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Orientation of Talitrus saltator to magnetic fields
Netherlands aTournalof Sea Research 15 (1): 23-32 (1981) ORIENTATION TO
OF TALITRUS SALTATOR MAGNETIC FIELDS by
M. C. A R E N D S E and C. J. K R U Y S W I J K
Laboratory of Comparative Physiology, State University of Utrecht, ffan van Galenstraat 40, 3572 LA Utrecht, The .Netherlands CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . II. Material and Methods . . . . . . . . . . . . . . . . . . . . . . 1. Further analysis of results published previously . . . . . . . . . . . 2. New experiments . . . . . . . . . . . . . . . . . . . . . . . 3. Statistical evaluation . . . . . . . . . . . . . . . . . . . . . . III. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. The number of radially directed animals . . . . . . . . . . . . . 2. The orientation of jumping and crawling animals . . . . . . . . . 3. Jumping ammals in the local geomagnetic field and in a compensated magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . IV. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Physical and social interaction . . . . . . . . . . . . . . . . . . 2. Water loss and mode of locomotion . . . . . . . . . . . . . . . . 3. Water loss and orientation . . . . . . . . . . . . . . . . . . . 4. Pre-experimental light conditions and orientation . . . . . . . . . . . V. Summary . . . . . . . . . . . . . . . . . .......... VI. References . . . . . . . . . . . . . . . . . . . . . . . . . . .
23 25 25 25 26 26 26 26 27 29 29 29 29 30 31 31
I. I N T R O D U C T I O N N o n - v i s u a l o r i e n t a t i o n in Talitrus saltator, the s a n d h o p p e r , is a controversial subject (for a discussion of this t e r m see Am~NDS~., 1980). VAN DEN BERCKEN et at. (1967) c l a i m e d to h a v e d e m o n s t r a t e d non-visual o r i e n t a t i o n in Talitrus. T h e y m a d e a statistical analysis o f their results, b u t this analvsis was criticized b y ENRIGHT (1972). ERCOLINI & SCAPINI (1972) did not find non-visual o r i e n t a t i o n in talitrids in several extensive series of tests. HARTWIeK (1976) shared their opinion, b u t did not give a n y e x p e r i m e n t a l evidence. W h i l e studying LEASK'S (1977) optical p u m p i n g m o d e l o f m a g n e t i c field detection ARENDS~. (1978) showed t h a t Talitrus saltator is able to orient to the local g e o m a g n e t i c field ( G M F ) a n d to reorient in total darkness to a n artificial m a g n e t i c field w i t h m a g n e t i c N o r t h at g e o g r a p h i c West. H o w e v e r , the f r e q u e n c y distributions o f direction choices he o b t a i n e d were b i m o d a l , b u t at t h a t time the factors u n d e r l y i n g this b i m o d a l i t y were less clear t h a n they are
24
M . C . A R E N D S E & C. ,]. K R U Y S W I J K
now (ARENDSE, 1980; ARENDSE & KRUYSWIJK,unpublished). Moreover, no orientation data were available .which had been obtained in a compensated magnetic field and would have completed the proof of magnetic orientation. This paper is intended to fill that gap. ARENDSE (1978, 1980) was of the opinion that the controw:rsial results were due largely to the considerable variance observed in most experiments. He could trace this variance back to the existence of two ditterent statistical populations of observations in his photographic recordings of orienting animals. He separated these populations on the basis of the angle (7) between the body axis of an animal and the radius from the centre of tile experimental vessel to the animal (as seen on a photograph). With the kind permission of Prof. Dr E. Ercolini and Dr F. Scapini, Florence, we were able to compare tile results ofERcoLIM & SCAPINX (1972) with ours (ARENDSE, 1980: experiments 1 and 2), after we had changed our arbitrarily chosen criterion 7 < ]30c't to ;, < 145°1, the criterion used by the Italian investigators. ARENDSE (1980) further hypothesized that one of tile ~ixtistical populations represented the behaviour of jumping animals, the other that of crawling ones. These categories have now been separated by direct visual observation. The orientation in the local G M F of the animals in tile two categories is compared. Finally the orientation of jumping animals in the local G M F is compared with their orientation in an artificial compensated field. Acknowledgements.---We are very grateihl to Prof. Dr F. J, Verheijen tor showing constant and stimulating interest and tor criticizing the manuscript, to Prof. Dr E. Batschelet for discussing and helping us with statistical problems, to Prof. Dr E. Ercolini and Dr F. Scapini tbr providing the detailed account of data and giving us permission to use them, and to Dr H. H. W. Velthuis tor many discussions. We should also like to thank Mr W. Maasse for constructing the set-up, D r J . D. A. Zijderveld and his co-workers of the Palaeomagnetic Laboratory tor providing the Helmholtz-coils and a workroom, Miss S. M. M c N a b for making so many improvements to the text and Miss F. E. M. van Vliet, Mrs T. Uytdehaage-van Vulpen, Miss S. J. van Cornewal and Mr C. Versteeg for their help in preparing the manuscript: Part of this investigation was supported by a grant made to the first author by the Netherlands Organization tbr the Advancement of Pure Research (Z.W.O.).
