious bchaviour of the larva is limited to the crawling and searching phase on the substratc. Then, after arrival on the substrate by current, the larva searches for another individual of the same species, and it will depend on length of time spent in searching, speed of crawling, and the density of previously settled individuals whether anothcr individual will be found or not. If it is assumed that on the scale of searching on the substrate (e.g. 12 mm for Balanus amphitrite, VISSCHER, 1928) the chance that a larva arrives on a particular spot of the substrate, by current, is random, then departure from randomness is due to the crawling of the larva over the substrate in search for other individuals of the same species, because then the chance to settle is greater when earlier arrivals are found than when they are not found. To prove or recognize gregariousness (page 95), a certain population density has to be present; the numerical value of this density depends upon the period and the intensity of searching. Anticipating data presented in Chapter V I I (page 91) it can be said here that gregarious behaviour influences the total number of settlers in Balanus crenatus only at a mean density greater than 2 to 4 individuals per cm ~. Gregarious behaviour has been found to exist in quite a number of marine sedentary species (KNIGHT JONES & MOYSE, 1961); among barnacles for Elminius modestus (KNIGHT JONES & STEPHENSON, 1950), Balanus balanoides and B. crenatus (KNIGHT JONES, 1953: 584). It is likely that it is a fairly common, if not universally valid, phenomenon

DISPERSAL

OF B A R N A C L E

83

LARVAE

a m o n g acorn barnacles. Opposite to, or simultaneous with gregariousness, but on a different scale, for some sedentary species a spacing-out

mechanism has been demonstrated ( C R I S P , 1961: 429; KNIGHTJONES & MOYSE, 1961: 78) for settling larvae, resulting in a minimum distance to be maintained between individuals. BARNES & POWELL (1950) have shown that under circumstances of heavy settlement this spacing-out mechanism breaks down in Balanus crenatus and B. balanoides. 4-2. LITERATURE ON THE

DATA

SETTLEMENT

ON ENVIRONMENTAL OF OYSTERS

INFLUENCES

AND MUSSELS

KORRINGA (1952) reviewed data on the settling of oysters; he quotes COLE & KNmHT JONES (1949): "The most critical phase in the life history of the oyster is the period of settlement. The fact that the sensory equipment and vigour of the larvae reaches its highest development immediately prior to settlement is no doubt correlated with the importance of the choice in ensuring survival", but warns that "we should, however, not presume that the inborn behaviour pattern of the settling larva safeguards it from making serious mistakes in choosing a site for its future sedentary life". KORRINGA then notes that light did not appear to be a factor of major importance in the settling of oysters in the rather turbid Oosterschelde, and neither did the colour of the substratum. On the other hand currents are considered to be of great importance in the settling of oysters under natural conditions; strong currents prevent the larvae from settling, and most of the settling is limited to the periods of slack water. Contrary to general experience oyster larvae in the Oosterschelde settled profusely on upper horizontal surfaces, while it was consistently found that settling is by far the greatest close to the bottom; floating collectors caught very little spat. Similar results have also been reported by COLE & KNIGHT JONES (1949) in Helford River and by LOOSANOFF& ENGLE (1940) in Long Island Sound: irrespective of place heaviest settling occurred near the bottom. COLE & KNmHTJONES (1949) found settlement in the quiet waters of a tank to be higher on under smfaces of collectors than on upper surfaces. KORRINGA (1952) remarked, however, that the prevalence of horizontal over vertical movements in open water, in contrast to tank conditions, made him hesitant to assume that oyster larvae have a preference for under surfaces. Gregarious behaviour in oyster larvae has further been described by COLE & KNIGHT JONES (1949), while KORRINGA (1952) also reports that he had experimental evidence for gregariousness.

84

P. D E W O L F

MEDCOF (1955) reported on oyster spatfall in "tideless" Gilles Cove, Bras d ' O r Lake, Nova Scotia for Ostrea virginica. The catch varied directly with depth, was heavier by day than by night, and the level of most intense spatfall was closer to the surface by day than by night. The catch on under sides was heavier than on upper sides, which difference was less by day than by night. MEDCOFF writes that readyto-settle oyster larvae have 3 behaviour characteristics; they are benthic, light stimulates them to settle and they settle most readily on under surfaces. Lastly SHELBOURNE (1957) working in the rivers Roach and Crouch, Essex found that planktonic oyster larvae do not accumulate in eddies, and he suggests that higher settlement in such eddies results from a water speed below the critical tbr settlement, for a longer period in each tidal cycle. Settlement data for larvae of Mytilus edulis are much rarer; apart from the unpublished data of ROOTH (1952) collected in the Wadden Sea, no data from the field are known. He found that settlement is nearly limited to the flood, independent of day and night, when total settlement is at a low level. When settlement is dense, it occurred on flood as well as on ebb periods; again no data indicating day and night influence could be obtained. BAYNE (1963, 1964) as has been mentioned earlier (page 10) showed in laboratory experiments that larvae of Mytilus react to pressure increases, and to light and gravity, by swimming upward. 4-3.

DISCUSSION

OF THE

PRESENT

RESULTS

Results obtained will now be discussed, and an attempt will be made to explain the differences in settlement in terms of light, current, gregariousness, and the groupwise occurrence of cyprid larvae in the water. The observations on the single row of panels stretching from the raft to the shore, from north to south, show that the number of settlers is usually highly variable. It will be clear that light cannot explain differences between panels; all panels facing in the same direction, either west or east, do receive equal amounts of light. True, changes in turbidity of the water do occur, but these changes are transistory and presumably more or less equal for all panels. There is a possibility that current plays a role, as current strength close to the shore will undoubtedly be less than further away from the shore (c.]: SI-IELBOURNE, 1957) ; but it appears unlikely that current differences are responsible for the large variations occurring from one panel to the next. Thirdly, gregariousness could influence the numbers of settlers present at a

DISPERSAL

OF BARNACLE

LARVAE

85

certain moment, and does this beyond doubt. Gregariousness, however, cannot explain, why considerable differences in numbers occur between panels when mean settlement density is lower than 2 to ¢ individuals per cm~; the chances of meeting a previously setded individual are then too low (page 91). It is therefore thought that, at least initial differences between panels are a result of different numbers of cyprids being carried to the panels; in other words, of the groupwise occurrence of the larvae in the water. It has been argued (page 48) that, theoretically, the groups of larvae in the water should have an elongated form. This fits with the present observations; a random distribution of small groups of whatever form would result in more or less equal numbers arriving on all panels. A random distribution in the water of large spheres of larvae (diameter of approximately 5 m) is impossible in view of the depth. As the panels have been exposed perpendicular to general current direction, a hit of a panel by a group of larvae results in high numbers in a large n u m b e r of cases; low numbers on panels possibly result from stray larvae outside groups as a result of breakdown of the groups. It was observed that over longer periods of exposure and settlement the rate of increase of numbers is not constant in relation to the place of the panel in the row, but is highly variable. This too, cannot be explained from differences in amounts of light between panels. Quantitatively it seems possible that it could be due to currents varying in time from place to place, but this would require great current variations and this appears highly unlikely. For the same reasons as given above, it cannot be due to gregarious behaviour of the larvae on the panels, as the phenomenon also occurs at population densities lower than the critical density for this behaviour. Again, groupwise occurrence of the larvae in the water can explain the observation. Occasionally the n u m b e r of settlers present decreases from one day to the next; it is clear that also natural mortality plays a role in determining the n u m b e r of settlers present after some time. This has been recognized earlier by KNIOHT JONES & STEPHENSON (1950: 287). From Fig. 15 follows that the variance of the mean number of settlers on the panels increases at first with increasing mean population density, reaches a maximum, and decreases subsequently at population densities of 25 barnacles per cm ~ and higher. This can be explained from territorial behaviour, as follows. A cyprid larva arriving on a~panel on which a fairly high n u m b e r of settlers is present, will have-a greater chance of leaving the panel after searching for a place to settle than a larva arriving on a panel with a lower n u m b e r of settlers. After a certain period of exposure all panels will have had chance to collect equal numbers of settlers, provided the supply is not

86

P. DE W O L F

too low. If, however, the n u m b e r of cyprids, arriving per tide is low, earlier arrivals would have grown in the meantime, occupying more space on the substrate and no equalization of the numbers on the panels can occur. It was found that the panels exposed near the dike always collected more settlers than the panels exposed over the m u d d y bottom between the dike and the raft. It is not seen how this could be explained by differences in light between the areas; differences in current are also unlikely. These differences in settlement density can be explained in the way KNIGHT JONES & STEPHENSON ( 1 9 5 0 ) e x p l a i n e d the differences in settlement on substrates over shelly, respectively m u d d y bottoms. If during the tidal cycle an increasing number of cyprids arrives amongst the adults present on the dike, there is a higher concentration in the water near the dike at the m o m e n t that the current becomes strong enough to transport them. A local high concentration of cyprids near the dike is then responsible for a high number of settlers on panels exposed here. It should be noted that the mechanism of group forming of the cyprids living free in the water, is also present in this case; in addition the adult population acts as a means to concentrate these cyprids on the bottom. Results obtained on the raft will be discussed on the basis of the 4 effects studied: depths, faces of the panel, frames and positions. As to depth it was always found that the numbers of settlers increased with depth of panels, whereas the numbers on each panel had an opposite distribution with more settlers on the upper half of the panel than on the lower halfi The first part of this observation fits with the vertical distributions of both species, as described by BOUSFIELD (1954: 132), PYEFINCH (1948: 482) and McDoUOALL (1943: 355). As noted before such distributions have been explained by photonegative swimming. In this respect it is necessary, however, to suppose that the phototaxis is reversed at the m o m e n t the larvae arrive at a panel, in order to explain why the upper half of the panel is more densily populated than the lower half. Further, it is known, that cyprid larvae, after searching for a place to settle, leave a substrate which is already too densily populated, and then it would be necessary to assume a return to photonegative behaviour. KNIOHT JONES & MOYSE (1961: 73) indeed indicate, for cyprids of Balanus balanoides, such frequent reversals of phototaxis (photopositive swimming alternating with photonegative periods to find dark rocks for settling), but it is not clear whether this has been observed or inferred. All these explanations for the vertical distribution of the species requires'iwell developed swimming possibilities~and a clear light preferendum. TttORSON (1966: 275) has already indicated that larvae, in the sea,

