Food and food uptake in arenicola marina

Netherlands Journal 13 (314) : 406-421

of Sea Research (1979)

FOOD AND FOOD IN ARENICOLA

UPTAKE MARINA

M. RIJKEN (Netherlands

Institute jbr Sea Research,

Texel,

The Netherlands)

CONTENTS I. Introduction . . . . . . . . . . . . . . . II. Field experiments on the mechanism of sediment ingestion. . 1. Methods . . . . . . . . . . . . . . . . . . . . . . . 2. Circulation rate of the sediment . . . . . . 3. The shape of the quick-sand column and the burrow . . 4. Downward movement of the sediment in the column . . . . 5. Utilization and abondonment of the funnel . . . . . . III. Laboratory experiments on the mechanism of sediment ingestion 1. Methods . . . . . . . . . . . . . . 2. Results. . . . . . . . . . . . . . . . . IV. Growth experiments on different diets . . . . . . . . . . . 1. Methods . . . . . . . . . . . . . 2. Feeding with bacteria . . . . . . . . . 3. Feeding with benthic diatoms . . . . . . . . . . . 4. Feeding with dried D’lva . . . . . . . . 5. Feeding with superficial sediment of a natural tidal flat . . . V. Discussion and Conclusions . . . . . . . . . . VI. Summary. . . . . . . . . . . . . . . VII. References . . . . . . . .

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406 410 410 410 411 412 412 413 413 413 415 415 416 417 417 417 418 419 419

I. INTRODUCTION

The mode of life, feeding behaviour and actual food sources of the lugworm (Arenicola marina L.) have been subject to many studies. The multitude of observations did not result, however, in a generally accepted hypothesis on food and feeding of the animal because the feeding behaviour was poorly understood. Most authors agree that Arenicola marina lives in an L- or J-shaped tube in the sediment (Fig. 1) (BOHN, 1903; THAMDRUP, 1935; LINKE, 1939; WELLS, 1945, 1963; KRUEGER,1959, 1971; ZIEGELMAYER,1964; JAKOBSEN,1967 ; DE WILDE, 1975). The tube is clothed with mucus and maintains its shape by internal pressure of the worm (TRUEMAN & ANSELL, 1969). Through the open end of the tube oxygen-rich water enters the tube and flows downwards by peristaltic movements of the worm. At the closed end the water is pressed through the sediment

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under considerable pressure (VAN DAM, 1938). According to WELLS (1945) the percolating water creates a column of a semifluid mixture of water and sediment straight upwards to the surface. Pumping capacities are discussed by KRUGER (1964) and BAUMFALK (1979). The lugworm swallows the sediment at the lower part of this column of quick-sand which gradually sinks down (THAMDRUP, 1935). At the top of the column the sinking sand is replaced by sand from the immediate surroundings, so that a funnel develops (Fig. la). JAKOBSEN (1967) showed, by means of coloured sand, that the circulation time of the sediment, from inside the funnel through the sand column and the intestine of the worm, ending with the ejection of the cast, was only 3 to 6 hours. The situation described is probably correct for the majority of sediments. However, doubt exists about its validity under all circumstances. THAMDRUP (1935) e.g. found that a layer of coloured sand, which was spread on the surface, gradually sank down to the head of the worm and obtained a cone-shape during this progress (Fig. 1b) . Similar ideas were developed by DE WILDE (1975) who observed that on certain tidal flats and in the laboratory the number of funnels sometimes was considerably lower than the number of castings, or even that in areas crowded with casts, the complementary funnels were virtually absent. Concerning the food uptake of Arenicola there has been a real evolution of ideas. THAMDRUP (1935) still believed the worm to be a completely aselect substrate feeder. Later WELLS (1945) and KRUGER (1962) observed that large particles are refused, and recently BAUMFALK (1979) observed that the lugworm shows a pronounced preference for small particles, which is ascribed to a difference in chance of adhesion of small and large particles to the papillae of the proboscis. Small particles, because of their relatively large surface area, can adsorb more organic matter per unit of weight than large particles. This can explain why faeces of Arenicola marina may contain even more organic matter than the substratum (KRUGER, 1959; JAKOBSEN, 1967; HYLLEBERG, 1975) when part of the organic matter is indigestible. Other hypothesis to explain the well-known feature of the high organic matter content of the faeces are the following: (1) Arenicola marina is a filter feeder as suspended particulate matter and micro-organisms are retained from the respiratory water by the sand at the blind end of the tube acting as a filter (KRUGER, 1959, 1962, 1964, 1971; HOBSON, 1967; POLLACK, 1979). The particles retained are swallowed together with the sand. However, JAKOBSEN (1967) showed this mechanism to be of little importance in the feeding of the worm, since insufficient organic matter is collected.