ORIENTATION TALITRUS
25
II. MATERIAL AND METHODS 1. F U R T H E R A N A L Y S I S OF R E S U L T S P U B L I S H E D P R E V I O U S L Y
All experiments were performed with Talitrus saltatorMont. (Crustacea, Amphipoda). For experimental procedures we refer to the original publications of ERCOLINI ~: SCAPINI ( 1 9 7 2 ) and ARENDSE ( 1 9 8 0 ) . ERCOLINI & SCAPINI collected the animals on the beach at Castiglione della Pescaia and performed one experiment on that beach and one at a laboratory in Florence. ARENDSE collected the animals for the beach experiment in Vlissingen, the Netherlands, and those for the laboratory experiment in Banyuls, France, and carried out the experiments in Vlissingen and Banuyls respectively (Table I). The photographed orientations are classified according to the angle 7 between the body axis of the animal and the radius from the centre of the experimental vessel to the animal (cf. ARENDSE, 1980: fig. 2). When 7 ~< 145°1 recordings are referred to as radiall]¢ directed, otherwise they are said to be nonradially directed. A Z~ test is used to find out whether there is any dift~rence between the results obtained by the different authors. 2. NEW EXPERIMENTS New experiments were also carried out with Talitrus saltator. The animals were collected on a sandy beach on the North bank of the river Scheldt, east of Vlissingen, the Netherlands. The seaward direction on this beach is 173 °, counting clockwise from magnetic North (0 °) for this and for all other directions. The animals were kept in a rectangular container (100 × 50 x 50 cm 3) in the Palaeomagnetic Laboratory in Utrecht. In the container there was an artificial water line running eastwest. An incandescent lamp (Philips, 40 W) was mounted outside the southwards facing glass wall of the container. A light-diffusing, opaline plastic screen was placed between the lamp and the container. No Other light could penetrate into the container. The test apparatus and test method were as described by ARENDSE VRINS (1975). The animals were put one at a time into the hemispherical experimental chamber in total darkness. The chamber was then illuminated by a 15 W incandescent lamp shining through a light-diffusing screen enabling us to record the positions of the animals. The lightfield in the bowl was uniform (cf. ARENDSE & VRINS, 1975: fig. 4). The set-up was surrounded by a system of Helmholtz-coils. The G M F could be compensated to within 1% of the original strength. The local G M F strength was H = 47 fiT, with components y = 43/tT, X = 16 p T and F= 9pT.
26
M, C. ARENDSE & C. J. K R U Y S W I J K
Positions were recorded by a human observer lying under the coils; his presence did not disturb the magnetic field inside the coils. The observer could see the animals through a transparent 12-sector disk and could thus record the position of the animal in a 12-class angular frequency distribution (AFD), counting clockwise with magnetic North between classes 12 and 1, Each animal was scored 12 times at intervals ranging from 3 to 27 s. Recording began after the animals had been exposed for 3 to 4 min to a relative humidity (RH) of 55 ± 5% at a temperature of 22 ° C. We classified the animals in two categories:jumpers and crawlers. A crawler mainly creeps around on the bottom of the vessel, or creeps round the vessel against the lower part of the inside surface. A jumper is an animal that jumps from the centre of the bottom towards the upper part of the inside surface of the bowl, hits it and then bounces or slides back to the bottom. 3. STATISTICAL EVALUATION The mean direction, 0 i (1 ~ i ~ N; N = total number of animals), was computed for each animal. To determine whether the distribution of the mean directions of the animals deviated from uniformity the Rayleigh test was applied as a second-order test (cf BATSCHELET, 1965, t978) ; test statistic Z- When the same data were subjected to a number (3) of tests the significance level (~) was altered accordingly Ccf. SAchS, 1978; and legend Fig. 1) to account for a 5% significance level for the sequence of tests. Differences in concentration around the mean direction are tested according to MARDIA (1972: 158); test statistic ~. III. RESULTS 1.
THE
NUMBER
OF RADIALLY
DIRECTED
ANIMALS
The data of ERCOLINI & SCAPINI (1972) and of ARENDSE (1980) were compared (Table I) and no significant difference between the proportions of radially and non-radially directed recordings was found. This holds for the experiments performed on the beach directly after the animals were collected as well for the experiments performed in the laboratories some days after the animals were collected. Despite this basic similarity of the data, ERCOLINI & SCAPINI did not demonstrate non-visual orientation in Tatitrus. 2.
THE
ORIENTATION
OF JUMPING
AND
CRAWLING
ANIMALS
In an experiment with a group of 69 animals 27 were classified as
27
ORIENTATION TALITRUS TABLE I
Comparison of the photographic recordings of Talitrus saltator dealt with in previous publications of EReOLINX& SeAPINX(1972) and At~NVSE (1980) respectively. Null hypothesis for the Xa test on which the numbers of expected (Exp) recordings of radially and non-radially directed animals are based: There is no significant difference between the numbers of observed (Obs) recordings obtained by the respective authors either in the beach experiments (X2 -----0.75, P> 0.10) or in the laboratory experiments (X2 = 1.58, P < 0.10). Orientation
ERCOLINI & SCAPINI, 1972
Castiglione, beach
radially non-radial~
Florence, laboratory
Obs
Exp
Obs
Exp
1435 1017
1 4 2 2 2 3 9 8 2391 1 0 3 0 1171 1178
ARENDSE, 1980
Vlissingen, beach
Banyuls, laboratory
Obs
Exp
Obs
Exp
83 72
90 65
102 51
103 50
jumpers and 42 as crawlers, as described in section II.2. This experiment was performed at the same R H as an experiment described earlier (ARENDSE, 1980: expt. 2). It is worth noting that the percentages of jumping and crawling animals (39% and 61% respectively), obtained here by visual inspection, were close to the percentages of radially and non-radially directed animals (41% and 59%) obtained in the earlier experiment from photographic recordings, using the separation criterion ? .< [30° I. This correspondence supports the hypothesis (Am~NDSE, 1980) that the radially directed photographic recordings are of the jumping Talitrus. The distribution of mean directions (0+) of jumping animals is significantly different from uniform, that of crawling ones is not (Fig. 1). On the assumption that only the jumping Talitrus are oriented, the statistic of concentration (ro) obtained froin these animals should be significantly larger than that obtained from the crawling specimens. The corresponding null hypothesis is rejected (~. = 2.08; P < 0.025; • = 2). We conclude that Talitrus saltator has a capacity for non-visual orientation; this confirms earlier results (ARENDSE, 1980). The mean direction of non-visual orientation is roughly parallel to the direction of the coast of origin of the animals (Fig. 1a). 3. J U M P I N G
A N I M A L S IN T H E L O C A L G E O M A G N E T I C IN A C O M P E N S A T E D M A G N E T I C F I E L D
FIELD AND
In view of the results of the foregoing tests we analyzed the recordings of jumping Talitrus only. In the local G M F the jumping Talitrus are again well orientated in a direction roughly parallel to that of their coast of
28
M. C. A R E N D S E
& C. J. K R U Y S W I J K
0
>
/
,
a
/¸
•
/'
i
-
/ ,
270
0 '
! j~
+
\
I
~9o
i
!