DISPERSAL

OF B A R N A C L E

LARVAE

87

will have no opportunity to compare substrates (with respect to environmental conditions) by swimming a short distance only. Once arrived upon a solid substrate they either settle or swim off again; in the latter case it may take a long time before they meet a solid substrate again. In Chapter I I I it has been made probable, that swimming is, in view of strong currents, not an important means of transport and it seems that it cannot be a means to reach a certain place for settlement either. Larvae, in the W a d d e n Sea, furthermore often remain for a long period in the cyprid stage, since solid substrates for settlement are rather rare. Therefore, following THORSON it might be supposed that larvae that are ready to metamorphose, will settle on any substrate to which they are brought by the current; even if the substrate is far from ideal for settling. Then, also, it is not necessary to suppose that larvae are very active swimmers, as would be necessary in the case of an active choice. Differences in the numbers of settlers at different depths can be explained from two entities: the concentration of larvae at each depth in question, and the period during which that concentration is present at that depth. It has been shown (page 41) that larvae are brought from the bottom to the surface by turbulence, and that subsequently they sink slowly. At greater depth the product of concentration of larvae and time is greater than at lesser depth. This might explain the vertical distribution as recorded without relying upon choice of light conditions and swimming activity. After initial settlement, thus when differences between panels at different depths are already present, gregarious behaviour can enhance such differences. It appears likely that the settling as observed on the different frames can be explained as follows. Groups of cyprids are carried to the raft on the flood, coming from the north (only occasionally south faces of panels do carry a high number of settlers). The majority of these do settle on north faces of panels, while a minority is diffused between the frames in relatively stagnant water, and settles wherever they are brought. Near the more southern frames less larvae are present in the water, and on the south side of the southern-most frame hardly any larvae settle, as no relatively stagnant water is present here. The differences between the numbers of settlers on panels on different east-west positions can be explained by assuming that the current does not flow exactly in a southern direction, but comes towards the raft from slightly north-western or north-eastern direction at times. When initial small differences between different positions have been realized, it can be attributed to gregarious behaviour that such differences are accentuated by new settlers. It remains to be explained that on each panel, at any depth, the

88

P. DE WOLF

upper half of the panel has a consistently higher number of settlers than the lower half. This may be due to a general upward movement of the larvae in the crawling phase, possibly as a result of a positive phototaxis or a negative geotaxis. In this stage, on a solid substrate, phototactic behaviour does make sense, as the larvae cannot be transported by water now. During the searching and crawling phase on the substrate the cyprids of Balanus crenatus and B. improvisus were observed to show a general upward movement on the substrate superimposed upon a more or less random walk (VAN ZALINGEN, personal communication). Summarizing, it is thought that transport of cyprid larvae to a substrate is largely passive; upon arrival by current they search the substrate for a place to settle. If they find such a place they settle; otherwise they swim off, and are carried by the current again. It appears likely that the decision to settle or to swim is a complicated process, influenced by the age of the larvae, and environmental factors, like light, colour of substrate, etc. Possibly the finicity of the larvae with regard to the environment decreases with increasing age, a supposition that needs more research, however. It would be expected that differences in numbers of settlers would be equalized in time. This aspect is treated in the next chapter in relation to the fact that settling takes place only during a short period of each tide (page 92). 4-4.

CRITICISM

OF

EARLIER

WORK

In view of the widely held opinion that it was proved for barnacle larvae and m a n y other marine larvae that their swimming is directed phototactically (either positively or negatively) it appears worthwhile to have a critical look at this literature, and see to what extent passive transport of groups of larvae can explain these observations. Explanations of settlement differences given in literature can nearly all be replaced by a simpler hypothesis when larvae of intertidal species are supposed to be buoyant, and in the crawling phase search photonegatively, while subtidal species are supposed to be heavier than water and crawl photopositively. VlSSCHER (1928) observed photonegative settling in Chthamalus fragilis, an intertidal species; this could, in view of the very small bowls used, easily be due to photonegative crawling. It seems that VlSSCHER'S observations on the subtidal Balanus improvisus contradict the hypothesis; these larvae were in fact photonegative as well. It is not unlikely, however, that these cyprids were actually B. amphitrite. JONES &

DISPERSAL

OF B A R N A C L E

LARVAE

89

CRISP (1954) indicated that it is nearly impossible to distinguish between the cyprlds of the 2 species. McDouOALL (1943: 355) also argues that the cyprids used by VISSCHER belonged to B. amphitrite, as B. improvisus cyprids are usually absent during summer in Beaufort, North Carolina. If the larvae used in the experiments were Balanus amphitrite, a mainly intertidal species (McDoUGALL, 1943), they do fit in with the hypothesis. The experiments on the influence of the colour of the substrate on settlement, as described by VISSCHER & LUCE (1928) can be explained on the basis of choice by the larvae during the crawling phase. In the small aquaria used the larvae could easily crawl from one side to the other. The observations of NEU (1933) on the choice of colours as substratum in field experiments can also be explained by arrival by current and subsequent choice for settlement or departure. The observations of POMERAT & REINER (1942) may largely be fitted into the present hypothesis. O n the shallow station with only a mild current practically no barnacles settled on vertical panels, while the numbers of barnacles on vertical panels on the deep station with an active current were generally much higher. M a n y barnacles on under surfaces of horizontal panels, hardly any on upper surfaces at a shallow station, and sometimes more barnacles on upper surfaces than on lower at a deep station, can be explained by a much thicker water layer at the deep station from which larvae can sink to the upper surface. The differences in settlement on black, opal and clear glass can be explained by arrival and subsequent choice. It is difficult to understand how light could be responsible for higher numbers of barnacles settling on the under surface of a horizontal clear glass panel. For McDouoALL'S (1943) experiments with black and white panels, choice between the 2 colours may have been made while the cyprids of Balanus eburneus were crawling. McDouGALL (1943) describes cyprids of Chthamalus fragilis as buoyant during the swimming phase and cites VlsscI-IER'S (1928) observations on the photonegativity of the species. The observations of both authors can be understood when the free-swimming phase indeed is buoyant, and the crawling phase of the larva is photonegative. McDoUGALL'S paper does not contain arguments for photopositive behaviour of Balanus improvisus; for the depth distribution of this species his results are much the same as the present ones, however. The observations of EDMONDSON • INGRAM (1939) on Balanus amphitrite on horizontal panels near the surface of the sea, show that in this position large numbers of barnacles settle on the under surface, while hardly any barnacle settles on the upper surface. McDoUGALL

90

P. DE

WOLF

(1943) described the species as mainly intertidal, and again buoyancy would bring the larvae to the under surface of the panels. On a horizontal panel at the surface photonegative crawling cannot play a role. Although WEiss (1947) thought to have shown that light influences settlement in Balanus improvisus, his observations can probably equally well be explained in terms of gregariousness, either on his panels, or on adjacent under water structures prior to transport to his panels. A further factor which should be regarded in explaining his data are the daily inequalities of the tides, while it is conspicuous that no consistent vertical distribution of settlement was found. His data do not contain any indication as to the photo-behaviour of the crawling stage. Lastly KNIGHT JONES & MOYSE described frequent reversals of photo-behaviour in the intertidal Balanus balanoides. This again, can be much simpler explained by buoyancy, and photonegative excursions. Buoyancy of cyprid larvae of B. balanoides has been described by RUNNSTR6M (1925: 13) who noted that in Norway in spring "fishermen take their boats out of the water when large numbers of cyprid larvae of Balanus balanoides are seen floating at the surface". The observations by CALLAME (1962) are left; they were made in a special situation, without tides, and probably without any current. Further the situation offers, relative to the amount of water, a large area of solid substrate, in the form of the walls of a submarine base. It appears that here light could play a decisive role in the settlement of the cyprids; this case is furthermore different from most other estuarine environments in that cyprids, when ready to settle can readily find a solid substrate to do so. As a result the planktonic cyprid population will have a relatively low mean age, leaving a large proportion of the larvae sufficient time to choose a place to settle. Furthermore in an environment without currents swimming is of significance. Thus, contrary to most authors it is thought that light plays an insignificant role in the settlement of populations of barnacles. The same hypothesis can be applied to observations on other organisms such as oyster and mussel larvae. KORRINGA (1952) has already indicated that light plays only a secondary role in the settlement of oysters in the Oosterschelde. Although COLE & KNIGHT JONES (1949) reported for a tank culture of oyster larvae that most oysters settled on under surfaces, and ascribed this to a negative phototaxis, it is not sure that these larvae were not transported by currents. MEDCOF (1955) also does not use the factor light to explain the settlement of oysters, although a significantly high proportion of settlement occurs on the under surface of horizontal collectors; and SHELBOURNE (1957) explains settlement of oysters in Essex rivers in terms of current and time.

DISPERSAL

OF BARNACLE

LARVAE

91

The observations of ROOTH (1952) on the settlement of Mytilus show that this is related to the tides only, and although BAYNE (1964) found in the laboratory that early larval stages of the mussel did show phototactic reactions to a limited extent, late stages of larvae, approachlng settlement, became photonegative (again, the magnitude of the response is small, amounting to a few centimetres swimming in 4 hours). v i i . OBSERVATIONS ON THE PATTERN OF SETTLEMENT OF BALANUS A M P H I T R I T E AND B. CRENATUS ON A SUBSTRATE I. INTRODUCTION

It has already been stated that gregarious behaviour of cyprid larvae on a substrate exerts no marked influence on the number of larvae settling on different panels when the population density remains below approximately 2 individuals per cm 2 (page 85). This statement was derived from the increase of the variance of the numbers of settlers at low densities and the leveling off of variance at a population density of 2 individuals per cm2; at higher densities variance decreases. Variances with a value higher than the mean indicate groupwise distributed populations; from Fig. 15 it could be derived that only dense populations ( > 25 individuals per cm ") would show an even distribution on the substrate. Now, gregarious behaviour alone cannot be held responsible for even distributions; in an extreme form it would lead to settling in one group of all cyprids arriving on a panel. CRISP (1961) and KNIGHT JONES & MOYSE (1961) have shown that barnacles also have a spacing-out mechanism causing that a minimum distance between individuals is maintained. (The term "territorial behaviour", as used by these authors, seems inappropriate. In its widest sense, a territory is "any defended area", but in the present situation newcoming cyprids avoid an occupied, but not defended area, and the even dispersal of settled barnacles is not due to aggressive behaviour of barnacles that were already present.) CRISP demonstrated the spacing out mechanism for Eliminius modestus, Balanus balanoides and B. crenatus, KNIGHT JONES & MOYSE for Eliminius and Balanus balanoides. The last authors, furthermore, showed that the phenomenon is species-specific: cyprid larvae only space out from individuals of their own species. KNIGHT JONES & MoYsE hypothesized that gregariousness and the spacing out mechanism together result in an even distribution on the substrate. It should be remembered that the hypothesis of KNIGHT JONES & STEPHENSON (1950), that gregariousness induces the free

92

P. D E W O L F

swimming cyprids to settle, should be rejected since it assumes them to swim against the current to which they are incapable (pages 52 and 63) ; most cyprids arrive on the early flood with the current. Thus, both gregariousness and the spacing out mechanism are only effective during the crawling phase on the substrate. As population density has an important effect on the dispersion of the population present on a substrate it was thought worthwhile to study this effect in view of the following considerations. The first larva, arriving on a bare substrate has to adhere on a randomly chosen place, but the probability that, after a long search, it will leave again seems high in the absence of other cyprids to orientate upon. If the first larva settles, a number of possibilities exist for the next one to arrive. After searching, this larva can settle near the first one, or, if that is not found, it again can leave the substrate or settle randomly. It will be clear that the dispersion of the young barnacles is determined by the intensity of their searching (time and speed) and the number of larvae arriving per unit time. Also the course of the larvae on the substrate could have an influence; this, however, can be excluded if the larvae choose their course randomly. Three possible results in the distribution after settlement can be expected : When the majority of searching cyprids did not find each other, they will be distributed randomly. A less sparse population could be more uneven than random, if the cyprids did find each other, or could be more even than random, if the cyprids did find each other, but if the spacing out mechanism becomes relatively important. When many larvae arrive in a short time, the dispersion would be even, due to the spacing out mechanism. In this chapter the relative importance of searching and spacing out is studied, for Balanus amphitrite and B. crenatus. 2. M E T H O D S