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M. RIJKEN

Fig. 1. A selection of feeding types found in Arenicola derived from and laboratory Availability and distribution of food sources physical properties the sediment determinative in respect. Often of different types occur. Normal Wadden type described this study; narrow quick-sand connects the with the of the upper sediment and supposedly food particles as somewhat dots) trapped the funnel rapidly transported the animal. Type described DE WILDE occurring in sediments; slowly transport in cone-shaped sediment the funnel food by and increased production by over an surface area; particles tend concentrate in centre of sediment this might a suitable for gardening 1975). c. and open reaching to living depth Arenicola marina found in and cohesive food is and obtained erosion of walls. d. and quick-sand are absent; type occurs nutrientrich at living originating from burried and mineralized algal or artificial in laboratory e. Filtration food particles the overlaying through the activity of as described KRUGER (1959) ; although the significance of this mechanism has never been proved, it may contribute in an additional way.

(2) The funnel combines a number of functions (DE WILDE, 1975). Firstly it acts as a sediment trap. Secondly, it provides an enlarged surface for primary production of microphytobenthos. Thirdly, during the downward moving of the sediment a concentration of food particles occurs.

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(3) The “concept of gardening” as described for Abarenicola paczjica (HYLLEBERG, 1975). According to this concept, an environment is created in the quick-sand column where digestable micro-organisms flourish upon the large quantities of indigestable organic matter present in the sediment. However, doubt exists whether there is sufficient time for a significant increase of micro-organisms in the quickly moving sand (HOBSON, 1967). When studying food uptake and potential food sources of Arenicola one should take into account that the possibilities of actual digestion are Iimited because of the short stay of the food in the intestine (KERMACK, 1955) and the limited number of enzymes, mainly carbohydrases, proteases and some lipases (LONGBOTTOM, 1970), available. This means that the food must be easily digestable. Six potential food sources of the worm were found mentioned: Detritus. Many organisms, also polychaetes, thrive on a diet of decomposed plant material only. It appeared, however, that Arenicola may possibly digest only 15y0 of natural tidal flat detritus (GEORGE, 1964). Half-decomposed fragments of Ulva and Monostroma even pass unchanged through the intestine of Abarenicola pacifica (HYLLEBERG, 1975). Bacteria. Heterotrophic bacteria are abundant in the upper sediment layers (MEADOWS & ANDERSON, 1968). According to HYLLEBERG ( 1975) Aberenicola paczjica would not digest any bacterial material at all. However, BOON et al. (1977) found organic compounds synthesized by Desulfovibrio desuljiiricans, in Arenicola marina tissues which suggests ingestion and assimilation of this bacterium living in anaerobic sediments. KRUEGER(197 1) suggested that bacteria associated with detritus might be filtered from the respiratory water to serve as food. Benthic diatoms. The fact that the peak in growth of the lugworm coincides in time with the maximum primary production of microphytobenthos, might indicate that benthic diatoms represent an important food source (CADEE, 1976). Moreover, HYLLEBERG (1975) found lower numbers of diatoms in the rectum of Abarenicola pacijica than in the crop, suggesting digestion. Assimilation of diatomaceous material was indicated by the presence of diatom-specific fatty acids in the tissues of Arenicola marina (BOON et al., 1978). However, KRUGER (1971: 161) o b served that diatoms also can pass through the worms’ intestines undamaged, and the enzymes needed to destruct the cell wall of diatoms were not found to be present in Arenicola. Meiofauna. Despite extensive investigations meiofauna organisms have never been found in the intestines of Arenicola marina (KRUEGER, 1971), or Abarenicola uagabunda (HYLLEBERG, 1975). This is surprising