=7o~
/
!
¢
.90 l
"
]
% \
/ "~N,
/
+~oZ~
,~o
b
0
O /,
Y
\
\,
.+.
270
- , ~ - -
t +°
--.../
/
i/£
,~o~
T~
-
Fig. 1. Orientation of jumping and crawling Talitrus saltator in relation to the geomagnetic field (GMF); indicated are the mean directions (0i) of single animals (dots), the seaward direction at the beach (triangle), and the ~ = 0.05/r significance level (where -- 2 in all figures) for the length (i'~) of the mean orientation vector (arrow~ in_the mean direction (0); Rayleigh tests applied: a. Jumpers in local GMF (N = 27, 0 - = 248 °, J+0= 0.46, 0.001 < P < 0.01). b. Crawlers in local GMF (N = 42, 0 -- 196 °, p0 = 0.1, P > 0.1). c. Jumpers in local GMF (N = 80, 0 = 272 ° , r~ - 0.35, P < 0.001). d. Jumpers in compensated magnetic field (N = 80, 0 = 283", r~ = 0.09, P > 0.1).
origin (Fig. l c). H o w e v e r , alter c o m p e n s a t i o n o f the G M F the distrib u t i o n of 0 i is u n i f o r m (Fig. l d ) . M o r e o v e r , there is a significant dit= terence b e t w e e n the statistics o f c o n c e n t r a t i o n o b t a i n e d in b o t h tests (~ - 2.37; 0.01 < P < 0.02; r = 2). T h i s e x p e r i m e n t confirms the c o n c l u s i o n d r a w n earlier (ARENDSE, 1978), n a m e l y that :the G M F is the d i r e c t i o n - i n d i c a t i n g s t i m u l u s in the n o n - v i s u a l o r i e n t a t i o n o f Talitrus saltator.
ORIENTATION
TALITRUS
29
IV. DISCUSSION 1.
PHYSICAL
AND SOCIAL
INTERACTION
In all previous experiments (VAN DEN BERCKENet al., 1967; ERCOLINI t~ SCAPINI, 1972; SCAPINI & ERCOLINI, 1973; ARENDSE, 1980) more than one animal was introduced into the experimental vessel in each test. Although this more or less hemispherical vessel, by its very form, prevented thigmotactic reactions to the inner surface, such reactions to the body of another Talitrus were still possible and could make the animals press close together. Social interaction might have the same effect. In any case, on photographic recordings small groups of Talitrus are regularly seen grouped together. By our method both reactions are prevented. 2.
WATER
LOSS AND MODE
OF LOCOMOTION
Grouping together may well produce a microclimate that is characterized by a high RH. This microclimate not only decreases water loss but also considerably reduces jumping (WILLIAMSON, 1952). As only jumping Talitrus appeared to be systematically oriented (Fig. 1) the fact that we could distinguish between jumpers and crawlers increased chances of detecting magnetic orientation. 3.
WATER
LOSS AND ORIENTATION
WILLIAMSON (1952) reported that in a closed vessel with a volume of about 1.8 l, containing a saturated magnesium chloride solution, the R H rose from the equilibrium value R H = 58% to R H = 70% within a quarter of an hour after the introduction of 3 Talitrus. From his fig. 7 it follows that an R H difference of 12% is just large enough to get a significant reaction in a spatial R H gradient. Although we did not take any measures to stabilize the R H in our smaller (1.2 l) vessel we introduced only one specimen into the vessel and kept it there for only 5 minutes. It is unlikely that the animal's reactivity changed during our experiments, unless the animal reacts more readily to a gradient in time than in space. In earlier experiments the animals were kept for l0 to 20 minutes or even longer in the bowl, although admittedly the bowl was not always completely closed (cf. ERCOLINI & S C A P I N I , 1972) and hence the R H might not have reached the level that was theoretically possible.
30
4.