T w o different methods can be used to evaluate the population dispersion in terms of gregariousness and spacing out mechanisms. The first method has been described by KmGHTJONES & MOYSE (1961) and consists of counting the numbers of organisms in a great number of small squares on the substrate. The effects of the 2 mechanisms then follow from the dispersion coefficients (page 95). The second method, described by the same authors and by CRmP (1961), consists of measuring the distances between neighbouring barnacles. The last method provides more information than the first. Under the circumstances of the present investigation, the counting of numbers of barnacles in squares is more practical. Populations of Balanus amphitrite have been collected in the harbour

93

DISPERSAL OF BARNAGLE LARVAE

of Haifa, Israel, from a depth of 1{ m, either from a raft or from a quay, during September 1965 and August 1966. B. crenatus populations were collected in the same way from the raft in the harbour of Den Helder during J u n e 1966 and J u n e 1967. On smooth PVC panels barnacles were allowed to settle until populations of the required density were present. Two different sampling schemes have been used. In the first scheme a great number of panels were exposed simultaneously; these panels were taken from the sea, one by one, and only after they had collected a suitable number of barnacles they were counted in the laboratory. In the second scheme only a few panels were exposed; after counting they were re-exposed for further cyprids to settle and metamorphose to obtain greater population densities. Both schemes have their drawbacks. The first assumes the development of the dispersion of the populations on the different panels to be comparable which possibly is not true. The second scheme has the disadvantage that, provided mortality plays a role in the determination of the dispersion pattern (discussed on page 100) the bringing of the populations to the laboratory, could have influenced the results. 3. RESULTS 3-1. BALANUS

AMPHITRITE

The observations made in September 1965 (Table X X X V I ) make clear that the distribution is nearly always significantly more regular than a r a n d o m distribution. There is, however, one exception; in this TABLE

XXXVI

The distribution of Balanus amphitrite on smooth PVC panels at Haifa, September 1965. Each time counts were made on a different panel in 5 × 5 mm squares. Exposition time in days

Number of squares counted

Mean numberof barnacles

1 1 2 2 4 5 7 9 12"

500 1000 1000 200 1000 1000 989 200 100

0.81 0.59 1.36 1.17 2.65 2.69 8.86 14.36 20.75

Variance

0.66 0.55 1.15 1.08 1.92 1.79 4.32 8.03 63.58

* Much Hydroides norvegica also present on panel.

CF

p

0.82 0.94 0.85 0.93 0.72 0.67 0.49 0.56 3.12

0.001 0.05 Z0.03 0.23 <<0.0003 <<0.0003 <<0.0003 <<0.0003 <<0.0003

94

P, DE W O L F

c a s e m a n y o t h e r o r g a n i s m s w e r e p r e s e n t , c h i e f l y Hydroides norvegica Gunnerus which showed an aggregated distribution. The barnacles s e t t l e d i n t h e o p e n s p a c e s b e t w e e n t h e g r o u p s o f Hydroides, a n d h e n c e also s h o w e d a n a g g r e g a t e d d i s t r i b u t i o n . I n t h e o b s e r v a t i o n s o b t a i n e d d u r i n g A u g u s t a n d S e p t e m b e r 1966 o n l y a few p a n e l s w e r e u s e d , t h a t were re-exposed after counting. The distributions found (Table XXXVII) are never significantly different from random. However, TABLE

XXX~I

The distribution of Balanus amphitrite on smooth PVC panels at Haifa, August 1966. Counts from 4 panels in 5 × 5 mm squares.

Exposition time in days

Numberof squares counted

Meannumber of barnacles

1 2 3 4 6 7 8 9 10 1 7 1 2 3 4 5 1 2 3 4 5 7 8 9

868 868 660 868 364 104 868 868 868 840 840 260 260 260 260 260 260 260 260 260 260 260 260 260

0.02 0.02 0.04 0.11 0.25 0.28 0.33 0.45 0.44 0.03 0.15 0.02 0.03 0.03 0.07 0.12 0.07 0.13 0.23 0.40 0.43 0.33 0.25 0.23

Variance

0.02 0.02 0.04 0.10 0.27 0.27 0.31 0.45 0.47 0.03 0.15 0.02 0.03 0.03 0.07 0.12 0.08 0.13 0.26 0.40 0.40 0.35 0.26 0.24

CF

p

0.98 0.98 1.04 0.91 1.05 0.99 0.94 1.01 1.04 1.05 0.98 0.98 0.97 0.97 1.06 1.02 1.04 1.00 1.13 1.02 0.93 1.04 1.02 1.03

n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.

the population densities were in this period much smaller than those i n S e p t e m b e r 1965. I n r e l a t i o n to m e a n d e n s i t y t h e B. amphitrite r e s u l t s m a k e c l e a r ( F i g . 17) t h a t for l o w d e n s i t y p o p u l a t i o n s ( u p to 0.5 s p e c i m e n s p e r 5 × 5 m m ~) t h e d i s t r i b u t i o n is r a n d o m , as t h e d i s p e r s i o n c o e f f i c i e n t CF is n o t s i g n i f i c a n t l y d i f f e r e n t f r o m 1. A t h i g h e r d e n s i t i e s t h e d i s t r i b u t i o n o f Balanus amphitrite b e c o m e s i n c r e a s i n g l y m o r e e v e n ; t h e h i g h e r t h e d e n s i t y is, t h e l o w e r t h e d i s p e r s i o n c o e f f i c i e n t .

D I S P E R S A L OF B A R N A C L E L A R V A E

95

A

/3\ •

. ; -,.

"1.0

•

.':

/

,/

\ o\

0.5

\o \ hol

o.b5 d.1

0'.5 ~

numbers

g 6

Fig. 17. Relation between mean population density and dispersion coefficient Cr for

Balanus amphitrite (0 and O) and B. crenatus (A and ~ ) . Open symbols indicate that CF is significantly different from unity.

3-2. B A L A N U S C R E N A T U S

For Balanus crenatus, contrary to B. amphitrite, significantly unevenly distributed populations are also encountered, chiefly at intermediate population densities (Table X X X V I I I ) . Dispersion coefficients (Fig. 17) show a considerable difference, and some agreement, with the data for B. amphitrite. At low densities both species have a dispersion coefficient, that is usually not significantly different from a random distribution (some exceptions will be discussed on page 99). Generally, however, the dispersion coefficient for B. crenatus shows greater variation at these low densities. At intermediate population densities of 0.5 to 2 barnacles per 5 X 5 m m 2, the distributions of B. crenatus are found to be significantly uneven, with dispersion coefficients ranging from 1.23 to 2.03. At still higher densities of more than 2 barnacles per 5 × 5 m m ~, the dispersion coefficients indicate a regularly distributed population. As in B. amphitrite the dispersion becomes increasingly more even with increasing population densities. There is some indication that this occurs more readily in Balanus crenatus than in B. amphitrite. 4. DISCUSSION

It follows from the results (Fig. 17) that settlement of both Balanus crenatus and B. amphitrite, in sparse populations (with settlement densities lower than one individual per 5 x 5 m m 2) is random, dispersion

96

P. D E

WOLF

T A B L E XXXVIII

The distribution ofBalanus crenatus on smooth PVC panels at Den Helder, June 1966 and 1967. Each time counts were made on a different panel, after varying periods of exposure, in 5 × 5 mm squares. Number of squares

Mean number of barnacles

263 251 263 266 266 252 252 266 266 262 243 266 264 236 266 250 231 250 233 266 264 238 266 266 266 266 234 266

4.04 2.78 0.64 0.14 0.38 0.07 0.04 0.05 0.19 0.40 0.91 0.51 0.84 0.72 11.69 0.68 5.54 0.82 6.22 0.07 0.03 0.59 0.01 0.04 0.12 1.27 0.12 1.76

Variance

2.79 2.26 0.83 0.15 0.40 0.07 0.04 0.04 0.22 0.36 1.40 0.73 1.20 0.82 3.91 0.93 2.38 1.14 3.07 0.09 0.03 0.72 0.01 0.04 0.13 1.17 0.15 3.57

CF

0.69~i.!~: 0.81 1.29 1.07 1.05 0.93 0.96 0.96 1.18 0.91 1.53 1.43 1.42 1.23 0.33 1.36 0.43 1.40 0.49 1.27 0.98 1.23 1.00 0.96 1.15 0.92 1.26 2.03

p

<0.001 0.03 <0.001 n.s. n.s. n.s. n.s. n.s. 0.05 n.s. < 0.001 <0.001 <0.001 0.03 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 n.s. 0.02 n.s. n.s. n.s. n.s. <0.001 <0,001

coefficients b e i n g h a r d l y ever significantly different from 1. T h e r e is a snag, however. CASSlE (1959b) has shown, t h a t unless overdispersion or u n d e r d i s p e r s i o n are h i g h e r t h a n average, this is not likely to be detected b y the C r test, unless: (1) the sub-sample (here: square of 5 × 5 m m ~) is large e n o u g h for the m e a n n u m b e r o f o r g a nisms in the sub-samples to exceed unity, or (2) a fairly large n u m b e r o f sub-samples is taken. CASSm gives a few examples from w h i c h it follows, t h a t the n u m b e r o f sub-samples should exceed 10,000 to show d e p a r t u r e from r a n d o m n e s s in low density populations. T h e present n u m b e r o f sub-samples t a k e n was a b o u t the highest t h a t could be h a n d l e d . A further difficulty is, t h a t a very great n u m b e r o f sub-samples

D I S P E R S A L OF B A R N A C L E L A R V A E

97

would mean the use of a large area. T h e n it is not impossible that the groupwise distribution of the larvae in the water influences different parts of the area in different ways. T h e n the homogeneity of the settlement over the panel which was assumed, no longer holds. It will be assumed that sparse populations of young settlers of Balanus crenatus and B. amphitrite are randomly distributed. There is clearly a large difference between the results for the 2 species at intermediate as well as at large densities (Fig. 17). At intermediate densities (1 to 1½ individuals per 5 × 5 m m ~) Balanus crenatus is repeatedly significantly uneven distributed which means that the animals occur in groups. B. amphitrite shows at these densities on the whole a significantly even distribution which means that the animals space themselves out over the substrate. No single explanation of the difference can be offered at present; 3 hypotheses can be put forward. (1) There could be a difference between the species in the intensity of searching behaviour on the substrate (difference in speed, difference in searching time, or both). IfB. crenatus cyprids search more efficiently for earlier settlers their chance to meet earlier settlers is greater, and they would settle in a higher proportion in their proximity than the cyprids of B. amphitrite. At higher densities, after a prolonged period of settlement, the open spaces between the groups would be filled up gradually, in a way as described for Elminius modestus (KNIGHT JONES STEPHENSON, 1950). (2) The species could arrive on the substrate in different numbers per unit time. If m a n y searching individuals are simultaneously present on the substrate, the chance 2 of them meet each other is greatly enhanced which means that more groups are formed. This hypothesis has been tested by simulation (page 114) which confirmed the possibility. As a further argument in favour of the hypothesis it was shown that Balanus crenatus populations in the W a d d e n Sea settle largely in a one hour period during each tide (page 63). For B. amphitrite it can be argued that in the nearly tideless Mediterranean this time restricting mechanism is absent which makes it possible that the arrival of the cyprids is more spread out in time. Also, if the cyprids are not transported by a tidal current, they will have only limited chances to meet a solid substrate. Hence, the free-living population of the cyprid larvae will be relatively old, and, following THORSON (1966: 275), can be supposed to be not very fastidious in their place for settlement. (3) There could be a difference in natural mortality of young settlers of the 2 species which influences the pattern that is found at a certain moment. In other experiments to be described it was found (page 106), that natural mortality in young settlers could be high, especially in the case of Balanus amphitrite in Haifa. MEADOWS (1969)