410

RIJKEN

in view of numbers of meiofaunal inhabitants in the upper sediment layers. Abarenicola pa&j&a, however, probably eats and digests some flagellates and ciliates (HYLLEBERG, 1975). Planktonic organisms. These organisms are mentioned as an important food source in the filter feeding hypothesis. However, they never have been found in the intestines of Arenicola (KRUGER, 1959, 197 1). Larger organisms. LINKE (1939) suggests that young individuals of Pygospio elegans are swallowed together with their sand tubes, which seems highly unlikely in view of the particle size selection of the worm. Curious is the observation of SAINT-JOSEPH (1894) of a half-digested Nereis in the intestine of Arenicola. Probably in this very case the Nereis was already dead before it was ingested. It is one of the aims of this investigation to define the difference in conditions at which funnels are made or are absent. In this connection it seems worthwhile to take the feeding habits of the animal into account, including food quality. Acknowledgements.-Thanks are due to J. W. de Blok, H. Postma, P. A. W. J. de Wilde and J. J. Zijlstra for improving the manuscript. II. FIELD

EXPERIMENTS SEDIMENT

ON THE MECHANISM INGESTION

OF

1. METHODS On three tidal flats along the east coast of Texel (location A north of the NIOZ harbour; location B in the small bay De Mok; location C south of De Eendracht salt marshes) with large numbers of distinct funnels, the experiment of JAKOBSEN (1967) on sediment circulation was repeated. Instead of artificially coloured sand, small industrial glass spheres (diameter 0.065 to 0.090 mm) were used as a marker. They have the advantage to be chemically inert and can easily be recognised with a binocular microscope, even in low concentrations. Besides, a size can be selected so that they are easily ingested by the worm (cJ BAUMFALK, 1979). A possible disadvantage could be the perfect spherical shape and the complete lack of adhering food substances. 2. CIRCULATION

RATE OF THE SEDIMENT

The funnels were filled with glass beads during low tide. Observations were made during the same and successive low tides. In 60 cases the period elapsing until the first appearance of glass beads in the faeces

b

h PLATE

I

Burrowing Arenicola marina demonstrated by filling the funnel with glass beads. a. Still no activity visible. b. First glass beads enter the sand column. c. The glass beads sink deeper. d. The worm is nearly reached. e. Complete quick-sand column filled. f. The lower part of the column still contains the glass beads, the upper part is filled again by natural sediment. g. Bent and at the lower end subdivided sand column. h. Sand column in sediment rich in shells.