M.C. ARENDSE & C.J. KRUY S WIJ K PRE-EXPERIMENTAL
LIGHT
CONDITIONS
AND ORIENTATION
ARENDSE • gRINS (1975) showed that in the daytime Tenebrio molitor oriented in a systematically changing geographic direction when the animals had been exposed to the sun for several weeks. This direction was predictable from and in general opposite to the sun's azimuth. However, when the culture container had been illuminated by a fixed light bulb, orientation was in a constant direction, and was predictable fi"om and generally opposite to the fight direction in the container. This reaction in the G M F develops during the exposure of the animals to the environmental illumination, in this case the illumination of the container. If in Talitrus too the direction preference in the G M F were determined by the systematically changing spatio-temporal relations between the directional properties of the light field and the magnetic field in the habitat, then the variance in the AFD's obtained with Talitrus kept tbr a relatively short time in the laboratory--where these relations are fixed---would increase considerably. Therefore, we ruled out this possibility by keeping the animals for more than 2 months in an artificial light field prior to the experiments. Although ENRmHT (t972) suggested that the animals showed a better moon orientation--measured as the percentage of distributions significant at the 1% level--- before rather than after they had been kept in the laboratory for 2 to 20 days, the effect of captivity does not necessarily hold for magnetic orientation. Moreover, ENRIGHT'S suggestion is based on distributions obtained alter the animals had been kept in captivity for not more than 21 days. This observation can in fact be interpreted as supporting the hypothesazed interaction between the light and the magnetic field in the orientation of Talitrus saltator. After a short period the animals may have acquired a reaction to the artificially changed spatial relation between both stimulus fields, without their reaction to the original relationship being completely extinguished. This interpretation of ENRIOHT'S ~1972) data made us opt tbr a period of captivity of at least 2 months. In our opinion the experimental design we chose on the basis of the effects discussed was crucial for the clear demonstration of magnetic orientation of Talitrus. Further studies are needed to find out whether the light and humidity conditions actually affect the magnetic orientation of talitrids. The phenomenon itself, however, is now based on experimental evidence. In the local G M F the orientation is in an ecologically important direction. In a compensated magnetic field no orientation could be detected. Finally, in an artificial field with the same vertical and horizontal field strength components but with magnetic N at geographic W the orientation vector also showed a 90 ~ change in direction CARENDSE, 1978 .
ORIENTATION TALITRUS
31
V. SUMMARY Earlier results relating to the n u m b e r s of radially and non-radially directed p h o t o g r a p h i c recordings o f Talitrus saltator, o b t a i n e d in tests on non-visual orientation, are shown to be consistent with the recordings o f o t h e r investigators. T h e hypothesis that the radially directed recordings were o b t a i n e d from j u m p i n g animals receives further support, in that ( 1 ) the percentages o f radially directed recordings and o f j u m p i n g animals are nearly equal a n d (2) the distribution o f the individual m e a n orientation directions o f j u m p e r s is significantly different from uniform, as was the distribution o f radially directed recordings o b t a i n e d earlier. Similar correspondences are found for the crawling individuals and the non-radially directed recordings. It is shown that the j u m p i n g animals are oriented in the local G M F b u t not after it has been compensated. This, together with the fact that Talitrus reorients in darkness to an artificial magnetic field in which magnetic N o r t h was at geographic West, completes the e x p e r i m e n t a l p r o o f that u n d e r the experimental conditions non-visual orientation in Talitrus is magnetic orientation. VI. R E F E R E N C E S ARENDSE, M. C., 1978. Magnetic field detection is distinct from light detection in the
invertebrates Tenebrio and Talitrus.--Nature, Lond. 274. 358-362. 1980. Non-visual orientation in the sandhopper Talitrus saltator (Mont).--Neth. J. Zool. 30. 535-554. ARENDSE,M. C. &J. c. M. VRXNS,1975. Magnetic orientation and its relation to photic orientation in TenebriomolitorL. (Coleoptera, Tenebrionidae).--Neth. J. Zool. 25: 407437. BATSCHEL~T,E., 1965. Statistical methods for the analysis of problems in animal orientation and certain biological rhythms.~Am. Inst. Biol. Sci., Washington D.C.: 1-57. - - , 1978. Second-order statistical analysis of directions. In: K. SCHMIDT-Ko~NIG& W. KEETON.Animal migration, navigation and homing. Springer, Berlin: 3-24. BERCKEN, J. VAN DEN, S. BROEKHUIZEN,J. RINGELBERG& H. H. W. VELTHUIS, 1967. Non-visual orientation in Talitrus saltator.--Experientia 23: 4~45. ENRIGHT,J. T., 1972. When the beach-hopper looks at the moon: The mooncompass hypothesis. In: S. GALLER, K. SCHMIDT-KOENIG,G. JACOBS & R. BELLEVILLE. Animal orientation and navigation. Symp. NASA, SP-264. U.S. Govt. Printing Office, Washington D.C. : 523-555. ERCOLINI~ A. & F. SGAPINI~ 1972. On the non-visual orientation of littoral amphipods.~Monitore zool. ital. (N.S.) 6: 7544. HARTWlCK, R. F., 1976. Beach orientation in talitrid amphipods: capacities and strategics.--Behav. Ecol. Sociobiol. 1: 447458. LEASK,M.J.M., 1977. A physico-chemical mechanism for magnetic field dctcction by migratory birds and homing pigeons.~Nature, Lond. 267: 144-145. MARDIA, K. V., 1972. Statistics of directional data. Academic Press, London: 1-357. SACHS, L., 1978. Angewandte Statistik. Springer, Berlin: 1-552. SCAPINI, F. ~. A. ERCOLINI, 1973. Research on thc non-visual orientation of littoral --,
32
M. C. ARENDSE & C. J. KRUYSWI,JK
amphipods: Experiments with young born in captivity and adults from a Somalian population of Talorchestia martensii Weber (Crustacea, Amphipoda).~ Monitore zool. ital. (N.S.) Suppl. 5; 23-30. W1LL1AMSON,D.J., 1952. Studies in the biology of Talitridae (Crustacea, Amphipoda) : Effects of atmospheric humidity.--J, mar. biol. Ass. U.K. ~tle~73-99.