98

P. D E W O L F

gave arguments for competition in Balanus balanoides as well as B. crenatus, in populations of animals of a few months old; BEVERTON & HOLT (1957: 47) listed a number of cases of competition for food as a source of mortality in fish-larvae. At the low level of food supply in the clear water of the harbour of Haifa competition for food amongst young settlers does not seem to be impossible (for the ways in which mortality can change a pattern of settled barnacles see page 104). There are thus 2 reasons why Balanus amphitrite is not distributed groupwise. In the first place they do not search very actively, secondly they probably do not arrive on tidal current, but over a longer period of time on wind driven current. Equal densities of the two species present on the substrate are thus not strictly comparable. Alternatively, from the observation that at intermediate densities Balanus amphitrite is more often evenly distributed than random could be concluded that the actual spacing out from other individuals is a different process, that furthermore occurs in old larvae. Three different mechanisms have been presented as an explanation for the differences in settlement pattern of the 2 species. It is not unlikely that all three operate at the same time. More work shall be necessary to evaluate their relative importance; this can partly be done in the laboratory with cultured larvae. In dense populations of the species (density over 1.5 barnacle per 5 x 5 mm ~) the distribution of the individuals over the substrate becomes increasingly even at increasing densities. Also for dense populations there appears to be a difference between the 2 species; the increase of evenness of the pattern, in Balanus crenatus, with increasing density, proceeds much faster than in B. amphitrite. The same 3 explanations can be considered operative again, they probably operate simultaneously. A difference in efficiency of searching of the crawling larvae of the species, could explain the observed differences in pattern. Again, the argument for the discrimination of places to settle relative to other individuals of the same species in its relation to larval age, can be applied. If larvae of Balanus amphitrite are generally older when arriving on the substrate, they will be less discriminative as regards the place to settle than larvae of Balanus crenatus*. Lastly, natural mortality differences between the 2 species can explain differences in settlement pattern at high densities. It can be * I t is p e r h a p s r e l e v a n t t h a t in l a b o r a t o r y experiments with isolated individuals, r e p r o d u c t i o n could be o b t a i n e d in Balanus amphitrite. This indicates t h a t the species is n o t a n obligatory crossfertilizer with the absolute necessity for its larvae to settle close to e a c h other. Possibly this also explains the success of the species in establishing itself in isolated locations w h e r e w a r m w a t e r is present (power station effluent).

DISPERSAL

OF BARNACLE

LARVAE

99

argued that natural mortality would have such an effect that a regular pattern would change towards a random pattern. O n the other hand, new settlers, which cannot be distinguished from earlier arrivals in the present experiments, would destroy this effect. However, if natural mortality is density-dependent, it would have an influence on the small differences present in the population which as a whole is regularly distributed, making the population more regular. A high natural mortality would in this respect have a stronger effect in a population of a higher density. Settlement, in the mean-time would be random and cannot have an effect upon the pattern evolving. Thus, a high density-dependent natural mortality, in Balanus amphitrite, and a lower mortality, density-dependent or not, in B. crenatus could explain the differences between the patterns of the species at high densities. U p till now it has been assumed that arrival of cyprids on a substrate is random in space; it appears necessary to look into this, since the cyprids do occur in groups in the water (page 41). The information on groupwise occurrence of the larvae in water is obtained in 200 1 samples; this means with e.g. 200 larvae per sample a mean distance between individuals of 10 cm. Information on groupwise occurrence on a smaller scale can only be obtained by taking smaller samples. Inherent to this is that much more samples should be taken to show departure from randomness (CAssIE, 1959b) which will be nearly impossible. Though the mechanism proposed for the groupwise occurrence of the larvae in water (page 41) acts on all scales it would be impossible to demonstrate it on this scale, especially at low settlement densities. At high densities it could perhaps be proved, but for the present problem this is not relevant. As settlement does occur mainly in a one hour period on the early flood, the current has already an appreciable velocity. The substrate is thus supplied by larvae, present in a long cylindrical water mass, and it has been argued (page 41) that it is difficult to see how larvae could make or maintain a group under these conditions. One further remark should be made. A possibility could occur that very uneven populations are formed, when the settlement rate of larvae on the substrate is extremely low (or mortality after arrival high), and growth in size rapid. Then a fast growing individual can soon take so much space of the sub-sample area, where no other individuals can settle, that the essential conditions for application of the Poisson distribution model are violated. In the present study growth, however, has not been studied, as natural mortality left no individuals to be measured after a few days; only populations of a few days old, that had grown hardly, were used.

100

P. DE

WOLF

VIII. N A T U R A L M O R T A L I T Y IN Y O U N G SETTLED BARNACLES I. INTRODUCTION

In the foregoing chapter it has been argued that natural mortality can have an influence on the pattern of settlement of young barnacles. It was thought that density-dependent mortality could possible change the pattern from uneven to even. In this chapter some experiments on the natural mortality of young barnacles will be described; the experiments were originally designed to answer 2 questions: (a) what is the magnitude of natural mortality in recently settled barnacles? (b) is natural mortality dependent or independent of population density at the settlement densities found ? Natural mortality in barnacles has been studied before by CONNELL in 2 intertidal species: Balanus balanoides (1961a) and Chthamalus stellatus (1961b) both in Millport, Scotland, and on a number of species in Friday Harbour, Washington, U S A (1969); and further by MEADOWS (1969) under conditions of continuous submergence for the species Balanus balanoides, B. crenatus and Elminius modestus, at 3 locations in Scotland. CONNELL (1961a) found for Balanus balanoides that available space and mortality during or just after settlement accounted for the population density obtained. In the intertidal area most mortality of young, metamorphosed barnacles was a result of damage by wavecarried material. The settlers, present at a certain moment could be distinguished into 2 groups: small early settlers, that did not survive very well, and bigger, late, settlers, that suffered an even heavier mortality. Growth of settlers of moderate settlement densities soon caused crowding, resulting in barnacles becoming undercut and displaced or crushed by growing neighbours. At low shore levels the growth rate was greater, as were crowding and mortality. In following years growth was slower, and older barnacles were affected by crowding only when young ones settled on the older ones and smothered them. Intraspecific competition for space has a great effect on survival during the first year of life. The mortality rates can be derived from the survival curves in CONNEL'S work; initially mortality is high, and ascribed to metamorphosis as such. As crawling and clinging cyprids were included in the census, however, there is a possibility that part of the mortality assumed has to be attributed to departure of cyprid larvae from the substrate. After a few days the mortality rate becomes rather constant for each weekly cohort of settlers. Different weekly cohorts, however

DISPERSAL

OF BARNACLE

LARVAE

10l

have different, though rather constant, mortality rates, that vary from a few per cent to about 20 per cent per week. Further it was found that (except for a few areas low on the shore) in young populations at settlement densities of over 10 individuals per cm ~ mortality varied directly with density over a period of 6 months. No consistent increase in mortality with increasing density was found in older age groups, but it should be remembered that then densities were much reduced already. In Chthamalus steUatus (CoNNELL,1961b) living in the intertidal zone at a higher level than Balanus balanoides, mortality rates were very low, and mortality as a result ofintraspecies competition for space was only rarely observed. At a lower level on the shore mortality was largely a result from competition of B. balanoides. In Balanus glandula (CONNELL, 1969) predation by Thais sp. accounted for a nearly 100% mortality of young barnacles on the lower shore in Friday Harbour, during mid and late summer. At high shore levels predation by Thais could not account for all mortality in summer, but in a u t u m n the Thais population moves in an upward direction, and it is probable that the consequently increased predation eliminates the barnacles here within the next year. CONNELL'S method in both species consisted in accurate daily mapping of animals present on stones; these were brought to the laboratory and replaced in the same position on the shore afterwards. Later (1969) photographs were used. The first method can be used in an intertidal species, that is adapted to periodic dry periods, and probably the method did not have too great an influence on the animals. MEADOWS' (1969) method consisted of exposing a number of panels on each of 3 different places in Scotland (Greenock, Mallaig and Rosyth). After one month at each place one panel was removed, after the second month the second panel, etc. The panels were exposed at a depth of 2 to 4 feet below mean low water spring; Balanus balanoides and B. crenatus settled at Greenock and Mallaig; Elminius modestus at Rosyth. Barnacles present were measured and counted, and allotted a certain age on basis of size. Though it is principally assumed that equal panels would collect equal numbers of barnacles, and (shown by analysis of variance) the n u m b e r of barnacles on a series of 5 panels exposed for a month at the same place is not significantly different, the variation between panels is large. Numbers of Balanus balanoides cyprids varied from 0 to 36, and spat from 20 to 103, while Balanus crenatus cyprids numbers ranged from 56 to 180, and spat from 88 to 232. Although the accuracy of the work may be high, the representativity probably leaves much to be desired, as can also be derived

102

P. DE WOLF

from the negative growth rates that were sometimes observed. The mortality rates during the second month, that follow from MEADOWS'S paper, vary considerably, they amounted forBalanus balanoides at Greenock 90%, and at Mallaig 83%; for Balanus crenatus at Greenock 78%, and at Mallaig 32%, for Elminius modestus at Rosyth 51% per month. As a further criticism against these figures should be mentioned the relatively long exposures used to designate a cohort. It is likely a possible time-difference in age of up to one month could also introduce bias. It thus seems that the 2 questions posed at the beginning of this chapter can be answered on the basis of literature data. The natural mortality of young settled barnacles can have a considerable magnitude, and can be highly variable. Further there are some indications that natural mortality is density-dependent at population densities of over 10 individuals per cm 2. As the purpose of the present paper was to study mortality in relation to pattern of settlement of young barnacles, that had not yet grown to the extent that direct contact between individuals did occur, a few experiments have been carried ont. No instances of predation on young settlers (up to 2 weeks old) have been encountered. 2. METHODS

Natural mortality has been studied in the species Balanus crenatus, in the entrance to the harbour of Den Helder, and B. amphitrite in the harbour of Haifa, Israel. Young barnacle populations were collected on a uniform smooth substrate; either rigid PVC panels as used in earlier experiments, or tin panels, covered with a nontoxic anticorrosive paint. Five series of experiments were done: (1) Two PVC panels were exposed in Haifa harbour from 15 to 22 September 1966 from a raft at a depth of 1.50 m. (2) A PVC panel was exposed from a raft, at a depth of 1.50 m, in the harbour of Den Helder, from 14June 1967 to 26 J u n e 1967. (3) Two such panels were exposed in the same way from 6 May 1968 to 20 May 1968. (4) Five painted panels were exposed in the harbour of Haifa from 8 to 24 May 1968, together with a series of similar panels coated with antifouling paints that had been aged before. Both series were, after cleaning off all organisms, exposed again in the harbonr of Den Helder from 10 to 27 J u n e 1968. (5) A further series of aged antifouling paints had been exposed in Haifa harbour from 16 April to 5 May 1967. In this chapter only the mortality on non-toxic substrates will be