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411

was measured. Only when the sand was very moist or the funnel happened to be situated in a shallow little pool, spheres started to appear in the faeces during the same low tide. In most other cases (34) the spheres were present at the beginning of the next low tide indicating their appearance in the castings during the first submersion period. Only in 4 observations the spheres did appear not earlier than the second period of submersion. The very first spheres in the faeces were observed after 13 hours, the majority after about 4 hours. In general, these results are in agreement with JAKOBSEN’S (1967) figures. During the experiment two interesting observations were made: Firstly, faeces consisted completely of glass spheres, almost without any other contribution. Only in some cases (69 out of 339) a faeces string contained a few centimeters of the normal anaerobic sediment, interspaced between parts, consisting of spheres. Secondly, in a number of cases glass spheres deposited in one funnel were found back in 2 or 3 different (in 200/, and 4% of the funnels respectively) castings. Apparently 2 or more individual worms can make use of the same funnel. 3. THE SHAPE OF THE QUICK-SAND COLUMN AND THE BURROW The quick appearance of the spheres in the faeces and the multiple use of funnels led to the following experiments in which the shape and operation of the quick-sand column were studied. A large number of funnels was filled with glass spheres. When the spheres appeared in the castings, the tube and sand column were opened with the aid of a special box corer ( ZIEGELMAYER, 1964). Very often the funnel appeared to pass into an only 5 mm wide column, filled with glass spheres going straight downwards for 10 to 20 cm. Then it gradually curved and made contact with the horizontal part of the burrow of the lugworm (Plate Ie) . By opening the tube and sand column some hours or days after the spheres were administered, other characteristics of the burrows were disclosed. In most cases the first centimetres of the tube were found to be surrounded by a thin layer of spheres. This confirms the hypothesis of WELLS (1945), supported by SEYMOUR (1971)) that the worm occasionally intrudes over some distance into the sand of the column and in retreating again pulls with its first three chaetigerous segments part of the surrounding sand downwards to the shaft. Also the gradual curve in the lower part of the sand column is difficult to explain when it was not shaped by the worm itself. Not rare were cases in which the sand column winds to “evade” barriers in the sediment, like shells, peat, etc. (Plate Ig and h). These

412

M.

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413

This would explain the quite common feature of two castings originating from only one funnel. On the densely populated location A even 200/, of 75 investigated funnels appeared to be connected with two castings, whereas 4% was connected with even three castings, as was shown by marking the funnels with glass beads. III.

LABORATORY

EXPERIMENTS ON THE OF SEDIMENT INGESTION

MECHANIShf

1. METHODS In two indoor experimental tidal mud flats (DE WILDE & KUIPERS, 1977) large numbers of Arenicola occurred at the time of the experiments. Although the sediment was obtained from a place close to location B, where funnels abound, funnels were rarely observed in the experimental tidal flat. Small areas of these indoor mudflats were quantitatively investigated in a similar way as in the outdoor experiments, again using an added layer of glass spheres as a marker sediment. 2. RESULTS The absence of funnels, combined with the fact that the faeces were almost exclusively black, suggested that the food uptake here was totally different from that found in the field. Apparently the entire substrate gradually sank down to replace the sand that was eaten from deeper layers, whereas the castings form the new top layer. This suggestion was tested in one of the experimental flats. Firstly, the production of faeces was determined on two 0.25 m2 squares, populated together by 56 or 57 worms. The faeces were collected during each low tide and the amount was measured in a measuring glass with sea water. The average daily production in a period of 6 days was approximately 1400 ml *m-s; or 12.3 ml per worm which amount is normal when compared to the observations of CADBE (1976).

Secondly, the sinking speed of the surface layer of the sediment was measured. Of three squares (25 x 25 cm) the surface was smoothened and covered with a 2 mm thick layer of glass beads. Soon this layer was covered by castings, but no white faeces appeared. During the first month each 2 or 3 days a sample of the faeces was collected for analysis; later samples were taken only incidentally. During the first month only occasionally glass beads were found in the faeces, after 150 days a slight increase was observed.

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RIJKEN

After 35 and 380 days the distribution of the glass spheres in the substrate was examined by taking a core in one of the squares. The core was subdivided in slices of 3 cm thickness and the vertical distribution of the glass spheres was determined (Table I). After 35 days the pearls had reached a depth between 3 and 6 cm. After 380 days they appeared to be more mixed with the substratum, but with a considerable concentration between 12 and 21 cm. Below a depth of 21 cm only few spheres were found. TABLE

I

Vertical distribution of glass beads in the sediment of an indoor experimental tidal flat inhabited by Arenicola marina, 35 and 380 days after a thin surface layer of these beads was administered. Number of glass beads per gram dry sand in the sieved fraction of 0.215 to 0.315 mm. Depth (cm) o-3 3-6 6-9 9-12 12-15 15-18 18-21 21-23 23-25