OF TALITRUS SALTATOR MAGNETIC FIELDS by
M. C. A R E N D S E and C. J. K R U Y S W I J K
Laboratory of Comparative Physiology, State University of Utrecht, ffan van Galenstraat 40, 3572 LA Utrecht, The .Netherlands CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . II. Material and Methods . . . . . . . . . . . . . . . . . . . . . . 1. Further analysis of results published previously . . . . . . . . . . . 2. New experiments . . . . . . . . . . . . . . . . . . . . . . . 3. Statistical evaluation . . . . . . . . . . . . . . . . . . . . . . III. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. The number of radially directed animals . . . . . . . . . . . . . 2. The orientation of jumping and crawling animals . . . . . . . . . 3. Jumping ammals in the local geomagnetic field and in a compensated magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . IV. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Physical and social interaction . . . . . . . . . . . . . . . . . . 2. Water loss and mode of locomotion . . . . . . . . . . . . . . . . 3. Water loss and orientation . . . . . . . . . . . . . . . . . . . 4. Pre-experimental light conditions and orientation . . . . . . . . . . . V. Summary . . . . . . . . . . . . . . . . . .......... VI. References . . . . . . . . . . . . . . . . . . . . . . . . . . .
23 25 25 25 26 26 26 26 27 29 29 29 29 30 31 31
I. I N T R O D U C T I O N N o n - v i s u a l o r i e n t a t i o n in Talitrus saltator, the s a n d h o p p e r , is a controversial subject (for a discussion of this t e r m see Am~NDS~., 1980). VAN DEN BERCKEN et at. (1967) c l a i m e d to h a v e d e m o n s t r a t e d non-visual o r i e n t a t i o n in Talitrus. T h e y m a d e a statistical analysis o f their results, b u t this analvsis was criticized b y ENRIGHT (1972). ERCOLINI & SCAPINI (1972) did not find non-visual o r i e n t a t i o n in talitrids in several extensive series of tests. HARTWIeK (1976) shared their opinion, b u t did not give a n y e x p e r i m e n t a l evidence. W h i l e studying LEASK'S (1977) optical p u m p i n g m o d e l o f m a g n e t i c field detection ARENDS~. (1978) showed t h a t Talitrus saltator is able to orient to the local g e o m a g n e t i c field ( G M F ) a n d to reorient in total darkness to a n artificial m a g n e t i c field w i t h m a g n e t i c N o r t h at g e o g r a p h i c West. H o w e v e r , the f r e q u e n c y distributions o f direction choices he o b t a i n e d were b i m o d a l , b u t at t h a t time the factors u n d e r l y i n g this b i m o d a l i t y were less clear t h a n they are
24
M . C . A R E N D S E & C. ,]. K R U Y S W I J K
now (ARENDSE, 1980; ARENDSE & KRUYSWIJK,unpublished). Moreover, no orientation data were available .which had been obtained in a compensated magnetic field and would have completed the proof of magnetic orientation. This paper is intended to fill that gap. ARENDSE (1978, 1980) was of the opinion that the controw:rsial results were due largely to the considerable variance observed in most experiments. He could trace this variance back to the existence of two ditterent statistical populations of observations in his photographic recordings of orienting animals. He separated these populations on the basis of the angle (7) between the body axis of an animal and the radius from the centre of tile experimental vessel to the animal (as seen on a photograph). With the kind permission of Prof. Dr E. Ercolini and Dr F. Scapini, Florence, we were able to compare tile results ofERcoLIM & SCAPINX (1972) with ours (ARENDSE, 1980: experiments 1 and 2), after we had changed our arbitrarily chosen criterion 7 < ]30c't to ;, < 145°1, the criterion used by the Italian investigators. ARENDSE (1980) further hypothesized that one of tile ~ixtistical populations represented the behaviour of jumping animals, the other that of crawling ones. These categories have now been separated by direct visual observation. The orientation in the local G M F of the animals in tile two categories is compared. Finally the orientation of jumping animals in the local G M F is compared with their orientation in an artificial compensated field. Acknowledgements.---We are very grateihl to Prof. Dr F. J, Verheijen tor showing constant and stimulating interest and tor criticizing the manuscript, to Prof. Dr E. Batschelet for discussing and helping us with statistical problems, to Prof. Dr E. Ercolini and Dr F. Scapini tbr providing the detailed account of data and giving us permission to use them, and to Dr H. H. W. Velthuis tor many discussions. We should also like to thank Mr W. Maasse for constructing the set-up, D r J . D. A. Zijderveld and his co-workers of the Palaeomagnetic Laboratory tor providing the Helmholtz-coils and a workroom, Miss S. M. M c N a b for making so many improvements to the text and Miss F. E. M. van Vliet, Mrs T. Uytdehaage-van Vulpen, Miss S. J. van Cornewal and Mr C. Versteeg for their help in preparing the manuscript: Part of this investigation was supported by a grant made to the first author by the Netherlands Organization tbr the Advancement of Pure Research (Z.W.O.).