DISPERSAL

OF BARNACLE

LARVAE

103

reported; the mortality on toxic substrates will be treated in Chapter X. To be able to distinguish age-cohorts among the barnacles present at a certain moment, exposed panels were photographed daily in the field, and re-exposed immediately. Only the first series of experiments was analyzed by direct visual observations. This practice was not continued because the panels had to be removed from the water for a long time. The size of the area studied on each panel varied. Photographs allowed for the recognition of individual metamorphosed barnacles. Cyprids, when present, were neglected in view of the fact that they might possibly leave the substrate. Living and dead settlers were recorded; it was found that many young, recently metamorphosed barnacles disappear without leaving a trace. For each panel and for each day the history of the individual barnacles was noted. Sometimes experiments could last only a few days before crowding occurred, or before other organisms (Enteromorpha linza and Hydroides norvegica in Haifa harbour) covered the barnacles. Such panels, on which individual barnacles could not be recognized with certainty, have not been analyzed. Mortalities will be given as mortality rates, following BEVERTON & HOLT (1957) and MEADOWS (1969: 87), and presented in the form of survival curves, in which the slope of the curve represents the mortality rate. As a day cohort consisted of the barnacles that settled during that day on the panel the size of the cohorts varied largely. Occasionally cohorts of a single individual occurred. It will be clear that in that case the precision of the mortality rate is very low, 0% or 100% mortality for a certain day. Cohorts consisting of small numbers of barnacles will therefore be neglected. Even then a comparison of mortality rates on a statistical basis meets with difficulties, as REGIER & ROBSON (1967) have pointed out. In fisheries literature, mortality rates are derived deterministically from the assumption that at any instant the rate of change in population number is directly proportional to that number. Thus:

dN

--

MN

dt

in which dVdenotes the number of organisms, t time, and M mortality rate. Since this model is deterministic it would be contradictory to seek to estimate the variance of M, when the latter is estimated from complete counts. It is reasonable, however, to estimate a variance between replicate experiments. In this paper mortality rates will be compared from the graphs. In addition to the considerations given in the introduction to this

104

P. DE WOLF

chapter some remarks that have been made by PIELOU (1969) are relevant. She discussed parameters for measuring the degree of aggregation of populations, mainly in relation to population density, and argued that any measurement of aggregation should be independent of that density. To visualize this an example is given of an aggregated population from which a proportion of the individuals is selected at random, killed and removed. The pattern of the survivors can then be thought of as having the same or a lesser degree of aggregation than the original population. O n the ground that the greatest number of deaths will occur in what were originally the most densely populated units, and thus make them less dense than before, it can be argued that the deaths have reduced aggregation. On the other hand, because the survivors are still at their original locations and the removal takes place at random, it can be argued that a measure of aggregation should not be affected by random deaths. R a n d o m deaths could be said to affect only the mean density of the population, leaving other aspects of its pattern unchanged (PI•LOU, 1969: 92). The choice of a proper parameter for aggregation is thus important. Such a parameter will enable a study of whether natural mortality, at a given population density, is density-dependent or not. PI~LOU (1969) indicated that LLOYD'S (1967) parameters mean crowding and patchiness meet the requirements. Frequency distributions of barnacle patterns were generally found to fit well negative binomial frequency distributions; as has been reported earlier by LLOYD (1967). Assuming that the counts are a random sample from a negative binomial distribution, "mean crowding" and "patchiness" as defined by LLOYD (1967) can be used to determine the influence of density upon mortality. Mean crowding is defined as the mean number per individual of other individuals in the same unit, thus crowding is a phenomenon that is experienced by each individual and depends on the total n u m b e r present (PW.LOU, 1969). Patchiness is the proportion of mean crowding over mean density, and is a property of a spatial pattern itself without regard to density. If it is supposed that mortality is density-dependent, exerting greater effect on the patches of local high density, then mean crowding will decrease proportionally more than the mean number and patchiness will decrease. In terms of mean (X) and variance (a 2) mean crowding is: m : X -I-

--i

To this a correction for sampling bias must be added 1950), giving:

(ANsCOMBE,

DISPERSALOF BARNACLELARVAE m=X+

--1

105

l-t-

The standard deviation for mean crowding is:

* V -1n (_~_)(~-)(* ÷ - ---S=) ~

SD (m) ~ and patchiness is:

m

X --1+

1

K --l+c

with standard deviation:

SD

s2 -2-

V

2m nX

It is realized that the efficiency of this m o m e n t method, as applied here for the calculation of K, is low (ANscOMBE, 1950). Correctly it is calculated by m a x i m u m likelihood method, although SHENTON & WALLINGTON (1962) have shown that even then the estimator of K will have some bias if the mean is small and K large. It will be seen that the results obtained by the m o m e n t method did not seem to warrant application of m a x i m u m likelihood methods, in view of the amount of labour involved. 3. RESULTS

Results for the first series of experiments, consisting of 2 PVC panels exposed in the harbour of Haifa, on Balanus amphitrite (Fig. 18a and b), show that mortality rates for each cohort, are generally low during the first few days; later mortality rates become high, leading to disappearance of the cohort in approximately 5 days. During the experiment settlement of other organisms (mainly Hydroides norvegica) was dense, and barnacles were smothered under the growing serpulids. The analysis for the B. amphitrite mortality in the fourth series of experiments in Haifa harbour (Fig. 18c) was done from photographs; the barnacles could not be recognized with certainty after 3 to 4 days due to a heavy growth of Enteromorpha. Although the panels were

106

P. DE WOLF 100 9

4 25

49 44 42

~I 65

6

loe:5c~

~

5O

O4

~5 16 o

,

~ ,

1'7 1'8 19 20

21 22 1966

5O

50

0~5 16

September

5C

,

~

May 1968 panelI

17 18 19 September 1966

50

50

,- Ot Mfly lg6B M~y 1968 1~3nel3 por,,ol4 ,

Moy 1968 pa,~el2

,

,

Moy 1968 ponel 5

c.

Fig. 18. Survival curves for daily settling cohorts of Balanus amphitrite in the harbour of Haifa, Israel, on P V C panels (a and b), area studied: 65 cm 2, in September 1966; and on panels painted with an anticorrosive paint (c) in M a y 1968, area studied 100 cm 2. T h e size of each cohort in n u m b e r of individuals is given at the 100% point of each survival curve.

exposed as close as possible survival in the larger cohorts varies greatly, from 31% to 80% after 2 days. The same panels, exposed in Den Helder, with one extra panel added to the series (Fig. 19a to f) show low mortality rates for the larger cohorts already during the first few days, that approach zero after a few days. Survival, after a period of a few days after settling, in the larger cohorts varies between 60 % and 100% daily; most of the curves maintain a survival of over 80%. The same kind of results were obtained in the 3rd series of experiments (Fig. 19g and h); here mortality is high immediately after settlement, but becomes low after a few days. In the larger cohorts (panel 1) survival rapidly reaches 70 to 80%. Quite different survival curves for Balanus crenatus in the entrance to the harbour of Den Helder resulted from the exposition of one PVC panel in J u n e 1967 (Fig. 19i) (second series of experiments). Mortality rates for all cohorts--all consisting of a large number of individuals

DISPERSAL 1

1

,4

~0

63

72

25

24

OF B A R N A C L E 7

4

0

107

LARVAE 2

19

22

71

44

47 17

10

3

2

1

5O-

4 b.

o

June 1968

10 11

"., . . . . . . . . . . . . .

12

13

14

15

15

17

18

19

20

21

22

23

24

15

25

37

4t

34

55

16

9

4

3

0

c,

,oo

1

\ \ -

25

26

'

1

27

6

June 19459 1

M a y 1968

. . . . . . . . . . . . .

7

Pa

9

10

11

12

13

14

15

16

17

h 305 480 619 454 155 100 808 1050 48~ 198 262

18

19 M e y 1988

6

1'%1'

~'~ ~b 1'4 ¢5 1'6-#

¢s 1~ 2'o 2'~ 2'z zg 2'4 ~5

,

50

J u n e 195~*

13

16

32

3 0 3"5 31

7

1

~

O

3

0 .

, 151 16

, 17

, 18

, 19

, 20

. 21

. . . . 22 23 2 4

. 25

25

Jul~e1~57

5

•

June ~958

Fig, 19. Survival curves for daily settling cohorts of Balanus crenatus in the entrance to the harbour of D e n Helder. a to f. Survival on 6 different panels painted with an anticorrosive paint, area studied on each panel 4 cm ~, J u n e 1968. g and h. Survival on P V C panels, area studied on each panel 90 cm ~, M a y 1968. i. Survival on a P V C panel, area studied 50 c m 2, J u n e 1967. T h e size of each cohort in number of individuals is given at the 100% point of each survival curve.

108

P. DE WOLF

are m o r e or less c o n s t a n t d u r i n g the 12 d a y period. T h e o n l y difference with the e x p e r i m e n t s described a b o v e is t h a t the t o t a l n u m b e r of b a r n a c l e s p r e s e n t at a n y o n e m o m e n t is m u c h h i g h e r t h a n in the o t h e r e x p e r i m e n t s . Densities u p to 59 b a r n a c l e s p e r c m 2 o c c u r r e d 22 J u n e 1967. I t a p p e a r s p r o b a b l e t h a t individuals do c r o w d e a c h o t h e r at such a density. O n the o t h e r h a n d , the cohorts w h i c h settled on 15, 16, 17 a n d 18 J u n e , show the s a m e t r e n d in the m o r t a l i t y r a t e as the c o h o r t o f 22 J u n e , a l t h o u g h the p o p u l a t i o n , t h e n present, was m u c h less dense. T h e p o p u l a t i o n o n this p a n e l has b e e n used to analyse w h e t h e r m o r t a l i t y is d e n s i t y - d e p e n d e n t . I t should be realized, h o w e v e r , t h a t the conditions for d o i n g so are u n f a v o u r a b l e , as the p o p u l a t i o n after a few days b e c a m e m o r e even t h a n r a n d o m ( T a b l e X X X I X ) . T h i s TABLE

XXXIX

Mean density (~), variance, dispersion coefficient GF, mean crowding m and patchiness for n squares of 0.5 × 0.5 cm *, on a PVC panel exposed in the harbour of Den Helder, June 1967. Date

Jgn8 15 16 17 18 19 20 21 22 23 24 25 26

n

)?

Vat.

CF

m

SD m

Patch.

S D patch.