Number of glass beads After 35 days

After 380 days

0 3433 34 0 0 0 0 0

(10 ? 38 11 554 957 419 52 8

The observations can be combined as follows. A daily faeces production of 1400 mI*m-2 equals a sinking speed of the substrate of 1.4 mm per day, so that the glass spheres can be expected to arrive at a depth of 3 to 6 cm after 35 days. After about 150 days they will reach a depth of 21 cm, which is the burrowing depth of the adult worm, therefore after that period the spheres should appear in the faeces. After 380 days the spheres have already been moved to the surface for the second time, but of course a considerable mixing with the sediment did occur. Agreement of faeces production with the overall sinking speed of the sediment indicates that there are no local differences in the sinking rate, as expected in the case of funnels. Also the presence of hidden funnels (as shown in Fig. lb) is not likely. This aberrant behaviour of the lugworms in the experimental tidal flats could not be caused by the sediment itself as it is similar as in location B in the field studies. Obviously the worms change their feedings habits in search for good food.

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ARENICOLA

Analysis of the chlorophyll a content of the sediment in the experimental tanks (SHUMAN & LORENZEN, 1975), showed surprisingly enough that the deeper layers contained as much chlorophyll a as the upper centimetre. Outside the living cell chlorophyll a is quickly converted into its derivates (SHUMANN& LORENZEN, 1975) which indicates that a large number of living algal cells, probably benthic diatoms, was present in the deeper layers of the experimental flats. Apparently the food uptake of Arenicola marina becomes adapted to this situation which is uncommon under natural conditions (CADBE, 1976). IV. GROWTH

EXPERIMENTS

ON DIFFERENT

DIETS

1. METHODS To test the growth possibilities of Arenicola on various food items, a number of potential food sources (DE WILDE, 1975) was mixed with sediment poor in nutrients (carbon content < 0.01%). Four diets were prepared consisting of bacteria, benthic diatoms, dried Ulva (as a controlled detritus) and natural rich sediment, respectively. Bacteria were separated with an ultra-centrifuge from cultures mainly consisting of a mixture of Desulfovibrio desulfuricans and Thiobacillus denitrijicans. Twenty grams of bacteria were mixed with 14 1 sand in which juvenile worms were grown. Benthic diatoms, in a natural mixture, were obtained from a nearby tidal mud flat and cultivated in the laboratory on a thin layer ( I 1 mm) of sand in sea water. After a few days sand and diatoms were collected to serve as food for young worms. Because of a technical failure no renewal of the sediment halfway the experiment was possible, therefore this experiment was ended a few days earlier than the other ones. As natural detritus not mixed with other potential food sources is difficult to obtain, a diet of dried fresh Ulva lactuca was preferred for this experiment. The Ulva was fragmentated and 100 g of the powder was mixed with 1.8 1 sand to form an experimental sediment. The natural superficial sediment layer was scraped off from a natural tidal flat. The layer consists of sand, clay, detritus, benthic microflora, bacteria and a11 kinds of other small organisms, and actually contains all of the earlier mentioned types of potential food. When funnels are present, Arenicola is supposed to feed upon this uppermost sediment layer. For the experiment it was mixed with sand in the proportion 1 : 2. These mixtures were offered to individual juvenile lugworms kept in small cuvettes (10 x 1 x 10 cm) as used by DE VLAS (1979). The

416

M. RIJKEN

cuvettes fitted just one Arenicola, and thus prevented waste of sediment. The cuvettes were placed in a container with running sea water of 10°C. At this temperature young worms still grow very well whereas mineralization processes by micro-organisms proceed relatively slowly (DE WILDE & BERGHUIS, 1979). The duration of the experiments was limited to avoid a significant decomposition of the food sources. Generally the experiments lasted 16 days. To avoid exhaustion of the substrate the upper 5 cm layer of the sediment was renewed halfway the experiment. Before and after the experiments the wet weight of the animals was measured (DE WILDE & BERGHUIS, 1979). The average initial weight of the worms was 0.2 to 0.3 g, with a range of 0.056 to 0.718 g. At beginning and end of the experiment the carbon content of sediment and faeces was measured with a Coleman 33 C-H analyser combining samples from cuvettes with the same sediment. The carbon that was still present in the faeces was thought to be a measure for the ability of Arenicola to digest the food. 2. FEEDING Growth bacterial

WITH BACTERIA

of the young Arenicola was considerable when fed with the mixture, and the faeces production was high (Table II).