ORIENTATION TALITRUS
25
II. MATERIAL AND METHODS 1. F U R T H E R A N A L Y S I S OF R E S U L T S P U B L I S H E D P R E V I O U S L Y
All experiments were performed with Talitrus saltatorMont. (Crustacea, Amphipoda). For experimental procedures we refer to the original publications of ERCOLINI ~: SCAPINI ( 1 9 7 2 ) and ARENDSE ( 1 9 8 0 ) . ERCOLINI & SCAPINI collected the animals on the beach at Castiglione della Pescaia and performed one experiment on that beach and one at a laboratory in Florence. ARENDSE collected the animals for the beach experiment in Vlissingen, the Netherlands, and those for the laboratory experiment in Banyuls, France, and carried out the experiments in Vlissingen and Banuyls respectively (Table I). The photographed orientations are classified according to the angle 7 between the body axis of the animal and the radius from the centre of the experimental vessel to the animal (cf. ARENDSE, 1980: fig. 2). When 7 ~< 145°1 recordings are referred to as radiall]¢ directed, otherwise they are said to be nonradially directed. A Z~ test is used to find out whether there is any dift~rence between the results obtained by the different authors. 2. NEW EXPERIMENTS New experiments were also carried out with Talitrus saltator. The animals were collected on a sandy beach on the North bank of the river Scheldt, east of Vlissingen, the Netherlands. The seaward direction on this beach is 173 °, counting clockwise from magnetic North (0 °) for this and for all other directions. The animals were kept in a rectangular container (100 × 50 x 50 cm 3) in the Palaeomagnetic Laboratory in Utrecht. In the container there was an artificial water line running eastwest. An incandescent lamp (Philips, 40 W) was mounted outside the southwards facing glass wall of the container. A light-diffusing, opaline plastic screen was placed between the lamp and the container. No Other light could penetrate into the container. The test apparatus and test method were as described by ARENDSE VRINS (1975). The animals were put one at a time into the hemispherical experimental chamber in total darkness. The chamber was then illuminated by a 15 W incandescent lamp shining through a light-diffusing screen enabling us to record the positions of the animals. The lightfield in the bowl was uniform (cf. ARENDSE & VRINS, 1975: fig. 4). The set-up was surrounded by a system of Helmholtz-coils. The G M F could be compensated to within 1% of the original strength. The local G M F strength was H = 47 fiT, with components y = 43/tT, X = 16 p T and F= 9pT.
26
M, C. ARENDSE & C. J. K R U Y S W I J K
Positions were recorded by a human observer lying under the coils; his presence did not disturb the magnetic field inside the coils. The observer could see the animals through a transparent 12-sector disk and could thus record the position of the animal in a 12-class angular frequency distribution (AFD), counting clockwise with magnetic North between classes 12 and 1, Each animal was scored 12 times at intervals ranging from 3 to 27 s. Recording began after the animals had been exposed for 3 to 4 min to a relative humidity (RH) of 55 ± 5% at a temperature of 22 ° C. We classified the animals in two categories:jumpers and crawlers. A crawler mainly creeps around on the bottom of the vessel, or creeps round the vessel against the lower part of the inside surface. A jumper is an animal that jumps from the centre of the bottom towards the upper part of the inside surface of the bowl, hits it and then bounces or slides back to the bottom. 3. STATISTICAL EVALUATION The mean direction, 0 i (1 ~ i ~ N; N = total number of animals), was computed for each animal. To determine whether the distribution of the mean directions of the animals deviated from uniformity the Rayleigh test was applied as a second-order test (cf BATSCHELET, 1965, t978) ; test statistic Z- When the same data were subjected to a number (3) of tests the significance level (~) was altered accordingly Ccf. SAchS, 1978; and legend Fig. 1) to account for a 5% significance level for the sequence of tests. Differences in concentration around the mean direction are tested according to MARDIA (1972: 158); test statistic ~. III. RESULTS 1.
THE
NUMBER
OF RADIALLY
DIRECTED
ANIMALS
The data of ERCOLINI & SCAPINI (1972) and of ARENDSE (1980) were compared (Table I) and no significant difference between the proportions of radially and non-radially directed recordings was found. This holds for the experiments performed on the beach directly after the animals were collected as well for the experiments performed in the laboratories some days after the animals were collected. Despite this basic similarity of the data, ERCOLINI & SCAPINI did not demonstrate non-visual orientation in Tatitrus. 2.
THE
ORIENTATION
OF JUMPING
AND
CRAWLING
ANIMALS
In an experiment with a group of 69 animals 27 were classified as
27
ORIENTATION TALITRUS TABLE I
Comparison of the photographic recordings of Talitrus saltator dealt with in previous publications of EReOLINX& SeAPINX(1972) and At~NVSE (1980) respectively. Null hypothesis for the Xa test on which the numbers of expected (Exp) recordings of radially and non-radially directed animals are based: There is no significant difference between the numbers of observed (Obs) recordings obtained by the respective authors either in the beach experiments (X2 -----0.75, P> 0.10) or in the laboratory experiments (X2 = 1.58, P < 0.10). Orientation
ERCOLINI & SCAPINI, 1972
Castiglione, beach
radially non-radial~
Florence, laboratory
Obs
Exp
Obs
Exp
1435 1017
1 4 2 2 2 3 9 8 2391 1 0 3 0 1171 1178
ARENDSE, 1980
Vlissingen, beach
Banyuls, laboratory
Obs
Exp
Obs
Exp
83 72
90 65
102 51
103 50
jumpers and 42 as crawlers, as described in section II.2. This experiment was performed at the same R H as an experiment described earlier (ARENDSE, 1980: expt. 2). It is worth noting that the percentages of jumping and crawling animals (39% and 61% respectively), obtained here by visual inspection, were close to the percentages of radially and non-radially directed animals (41% and 59%) obtained in the earlier experiment from photographic recordings, using the separation criterion ? .< [30° I. This correspondence supports the hypothesis (Am~NDSE, 1980) that the radially directed photographic recordings are of the jumping Talitrus. The distribution of mean directions (0+) of jumping animals is significantly different from uniform, that of crawling ones is not (Fig. 1). On the assumption that only the jumping Talitrus are oriented, the statistic of concentration (ro) obtained froin these animals should be significantly larger than that obtained from the crawling specimens. The corresponding null hypothesis is rejected (~. = 2.08; P < 0.025; • = 2). We conclude that Talitrus saltator has a capacity for non-visual orientation; this confirms earlier results (ARENDSE, 1980). The mean direction of non-visual orientation is roughly parallel to the direction of the coast of origin of the animals (Fig. 1a). 3. J U M P I N G
A N I M A L S IN T H E L O C A L G E O M A G N E T I C IN A C O M P E N S A T E D M A G N E T I C F I E L D
FIELD AND
In view of the results of the foregoing tests we analyzed the recordings of jumping Talitrus only. In the local G M F the jumping Talitrus are again well orientated in a direction roughly parallel to that of their coast of
28
M. C. A R E N D S E
& C. J. K R U Y S W I J K
0
>
/
,
a
/¸
•
/'
i
-
/ ,
270
0 '
! j~
+
\
I
~9o
i
!