232 233 300 297 292 72 72 72 72 72 72 72

0.43 1.33 3.47 4.51 6.41 8.74 10.00 9.83 9.19 8.72 7.12 6.94

0.46 2.20 4.80 4.06 3.64 4.08 6.72 7.72 6.48 7.54 4.60 6.69

1.07 1.65 1.38 0.90 0.57 0.47 0.67 0.78 0.70 0.86 0.65 0.96

0.50 1.98 3.85 4.41 5.98 8.21 9.67 9.61 8.89 8.58 6.77 6.90

0.12 0.24 0.18 0.12 0.11 0.27 0.31 0.34 0.31 0.35 0.26 0.34

1.16 1.49 1.11 0.79 0.93 0.94 0.97 0.98 0.97 0.98 0.95 0.99

0.11 0.16 0.11 0.07 0.04 0.07 0. I 1 0.12 0.11 0.15 0.10 0.16

m e a n s t h a t the differences b e t w e e n squares are relatively small, a n d the effect to be m e a s u r e d m a y be difficult to detect. M e a n c r o w d i n g a n d patchiness h a v e b e e n c a l c u l a t e d , a n d m e a n c r o w d i n g has also b e e n p l o t t e d a g a i n s t m e a n density (Fig. 20). Patchiness is seen not to c h a n g e in t i m e ( T a b l e X X X I X ) , while neither m e a n c r o w d i n g changes relative to m e a n density (Fig. 20). I t thus a p p e a r s t h a t m o r t a l i t y is not d e p e n d e n t o n density in this p o p u l a t i o n ; at least it c a n n o t be c o n c l u d e d f r o m the m e t h o d s followed here. W h e t h e r the m e t h o d is suitable for the present case will be discussed in the n e x t section.

DISPERSAL

OF

BARNACLE

LARVAE

109

10.

8"

'

:~

'

4

'

~

'

~

'

lb

Fig.20. Relation between m e a n density ~', and mean crowding rn for a population of Balanus crenatus on a P V C panel; J u n e 1967. 4" D I S C U S S I O N

When comparing the mortalities of the 2 species of barnacles studied, it is obvious that the mortality rate of Balanus amphitrite at Haifa is higher than that of B. crenatus at Den Helder, as far as can be judged from the limited data. In September 1966 (Fig. 18a and b) mortality in B. amphitrite took a heavy toll; in all populations that were followed more than 5 days over 80% died. It is thought that this heavy mortality is due to smothering of the settling barnacles by other species, mainly Hydroides norvegica. Probably the same situation holds for the beginning of M a y 1968 (Fig. 19) when cohorts could be followed only for a few days, due to the development of a dense mat of Enteromorpha which could have the same effect as the Hydroides cover. In this case the mortality, of the larger cohorts, varied from 30 to 70% in a period of 2 days. In not too dense populations ofBalanus crenatus (Fig. 19a to f), where no crowding or smothering by other organisms occurs, mortality during the 2 or 3 days following metamorphosis accounts for nearly all mortality observed. After a few days survival curves tend to become horizontal straight lines. This is quite different in dense populations (Fig. 19i), where mortalities become much higher, attaining values of 90% in 10 days. Survival curves indicate a continuing logarithmic decrease of the numbers of barnacles in each cohort. Thus, it seems demonstrated that young settled populations are subject to a mortality in early life, possibly as effects of metamorphosis. After this period mortality becomes small, unless crowding by the same or another species takes toll. There are deviations from the general trend in this way that on the

110

P. DE WOLF

same day and the same panel different cohorts can show markedly different mortality rates. Examples are the mortality rates for the cohort of 17 June, in its third day and of the cohort of 18 J u n e in its second day (Fig. 19i). Another example is afforded by the cohorts of 19 and 20 J u n e between 20 and 21 J u n e 1967. On the other hand the mortality rates for the cohorts of 19, 22, 23 and 24 J u n e are more or less equal between 24 and 25 June. It occurs to me that besides an age-specific mortality there might exist an environment-specific mortality, as well as an interaction between these two. Comparison of the mortality rates found with those presented by CONNELL (1961) and MEADOWS (1969) shows that for populations of comparable densities mortalities come in the same range which is not amazing in view of the size of the range. As to the question whether mortality in a population of barnacles is density-dependent or not it should be remarked that the differences between survival curves in Figs 19h and 19i are a clear indication that mortality is indeed density-dependent. On the other hand, the parameters developed by LLOYD (1969) to measure mean crowding and patchiness, failed to indicate density-dependency of mortality in the case. T w o criticisms can be brought forward against the use of LLOYD'S parameters in the present case. Firstly, a dense barnacle population has an even distribution over the substrate, and density differences between units (squares) are rather small (Chapter VII). The value for mean crowding will thus be generally lower than the value for mean density, and the effect to be measured must be large, before it can be observed. Secondly, it is doubtful whether the use of mean crowding and patchiness in the present case is justified. This doubt arises principally from the same phenomenon as described above, as can best be elucidated from an example. In a population of groupwise (patchy) distributed barnacles on a substrate in which mortality is densitydependent, the patchiness of the population will decrease. But newly arriving barnacle larvae will space out over the substrate; the effect of the newcomers on the patchiness parameter will be dependent, amongst others, upon the density of the population (Fig. 19) and cannot be predicted. Thus to determine the influence of density upon mortality, and the results of this influence on the pattern on the substt ate, it would be better to isolate a patchy one-day cohort on a substrate from new arriving barnacles, and determine the patchiness as indicated earlier. Decreasing patchiness would then indicate densitydependent mortality. Isolation of the cohort could be done by keeping the substrate in the laboratory or in a cage of plankton gauze in the sea. Lastly, a remark should be made regarding the method followed here to recognize cohorts of barnacles, by means of photographs, taken

DISPERSAL

OF BARNACLE

LARVAE

111

daily of a population. The method has its drawbacks, as normal photographic material is not true to measure, and furthermore the method is extremely laborious. It seems, however, that the automated and computerized version of a method used in astronomy for the search of nova's might be fruitfully applied here (WILLIAMS, 1969). IX. A S I M U L A T I O N M O D E L F O R S E T T L E M E N T D I S P E R S I O N I. INTRODUCTION

In Chapter V I I I it was seen that the dispersion of settling barnacles on a even, smooth substrate is dependent upon the density of the population. In sparse populations the settlement cannot be distinguished from a random distribution. In denser populations the character of the dispersion is dependent upon the species: At increasing population densities Balanus amphitrite settles randomly until a certain threshold is reached, after which the dispersion becomes increasingly more even. At intermediate densities B. crenatus settles in groups. These groups merge with a further increase of the density and then an even dispersion of the settled young barnacles results. It has been postulated earlier (page 97) that this difference between the 2 species could be due to several factors. On of these, the intensity of searching for earlier settlers of the same species, was thought to be related to the age of the larvae. The intensity of searching depends on the speed of searching, and time spent in searching. The age of the larvae could be relevant in the sense mentioned earlier (page 88), older larvae being less discriminative as to their settlement place. It has been inferred that these factors would be difficult to analyse. Although an attempt to measure searching (VANZALINGEN,personal communication) has been made for B. crenatus this proved to be very laborious. Further it was found difficult or impossible to determine the age of the cyprid larvae. When reared, or caught and brought to the laboratory, cyprid larvae usually settle soon, at least when compared with the time lag they could show in the sea (page 54). For these reasons it was decided to try to simulate settlement behaviour in a stochastic computer simulation programme. The simulation model was made rather complicated, as it was meant to test a number of hypotheses. For the moment not all its possibilities have been fathomed. A description of the model in general terms will be given (section 2), together with an example of an application where it has clearly influenced experimental work in the field (section 3) : the experimental results, presented in Chapter V, were "predicted" by the simulation model.

112

P. DE WOLF 2. G E N E R A L

DESCRIPTION

OF THE

MODEL

The settlement of barnacles is simulated on a 10 × 10 cm grid upon which cyprids are though to arrive, search for a suitable place to settle, and upon finding such a place to settle, metamorphose, grow and built a shell. Failing to find a suitable site, they are thought to swim away. Furthermore, cyprids and metamorphosed barnacles are thought to be subject to mortality during this time. Each square of the grid is thought to be devided into 100 × 100 places on which, at each moment, a cyprid or a metamorphosed barnacle can be present. In the programme a number of variables have been introduced. A cyprid arrives on the grid on a randomly chosen place, indicated by 2 pseudo-random numbers, generated in the computer, and representing an X and an Y co-ordinate in the grid. After arrival the cyprid starts to search for a previously settled barnacle in order to settle nearby. Searching in the model, is done by carrying out a step to a different set of X, Y co-ordinates; the step is done in a randomly chosen direction and is of random length within certain limits. After arrival on the new place the cyprid tests its immediate environment for the presence of other barnacles. When other barnacles present are too close or too far away searching proceeds. When other barnacles are at the right distance the cyprid settles, and metamorphoses into a barnacle. The model deviates from reality with respect to the edges of the grid. In reality a substrate will usually be much larger than the grid used here, and searching larvae can enter the study area when searching. In the simulation model cyprid larvae that, when searching, leave the grid across the edge are lost, while no larvae from outside the grid can enter. This results in a scarcity of cyprids (and barnacles) near the edges of the grid. This difficulty was overcome by limiting the choice of random directions for a next step for cyprids approaching the edge of the grid; after reaching the edge they were allowed to search into the grid only. Now the question arises if cyprids search for an infinite period of time to find another searching individual. Then after settlement of the first individual in the grid all later arriving larvae will settle close to this individual, and one cluster of young settlers will gradually spread out over the grid. But this is not conform reality (Chapter VII), and to meet this difficulty time limits were introduced. A cyprid on arrival on the grid, was within limits allotted a randomly chosen age within a limited life period. Searching time was made to vary with the age of the cyprids; young larvae searching for a short period only, while for larvae at the end of larval life, searching time can approach the

DISPERSAL

OF BARNACLE

LARVAE

113

m a x i m u m time of 1800 seconds. An old larva will thus have a greater chance to find an earlier settled barnacle than a young one. Furthermore, the old larvae have a chance that their larval life period runs out during their searching period; in that case they either die, or settle irrespective of their position relative to other barnacles. In this way some cyprids settle randomly over the grid, and by this way a cyprid can become the first settler on the grid. Further variables in the searching model are the speed of searching, usually taken to be 0.03 mm/sec, and the distance of settling kept between individuals, usually taken to be 2 m m (cf. CRISP, 1961). Both variables are of relatively little importance. If the speed of searching is increased this is equivalent to an increase of the duration of searching. If the distance of settling is made larger the grid will ultimately contain less barnacles, an effect equivalent to the use of a grid of a smaller size. A few other variables were built into the simulation programme, the number of arrivals per 24 hours, the final population density, growth, and mortality. The number of cyprid larvae arriving per day will be treated in detail in the next section. The time periods between successive arrivals are varied randomly around the mean value. The final density of the simulated population could be varied. Usually simulations were carried out to the final density of 600 barnacles in the grid, a density largely determined by the available memory of the computer. However, even if the memory had been larger, there would have been difficulties in reaching much higher densities in the simulations, as with increasing densities it became increasingly more difficult for forther barnacles to find a place to settle; this was reflected in the increase of computer time used. The growth and mortality of the settled barnacles could be varied also in the model. As until now no simulations over periods longer than a few days have been done, these variables so far have not been taken into account, although, as indicated earlier (Chapter V I I I ) , they could be important factors determining the nature of the dispersion of the population. The output of the results of the simulation experiments was realized by means of an electric typewriter which so far is not ideal as the spacing of the letters and lines of the typewriter can introduce deviations from the computer results for the simulated population. 3" T W O

EXAMPLES

OF SIMULATION

EXPERIMENTS

As has been stated before the work with the simulation model is in-

114 complete; only 2 experiments: spacing in time the simulation presented.