TABLE

Growth

II

experiments with Arenicola marina specimens kept in separate boxes in which 4 different potential food sources were added to the inert sediment. Measurements

Number of animals at start at end Duration experiment (days) Sediment renewed Mean weight of worms (g) at start at end Increase in weight (%) (SD) Carbon content sediment (%) at start at end Carbon content faeces ( yb) first castings at end Faeces production

Benthic diatoms

Bacteria

(5) 5 13-14 no

(14) 13 16 yes

(17) 5 16 yes

0.2777 0.4562

0.3047 0.5505

0.2599 0.3750

91(&31)

55(&41)

63(&

10)

0.15 0.07

? 0.06 high

UlVZ

Surface sediment

(14) 11 16 yes 0.2210 0.5304 140( *67)

0.15 0.04

0.99 0.69

0.44 0.34

0.08 0.02 very high

? 0.28 very low

0.68 0.61 low

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UPTAKE

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ARENICOLA

About half of the organic matter present in the sediment was digested. This was still so at the end of the experiment when the carbon content of the substratum was reduced to low levels. 3.

FEEDING

WITH

BENTHIC

DIATOMS

In the young Arenicola fed with benthic diatoms faeces production was high from the beginning. The increase in weight was considerable despite the shorter duration of the experiment. However, at the end of the experiment it appeared that the utilization of the offered sediment was poor and there was some doubt whether the diatoms were digested indeed. For verification this was tested in an analogue experiment. Here after some days the proportion of chlorophyll a of the total chlorophyll present was determined (SHUMAN & LORENZEN, 1975) in sediment and faeces. In the original sediment values of 25 to 60% chlorophyll a were found indicating that a considerable amount of the diatoms was already dead. In the faeces however, the proportion dropped to only 2 to 504. Thus most diatoms were killed during the passage of the intestines of the lugworms. 4.

FEEDING

WITH

DRIED

ULVA

With the Ulua powder added as a food source defecation started only slowly and the castings were minute. Remains of Ulna could still be distinguished in the faeces. During the experiments 12 of the 17 worms died. On the surface of the 5 other cuvettes mould developed, indicating that mineralization had started. Towards the end of the experiment the production of faeces increased, but was always far below normal. The relatively low carbon content of the faeces may partly be caused by particle selection : a concentration of Ulna particles was observed near the bottom of the cuvettes. 5.

FEEDING OF

WITH

SUPERFICIAL

A NATURAL

TIDAL

SEDIMENT FLAT

Growth on rich natural sediment showed to be quicker than in any of the other feeding experiments, although the faeces production was rather low. Obviously this sediment was so rich that small amounts were sufficient to cope with the food requirements of the animals. The high carbon content of the faeces is ascribed to particle selection. Microscopic analysis of the faeces showed that the share of sand grains was much lower than in the original sediment.