=7o~
/
!
¢
.90 l
"
]
% \
/ "~N,
/
+~oZ~
,~o
b
0
O /,
Y
\
\,
.+.
270
- , ~ - -
t +°
--.../
/
i/£
,~o~
T~
-
Fig. 1. Orientation of jumping and crawling Talitrus saltator in relation to the geomagnetic field (GMF); indicated are the mean directions (0i) of single animals (dots), the seaward direction at the beach (triangle), and the ~ = 0.05/r significance level (where -- 2 in all figures) for the length (i'~) of the mean orientation vector (arrow~ in_the mean direction (0); Rayleigh tests applied: a. Jumpers in local GMF (N = 27, 0 - = 248 °, J+0= 0.46, 0.001 < P < 0.01). b. Crawlers in local GMF (N = 42, 0 -- 196 °, p0 = 0.1, P > 0.1). c. Jumpers in local GMF (N = 80, 0 = 272 ° , r~ - 0.35, P < 0.001). d. Jumpers in compensated magnetic field (N = 80, 0 = 283", r~ = 0.09, P > 0.1).
origin (Fig. l c). H o w e v e r , alter c o m p e n s a t i o n o f the G M F the distrib u t i o n of 0 i is u n i f o r m (Fig. l d ) . M o r e o v e r , there is a significant dit= terence b e t w e e n the statistics o f c o n c e n t r a t i o n o b t a i n e d in b o t h tests (~ - 2.37; 0.01 < P < 0.02; r = 2). T h i s e x p e r i m e n t confirms the c o n c l u s i o n d r a w n earlier (ARENDSE, 1978), n a m e l y that :the G M F is the d i r e c t i o n - i n d i c a t i n g s t i m u l u s in the n o n - v i s u a l o r i e n t a t i o n o f Talitrus saltator.
ORIENTATION
TALITRUS
29
IV. DISCUSSION 1.
PHYSICAL
AND SOCIAL
INTERACTION
In all previous experiments (VAN DEN BERCKENet al., 1967; ERCOLINI t~ SCAPINI, 1972; SCAPINI & ERCOLINI, 1973; ARENDSE, 1980) more than one animal was introduced into the experimental vessel in each test. Although this more or less hemispherical vessel, by its very form, prevented thigmotactic reactions to the inner surface, such reactions to the body of another Talitrus were still possible and could make the animals press close together. Social interaction might have the same effect. In any case, on photographic recordings small groups of Talitrus are regularly seen grouped together. By our method both reactions are prevented. 2.
WATER
LOSS AND MODE
OF LOCOMOTION
Grouping together may well produce a microclimate that is characterized by a high RH. This microclimate not only decreases water loss but also considerably reduces jumping (WILLIAMSON, 1952). As only jumping Talitrus appeared to be systematically oriented (Fig. 1) the fact that we could distinguish between jumpers and crawlers increased chances of detecting magnetic orientation. 3.
WATER
LOSS AND ORIENTATION
WILLIAMSON (1952) reported that in a closed vessel with a volume of about 1.8 l, containing a saturated magnesium chloride solution, the R H rose from the equilibrium value R H = 58% to R H = 70% within a quarter of an hour after the introduction of 3 Talitrus. From his fig. 7 it follows that an R H difference of 12% is just large enough to get a significant reaction in a spatial R H gradient. Although we did not take any measures to stabilize the R H in our smaller (1.2 l) vessel we introduced only one specimen into the vessel and kept it there for only 5 minutes. It is unlikely that the animal's reactivity changed during our experiments, unless the animal reacts more readily to a gradient in time than in space. In earlier experiments the animals were kept for l0 to 20 minutes or even longer in the bowl, although admittedly the bowl was not always completely closed (cf. ERCOLINI & S C A P I N I , 1972) and hence the R H might not have reached the level that was theoretically possible.
30
4.