P. DE WOLF a few preliminary results can be given. They concern one on the searching procedure, and the other on the of the arrivals of the larvae. A few general remarks on programme will be made accompanying the results

The results concerning the searching procedure in the simulation programme can be illustrated in 2 cases made with 200 cyprids arriving per 24 hours, and a distance between settled barnacles of 2 mm. In the first case the simulation is carried on up to a density of 300 barnacles (Fig. 21a), in the second case (Fig. 21b) up to 600 barnacles present on the grid. It follows that initially isolated patches of barnacles are formed while at higher densities the open spaces between these patches gradually become occupied. It can be observed that the distances between barnacles are generally much more uniform than can be expected for a random distribution of points on a grid (Fig. 21c). However, when comparing the results of the simulation with experimental observations, the impression is obtained that barnacles do search better. Especially on the right hand side of Fig. 21a it seems that barnacles often settle in rows, a phenomenon never observed in reality (on a smooth substrate). It suggests that the simulated searching is less complicated than the behaviour of the larvae in nature. If in reality a larva settles at equal distances of 2 barnacles previously settled at a similar distance from each other, such rows could not be obtained. In increasingly dense populations (Fig. 21b) the rows are gradually obscured by newly arriving cyprids. In a few instances it can be observed that cyprids settled at distances smaller than 2 mm from previously settled barnacles (upper left hand corner of Fig. 21a and b). This probably illustrates the supposition of settlement of larvae, whose larval life period is running out. The second series of simulation experiments concerns the spacing in time of the arrival of cyprids and its influence on the pattern of settlement. In this series of experiments, 600 larvae arrive over a period of 24 hours, with a random spacing, neglecting growth, and keeping other variables at a fixed level. In reality (Fig. 17) the dispersion coefficients CF for populations with a mean density of 6 barnacles per cm ~ range from < 1 to 1.4, while in the simulation experiments this dispersion coefficient was nearly always higher than 1.8. Furthermore, the dispersion coefficients of equally dense populations are generally lower when high numbers of arrivals per day are simulated as compared to low numbers per day (Fig. 21b and d). Considering the last result firstly, it is evident that an arriving cyprid

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Fig.21. Computer simulated settlement dispersion of barnacles on a smooth subs t r a t e , a. D e n s i t y o f t h e p o p u l a t i o n 3 0 0 i n d / 1 0 × 10 c m ~ ; n u m b e r o f c y p r i d l a r v a e

116

P. DE WOLF

can settle in 2 different ways only. It can do so in reaction to one or more other cyprids, thus adjoining a patch, or it can settle on its own for lack of time. A less patchy dispersion has relatively small and many patches of barnacles which suggests that initially many first settlers settled. A large number of new arrivals suggests on the one hand that this population would contain a larger number of larvae which settled because their larval fife span was over, but as it is statistically unlikely that this number would not be relatively high, this phenomenon cannot explain that the populations becomes relatively even. On the other hand, when the number of new arrivals is large a great number of searching larvae will be present at the same moment, and thus the chance that 2 searching larvae meet will be greater than when the number of arrivals is low. To explain how dispersion coefficients for real populations could be consistently lower than for equally dense populations in a simulated experiment, an arrival of the larvae restricted to a certain stage of the tide, was considered. Consequently a simulation experiment similar to the one described above, was carried out with a slight variation. With all other variables kept constant, the same number of arrivals was spaced over a period of 12 hours (Fig. 21e), and again over a period of 1 hour (Fig. 21f), showing that in the second case the population is much more even than in the first case. Subsequent field experiments (Chapter V) showed that in accordance with the hypothesis in nature settlement of cyprids is limited to a certain period of the tide, at least to a large extent. X. A P P L I C A T I O N

OF RESULTS

INANTIFOULING

RESEARCH

I, I N T R O D U C T I O N

In Chapter I it was stated that the present research originated from the fouling problem. Some results obtained are of importance for antifouling research, and in the following sections a number of applications arriving 200/24 h ; distance between barnacles 2 mm; dispersion coefficient CF -2.61. b. Density of the population 600 ind/10 × 10 cm~; number of cyprid larvae arriving 200]24 h ; distance between barnacles 2 ram; CF = 1.79. e. Random dispersion of 600 ind/10 x 10 cm 2 (it should be born in mind that the spacing of the typewriter symbols may possibly have caused a deviation from the true position of the simulated barnacles in the grid) ; C~ = 1.0. d. Density of the population 600 ind/10 × 10 cm2; number of cyprid larvae arriving 450/24 h ; distance between barnacles 2 ram; CF = 1.46. e. Density of the population 600 ind/10 × 10 cm2; number of cyprid larvae arriving 3600/12 h ; distanee between barnacles 2 ram; CF = 1.15. f. Density of the population 600 ind]10 × 10 cm*; number of cyprid larvae arriving 3600/h; distance between barnacles 2 ram; CF = 0.93.

DISPERSAL

OF BARNACLE

117

LARVAE

of these results, useful for the testing of antifouling paints, will be given. T h e y concern the raft testing ofantifouling paints, the patchiness of barnacles on aged antifouling paints, and a proposal for a new test method. 2. ON T H E N U M B E R OF R E P L I C A T E S I N T E S T I N G A N T I F O U L I N G P A I N T ON A R A F T

AN

Antifouling paints are intended to prevent the settlement of organisms on man-made structures. The mode of action of such paints is the release of poison from a supply in the paint layer into the adjacent sea-water (ANONYMOUS, 1952), or directly into the organisms (DE WOLF, 1964; K~HL, 1968). The release of poison from the paint causes the supply to diminish; this process is called the aging of the paint, and it puts a limit on the efficiency of the paint in time. To test the efficiency of newly developed paints, these are usually aged first, either by merely submergence from a raft in the sea, or by chemical or physical aging. Apart from aging, the efficiency of paints is usually tested by exposing them to settling organisms in the sea, in duplicate with non-toxic controls, and standard paints of well-known performance. It is customary then to judge the quality of the paints on the basis of the number of organisms (barnacles) settling successfully. This practice has been found to lead to extreme variation in the apparent performance of a single paint (DE WOLF & VANLONDEN, 1966). It follows from Chapter VI that the numbers of barnacles settling on a series of identical, non-toxic substrates can vary to a large extent, and it is thought that this variation may, at least in part, be responsible for the variations observed in the testing of paints. As a practical approach to remove this variation it was decided to estimate the number of replicates necessary to obtain a mean value that is representative for settlement on the whole raft. Mean values for such series of replicates would then be comparable. It should be mentioned here, anticipating the next 2 sections, that nearly all barnacles attempting to settle upon an effective antifouling paint are killed during or after metamorphosis; this is called post-attachment mortality (CRISP & AUSTIN, 1960; DE WOLF, 1964). This means that newly arriving barnacle larvae may find low numbers of recently metamorphosed barnacles to orientate upon for settlement. In the worst possible situation new arrivals on successive days meet a substrate which lacks barnacles. For this reason the determination of the number of replicates was done on a series of new non-toxic PVC panels, and a 1 day cohort was used. The cohort selected for this purpose from the experiments described in Chapter VI (page 75) had the most uneven distribution that was

118

P. Dig W O L F

observed in the series of experiments (which not necessarily means that more uneven distributions could not occur). Settlement data of 1 J u l y 1966 (Table X X X I ) have been used; all 144 panel faces were numbered, and, by means of a table of random numbers, were consecutively divided into 2 samples of 72 faces, 3 samples of 48 faces, 4 samples of 36 faces, etc. Means and standard deviations for the numbers of barnacles on the faces of the panels have been calculated for all samples (Fig. 22). For each sample size, it has been tested to what

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Fig.22. 144 panels randomly divided into equally sized groups; group-size (number of panels) subsequently decreased and number within groups inversely increased, a. Mean number of barnacles per panel in relation to number of panels in group, b. Standard deviation of mean numbers of barnacles per panel in relation to number of panels in group; horizontal line indicates population standard deviation.

extent the outermost means and standard deviations are significantly different from each other by t-test (means) (at p ---- 0.05 two-sided

DISPERSAL

OF

BARNACLE

LARVAE

119

test) or ;(2 test (variances) (atp = 0.05 a n d p = 0.95). It followed that outermost means start to be significantly different at samples of 13 replicates, and that variances are significantly different from population variance (all panels) at sample sizes of 12 replicates. The procedure as executed is statistically not very sound; because the number of panels is finite, making that samples can not be drawn completely at random; the last sample in a lot consists of those panel faces that have not been drawn earlier. To remain safe it is supposed that to have a representative sample of panel faces, at least 15 faces should be exposed, and counted. This is much more than the usual duplicate, and in view of the amount of work involved and the capital investment in a raft this number is considered to be prohibitive. Further, in the study of new materials often not sufficient material is available. Therefore, a test method for evaluating antifouling paints independent of the numbers of barnacles settling is highly desirable. 3" P A T C H I N E S S

OF BARNACLES

ON AGED

ANTIFOULING

PAINTS

It has been shown in Chapter V I I that the dispersion of barnacles on a smooth, non-toxic, substrate is usually not significantly different from a random distribution at low population densities. At high population densities the dispersion is usually significantly more even than a random distribution, and the evenness increases with increasing density. At intermediate densities different species behave differently with regard to this dispersion, and it is thought that these differences between species might be due to differences in the intensity of searching of the cyprids for a spot to settle. In earlier papers (DE WOLF, 1966, 1968) on the dispersion of barnacles (Balanus crenatus and B. amphitrite) on aged antifouling paints it was found that barnacles are often unevenly distributed at population densities, that are evenly dispersed on non-toxic substrates. It was further observed that barnacle spat on a toxic substrate initially settle in a manner that is not different from a pattern on a non-toxic substrate; subsequently, however, the barnacles on some parts of the substrate die, leaving behind a patchy dispersion of the survivors. It was concluded that the toxic substance had an uneven dispersion in the paint, or the flux of toxic material from the paint to the water was irregular; in both cases the unevenness of the dispersion of the toxic material would be present at a scale larger than the mean distance between barnacles. It has also been tried (DE WOLF, 1968) to explain the uneven dispersion in terms of variation in sensitivity of individual barnacles for the poison. As it has to be assumed, however, that the

120

P. D E W O L F

sensitivity for poisons is randomly distributed in the population, a patchy dispersion then could result only from the settling of nonsensitive larvae in close proximity to one another, which seems unlikely. Such a behaviour would moreover necessitate the assumption of a mechanism sorting the larvae according to their sensitivity for poisons. 4" A N E W T E S T M E T H O D

As indicated the customary way of testing antifouling paints meets difficulties, because of the groupwise distributions of barnacles attempting to settle. The use of a sufficiently large number of replicates would increase the amount of work considerably. Two further objections should be mentioned; the first is of practical nature, and consists of difficulties in the preparation of test panels. The application of paint by brush or other methods is clearly not well reproducible; thickness variations in the paint layer on a panel can be large, which limits the sense of using replicate panels. The second objection is of a more principal nature; counting of fouling organisms that successfully settled on an antifouling paint is a contradiction in itself. True, usually the successful life period of the paint is determined by extrapolating back from a number of counts, but it has been shown that the rate of increase in numbers of barnacles on aged paints can vary for different paints (DE WOLF, 1968). For these reasons a different method is thought to be useful; namely the determination of the survival curve of a not too small one day cohort of settlers. To illustrate this a number of survival curves of barnacles on aged antifouling paints have been given (Fig. 23). A number of antifouling paints have been applied to test panels, aged on a rotor apparatus (VAN LONDEN, 1964) for periods equivalent to sailing 4100, 7300 and 10,200 miles, and exposed to settling barnacles in Haifa (Balanus amphitrite) and Den Helder (B. crenatus). Survival curves have been determined as described in Chapter V I I I (page 105). Comparison survival curves for Balanus crenatus on a thin layer of a poor quality antifouling paint aged for 10,200 miles (Fig. 23a) with data from non-toxic substrate (Fig. 19a-h) shows that these survival curves are not different. However, survival curves for day cohorts on 3 different panels, aged for respectively 4200, 7300 and 10,200 miles for a better quality antifouling paint (Fig. 23b, c and d) are clearly different from those obtained on a non-toxic control (Fig. 19a-h). Mortality rates are generally high, and on the panels aged for 4200 and 7300 miles hardly any barnacles survive; it seems likely that those surviving are located on less toxic sites.