418

M. RIJKEN V. DISCUSSION

AND CONCLUSIONS

In most cases at the investigated localities Arenicola marina makes use of its effective mechanism to obtain the nutrient-rich surface layer of the sediment for consumption. The field experiments confirm in broad outline the ideas of THAMDRUP (1935) and WELLS (1945), although in WELLS’ laboratory experiments the sand columns were wider than the columns found in the field by ZIEGELMAYER ( 1964) and in this study, but wider columns are also known from the western Wadden Sea (cj Fig. 1). The small diameter of the column found in the present experiments explains the quick appearance of the funnel content in the castings. It remains uncertain whether quick-sand columns can be present in situations where because of waves and currents or the nature of the sediment no funnels can develop, although this would seem likely. New is the fact that Arenicola can feed on the sediment at living depth without the use of a moving sand column as was found for the indoor experimental mud-flats. In that case the lower sediment layers contained sufficient amounts of food. Obviously conditions occur that Arenicola marina is not dependent on nutrient-rich surface layers and can refrain of the formation of funnels. This demonstrates a surprisingly high adaptability in the feeding behaviour of the worm. In the Wadden Sea, however, Arenicola will be as a rule dependent on the food sources present in the surface layer of the tidal flats. The question remains: what is good food for Arenicola? Tidal flat mire proper indeed induced an excellent growth of the juveniles investigated. Benthic diatoms as well as bacteria, also gave good growth at a much lower carbon content of the diet sediment. Low carbon values in the faeces showed that bacteria and diatoms were digested, as was also indicated by the results of BOON et al (1978). Digestion of diatoms is surprising because of the limited number of enzymes found for Arenicola. A decrease in chlorophyll a from ingested sediment to the faeces nevertheless confirms this result. When comparing benthic diatoms and bacteria, bacteria are indicated to be a better food because of a better growth, a higher faeces production, and a more complete utilization of the carbon content of the sediment. Grinded dead Ulva as an artificial detritus, gave poor growth results and a very low faeces production. The occurrence of some growth probably results from a starting mineralization and the deveIopment of microorganisms. Probably also natural detritus will be an uncertain source of food. Its main function may be that of a carrier of digestable microorganisms. Whether other than the investigated food sources play a part in the menu of Arenicola marina remains still unknown.

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The high sinking rates of the sediment in the quick-sand column, largely excludes the concept of found in the present experiments, gardening (HYLLEBERG, 1975) as the short stay of detritus in the sand column hardly allows the development of microorganisms. Particle selection will play a major part in the feeding behaviour of the lugworm. Its importance is illustrated in the growth experiments with mud flat surface sediment, in which the carbon content of the faeces exceeded the carbon content of the original sediment with more (KRUGER, 1959 ; than 50yb, in accordance with earlier observations JAKOBSEN, 1967; HYLLEBERG, 1975). VI. SUMMARY Marking with glass spheres, showed the lugworm to live over large tidal flat areas in J-shaped tubes of which the blind end is connected with the funnel in the sediment surface by means of a perpendicular sediment (quick-sand) column of only ca 5 mm wide and 10 to 20 cm long. When the worm ingests the sediment at the lower end of the column, the surface sediment of the funnel will reach the worm in only a few hours because of this small diameter of the column. In this way the worm utilizes the nutrient-rich material from the uppermost surface layers. A different feeding behaviour of the lugworm appeared in the laboratory where in an indoor tidal flat a homogeneous rich sediment was present. No funnels occurred in this situation; the animals ingest the sediment present at living depth. Feeding experiments showed good growth in Arenicola marina on sediments with an addition of benthic diatoms or bacteria. Both food items are really digested; the latter perhaps more effectively. Dead and minced Ulna lactuca, as an artificial detritus, showed a poor utilization. On sediments with this detritus the faeces production of the lugworms was poor, but increased when mineralization of the material started. Highest growth was observed with a rich sediment, collected from the surface of a natural tidal mud flat. VII. REFERENCES BAUMFALK, Y. A., 1979. Heterogeneous grain size distribution in tidal flat sediment caused by bioturbation activity of _4renicola nmrina (Polychaeta).-Neth. J. Sea Res. 13 (3/4): 428-440. BOHN, G., 1903. Observations biologiques sur les ArPnicokJ.-Bull. hlus. natn. Hist. nat., Paris 9: 62-73. BOON, J. J., W. LIEFKENS, W. I. C. RIJPSTRA, M. BAAS & J. W. DE LEEUW? 1978. Fatty acids of Desulfovibrio deszdfimhns as marker molecules in sedimentary

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