M.C. ARENDSE & C.J. KRUY S WIJ K PRE-EXPERIMENTAL
LIGHT
CONDITIONS
AND ORIENTATION
ARENDSE • gRINS (1975) showed that in the daytime Tenebrio molitor oriented in a systematically changing geographic direction when the animals had been exposed to the sun for several weeks. This direction was predictable from and in general opposite to the sun's azimuth. However, when the culture container had been illuminated by a fixed light bulb, orientation was in a constant direction, and was predictable fi"om and generally opposite to the fight direction in the container. This reaction in the G M F develops during the exposure of the animals to the environmental illumination, in this case the illumination of the container. If in Talitrus too the direction preference in the G M F were determined by the systematically changing spatio-temporal relations between the directional properties of the light field and the magnetic field in the habitat, then the variance in the AFD's obtained with Talitrus kept tbr a relatively short time in the laboratory--where these relations are fixed---would increase considerably. Therefore, we ruled out this possibility by keeping the animals for more than 2 months in an artificial light field prior to the experiments. Although ENRmHT (t972) suggested that the animals showed a better moon orientation--measured as the percentage of distributions significant at the 1% level--- before rather than after they had been kept in the laboratory for 2 to 20 days, the effect of captivity does not necessarily hold for magnetic orientation. Moreover, ENRIGHT'S suggestion is based on distributions obtained alter the animals had been kept in captivity for not more than 21 days. This observation can in fact be interpreted as supporting the hypothesazed interaction between the light and the magnetic field in the orientation of Talitrus saltator. After a short period the animals may have acquired a reaction to the artificially changed spatial relation between both stimulus fields, without their reaction to the original relationship being completely extinguished. This interpretation of ENRIOHT'S ~1972) data made us opt tbr a period of captivity of at least 2 months. In our opinion the experimental design we chose on the basis of the effects discussed was crucial for the clear demonstration of magnetic orientation of Talitrus. Further studies are needed to find out whether the light and humidity conditions actually affect the magnetic orientation of talitrids. The phenomenon itself, however, is now based on experimental evidence. In the local G M F the orientation is in an ecologically important direction. In a compensated magnetic field no orientation could be detected. Finally, in an artificial field with the same vertical and horizontal field strength components but with magnetic N at geographic W the orientation vector also showed a 90 ~ change in direction CARENDSE, 1978 .
ORIENTATION TALITRUS
31
V. SUMMARY Earlier results relating to the n u m b e r s of radially and non-radially directed p h o t o g r a p h i c recordings o f Talitrus saltator, o b t a i n e d in tests on non-visual orientation, are shown to be consistent with the recordings o f o t h e r investigators. T h e hypothesis that the radially directed recordings were o b t a i n e d from j u m p i n g animals receives further support, in that ( 1 ) the percentages o f radially directed recordings and o f j u m p i n g animals are nearly equal a n d (2) the distribution o f the individual m e a n orientation directions o f j u m p e r s is significantly different from uniform, as was the distribution o f radially directed recordings o b t a i n e d earlier. Similar correspondences are found for the crawling individuals and the non-radially directed recordings. It is shown that the j u m p i n g animals are oriented in the local G M F b u t not after it has been compensated. This, together with the fact that Talitrus reorients in darkness to an artificial magnetic field in which magnetic N o r t h was at geographic West, completes the e x p e r i m e n t a l p r o o f that u n d e r the experimental conditions non-visual orientation in Talitrus is magnetic orientation. VI. R E F E R E N C E S ARENDSE, M. C., 1978. Magnetic field detection is distinct from light detection in the
invertebrates Tenebrio and Talitrus.--Nature, Lond. 274. 358-362. 1980. Non-visual orientation in the sandhopper Talitrus saltator (Mont).--Neth. J. Zool. 30. 535-554. ARENDSE,M. C. &J. c. M. VRXNS,1975. Magnetic orientation and its relation to photic orientation in TenebriomolitorL. (Coleoptera, Tenebrionidae).--Neth. J. Zool. 25: 407437. BATSCHEL~T,E., 1965. Statistical methods for the analysis of problems in animal orientation and certain biological rhythms.~Am. Inst. Biol. Sci., Washington D.C.: 1-57. - - , 1978. Second-order statistical analysis of directions. In: K. SCHMIDT-Ko~NIG& W. KEETON.Animal migration, navigation and homing. Springer, Berlin: 3-24. BERCKEN, J. VAN DEN, S. BROEKHUIZEN,J. RINGELBERG& H. H. W. VELTHUIS, 1967. Non-visual orientation in Talitrus saltator.--Experientia 23: 4~45. ENRIGHT,J. T., 1972. When the beach-hopper looks at the moon: The mooncompass hypothesis. In: S. GALLER, K. SCHMIDT-KOENIG,G. JACOBS & R. BELLEVILLE. Animal orientation and navigation. Symp. NASA, SP-264. U.S. Govt. Printing Office, Washington D.C. : 523-555. ERCOLINI~ A. & F. SGAPINI~ 1972. On the non-visual orientation of littoral amphipods.~Monitore zool. ital. (N.S.) 6: 7544. HARTWlCK, R. F., 1976. Beach orientation in talitrid amphipods: capacities and strategics.--Behav. Ecol. Sociobiol. 1: 447458. LEASK,M.J.M., 1977. A physico-chemical mechanism for magnetic field dctcction by migratory birds and homing pigeons.~Nature, Lond. 267: 144-145. MARDIA, K. V., 1972. Statistics of directional data. Academic Press, London: 1-357. SACHS, L., 1978. Angewandte Statistik. Springer, Berlin: 1-552. SCAPINI, F. ~. A. ERCOLINI, 1973. Research on thc non-visual orientation of littoral --,
32
M. C. ARENDSE & C. J. KRUYSWI,JK
amphipods: Experiments with young born in captivity and adults from a Somalian population of Talorchestia martensii Weber (Crustacea, Amphipoda).~ Monitore zool. ital. (N.S.) Suppl. 5; 23-30. W1LL1AMSON,D.J., 1952. Studies in the biology of Talitridae (Crustacea, Amphipoda) : Effects of atmospheric humidity.--J, mar. biol. Ass. U.K. ~tle~73-99.