DISPERSAL

o

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121

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1 c

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Fig.23. Survival curves for daily cohorts of barnacles on aged antifouling paints. Numbers of barnacles in each cohort have been given at the 100% point of each survival curve, a. Balanus crenat~ on a poor quality antifouling paint, aged for 10200 miles, Den Helder, J u n e 1968. b. B. erenatus on a n antffouling paint, aged !for 4200 miles, Den Helder, J u n e 1968. c. As b, b u t aged for 7300 miles, d. As b, but aged for 10200 miles, e. B. amphitrite on the same panel as b, in Haifa. f. B. amphitrite on the same panel as c, in Haifa. g. B. amphitrite on the same panel as d, in Haifa.

It is difficult to compare the results, obtained for Balanus amphitrite on the same panels in Haifa (Fig. 23e, f and g) with the survival rates for these species on a non-toxic control (Fig. 18c), as the latter results could be collected a few days only due to hindrance by other settling organisms. These organisms are lacking or much lower in numbers on the antifouling paints, while the numbers of settling barnacles are nearly the same. The impression is obtained, however, that the initial mortality rate on an antifouling paint is generally much higher than on a non-toxic control. There is no difference between panels, that had been aged for different periods, as observed in Den Helder. Either

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the natural mortality of B. amphitrite interacts with toxic mortality, or young B. amphitrite are more sensitive to the poisons used in the paint. A fuller account of these observations, which are an example from a larger material, will be published elsewhere. In view of the difficulties encountered in the testing of antifouling paints it seems appropriate to mention this test method already briefly. xI. S U M M A R Y This paper is concerned with the dispersion of V l t h stage nauplius larvae, free swimming cyprid larvae, crawling cyprid larvae and recently settled barnacles of the species Balanus crenatus, B. improvisus, B. amphilrite and Elminius modestus. It is shown (Chapter III) that numbers of VIth stage nauplii and cyprids of B. crenatus and B. improvisus and cyprids of Elminius modestus per unit volume show a tidal variation in the Western Wadden Sea. In addition to this there is a short period variation; it is thought that larvae do occur in groups in the water. The observations are concomitant with the assumption that cyprids are transported by tidal currents, that they sink to the bottom during periods of low current velocity, and that they are redispersed again in the water column when current increases. Mechanisms for group formation are discussed and it is postulated that groups are being formed by currents and accompanying turbulence. It is shown that numbers of larvae show a correlation with the amount of suspended matter in the water column. For this reason it is argued that retention of larvae in estuaries can be explained by a fully mechanical process, and that there is no need to suppose that the swimming behaviour of the larvae, induced or released by a tide-coupled environmental factor, influences retention. It is further shown that the field data from a number of authors, on oyster larvae, mussel larvae, barnacle larvae and medusae are not sufficient to sustain the conclusions that these animals further their retention in the estuaries by swimming. Attempts to characterize groups o f c y p r i d larvae as to size were not successful. Laboratory experiments on the sinking, and redispersion of cyprid larvae by current have been described (Chapter IV). The majority of cyprids sink with a speed of 10 to 32 cm/min. Horizontal swimming was hardly ever observed. Redispersion into the water column occurred at current velocities of 35 to 67 cm/sec. In view of the large tidal variations in the numbers of larvae in the water column it would be reasonable to expect variations in the settlement of cyprid larvae with the tide. Experiments to this end have been described (Chapter V); the results show that settlement occurs

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throughout the tidal cycle in low numbers, but a large proportion of total settlement takes place in I or 2 hours during early flood. During later flood, numbers of larvae in the water are higher, but greater current velocities prevent settlement. It is to be expected that the groupwise distribution of the cyprids in the water induces differences in settlement from place to place. Observations on such differences have been given (Chapter VI): firstly on panels in a linear row, secondly on panels exposed in a regular 3 dimensional grid suspended from a raft. The linear series of panels showed that the numbers of settlers can be highly variable, even at distances as small as 5 m. This can be ascribed fully to the groupwise distribution of the larvae in the water. For panels on the raft differences in settlement can similarly be explained by transport of larvae by current. Publications of other authors, explaining differences in settlement in terms of light, current and swimming behaviour of larvae have been criticized, although it is shown that crawling behaviour of larvae may influence settlement density. Observations have been described on the pattern of settlement, especially the development of an even distribution on the substrate (Chapter VII). Even distributions develop only after a certain population density has been reached; the intensity of searching by the crawling larva and the spacing-out mechanism are of importance. It follows from the observations that these are different in Balanus amphitrite and B. crenatus; searching for previously settled individuals seems to be more intense in larvae of B. crenatus, although simulation experiments indicate (Chapter IX) that a large settlement during a short period may be of importance in this respect. It has been argued that natural mortality of young settled barnacles m a y have an influence on the settlement pattern; it is thought that density-dependent mortality could possibly change the pattern from uneven to even. Experiments to investigate this have not been very successful. Too heavy settlement developed unsuitable even patterns. A few factors, thought to be of importance in the development of settlement patterns, which are, however, difficult to investigate experimentally, such as intensity of searching of the crawling cyprids, age of cyprids, and numbers of settlers arriving per unit time, have been studied in a simulation model in a computer (Chapter IX). Some results of simulations influenced experimental work in the field, especially in the case of Balanus crenatus, where a high number of arrivals per unit time is necessary to develop an even distribution on the substrate. This was found in the field in accordance to earlier indications by simulation. Lastly, a number of applications of the results to the testing of anti-

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fouling paints has been given (Chapter X) ; these concern the number o f r e p l i c a t e s n e c e s s a r y for t h e t e s t i n g , t h e p a t c h i n e s s o f b a r n a c l e s o n aged antifouling paints, and a new test method based upon mortality r a t e s o f y o u n g s e t t l e r s o n a g e d a n t i f o u l i n g p a i n t s as c o m p a r e d to t h o s e on non-toxic controls. XII. REFERENCES

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CRISP, D.J. & A. P. AUSTIN,1960. The action of copper in antifouling paints.--Ann. appl. Biol. 48: 787-799. CRISP, D.J. & P. A. DAVIES, 1955. Observations in vivo on the breeding of Elminius modestus on glass slides.--J, mar. biol. Ass. U.K. 34: 357-380. CUSHING,D. H., 1955. Production and a pelagic fishery. Fishery Invest., Lond. (2) 18 (7) : 1-104. DOOCHIN, H. D., 1951. The morphology of Balanus improvisus Darwin and Balanus amphitrite niveus Darwin during initial attachment and metamorphosis.--Bull. mar. Sci. Gulf Carrib. 1 (1): 15-39. EDMONDSON,C. H. & W. M. INGRAM, 1939. Fouling organisms in Hawaii.--Occ. Pap. Bernice P. Bishop Mus. 14:251-300. FISH, C.J., 1925. Seasonal distribution of the plankton of the Woods Hole r e g i o n . Fishery Bull. Fish Wildl. Serv. U.S. 41 • 91-179. FREIBEROER,A., 1965. Rearing barnacle cyprids in the laboratory for marine fouling studies. Progr. Rep. No. 1, U.S. Naval Appl. Sci. Lab. New York (mimeD). FROLANDER, H. F., 1968. Statistical variation in zooplankton numbers from subsampling with a Sternpel pipette.--J. Wat. Pollut. Control Fed. 40: R82-R88. GRAHAM,H. W. & H. GAY, 1945. Season of attachment and growth of sedentary marine organisms at Oakland, California.--Ecology 26: 375-386. GRAVE, B. H., 1933. Rate of growth, age at sexual maturity and duration of life of certain sessible organisms at Woods Hole, Massachusetts.--Biol. Bull. mar. biol. Lab., Woods Hole 65" 375-386. GREGG, J. H., 1945. Background illumination as a factor in the attachment of barnacle cyprids.--Biol. Bull. mar. biol. Lab., Woods Hole 88: 44-49. GREIG-SMITH,P., 1964. Quantitative plant ecology. Butterworths, London. GROENEWEGEN,J.A. W., 1922. Cirripedien. In: H. REDEKE. Flora Fauna Zuiderzee, Den Helder: 311-315. GRY,H., 1942. Das Wattenmeer bei Skallingen, No. 1. Quantitative Untersuchungen fiber den Sinkstofftransport durch Gezeitenstromungen.--Foliageogr, dan. 11: 1-138. HARDER, W., 1957. Verhalten von Organismen gegenfiber Sprungschichten.-Ann6e biol. 33: 227-232. HARDY, A. C. & R. BAINBRIDGE, 1954. Experimental observations on the vertical migrations of plankton animals.--J, mar. biol. Ass. U.K. 33" 409-448. HARTOG, C. DEN, 1953. Immigration, dissemination and ecology of Elminius rnodestus Darwin in the North Sea, especially along the Dutch coast. Beaufortia 4 (33) : 9-20. HATTON, H. & E. FlSCHER-PIETTE, 1932. Observations et experiences sur le peuplemerit des c6tes rocheuses par les Cirrip~des.--Bull. Inst. ocdanogr. Monaco 592: 1-15. HERZ, L. E., 1933. The morphology of the later stages of Balanus crenatus Brugi+re, Biol. Bull. mar. biol. Lab., Woods Hole 64: 432~,42. HOUGHTON, D. R. & H. G. STUBmNGS, 1963. On the vertical distribution of Elminius modestus Darwin.--J. Anim. Ecol. 32: 193-201. IWAO, S., 1968. A new regression method for analyzing the aggregation pattern of animal populations.--Res. Popul. Ecol. 10: 1-20. IWAO, S. & E. KUNO, 1968. Use of the regression of mean cIowding on mean density tbr estimating sample size and the transformation of data for the analysis of variance.~Res. Popul. Ecol. 10:210-214. JONES, L. W. G. & D. J. CRISP, 1954. The larval stages of the barnacle Balanus improvisus Darwin.--Proc. zool. Soc. Lond. 123: 765-780. JosEPH,J, 1954. Die Sinkstofffihrung yon Gezeitenstr6men als Austauschproblem.--

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