P. S . Z. N. I: Marine Ecology, 4 (3): 263-274 (1983) 0 1983 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0173-9565/InterCode: MAECDR

Accepted: 24.2. 1983

The Effect of an lnfaunal Suspension Feeding Bivalve Mercenaria mercenaria (L.) on Benthic Recruitment* DON MAURER ’

Southern California Ocean Studies Consortium, California State University, Long Beach, California 90840, U.S.A.

With 2 tables

Key words: Infaunal, suspension feeding, benthic recruitment.

Abstract. Animal-animal interactions we recognized as critical i n benthic invertebrate recruitment. The hypothesis tested was that benthic recruitment is more successful in low densities of infaunal suspension feeders than in high densities. Densities of young hard clams Mercenaria mercenuriu (L.) ranging from 82.5 to 3301n-~ were placed in defaunated boxes of sand. The mean number of species, mean number of individuals, mean wet weight biomass, mean species richness and mean dominance index per sample were calculated per density of hard clams from May to October. The experiment did not support the hypothesis. It was concluded that feeding behaviour of dense populations of M. mercenariu did not preclude successful recruitment of other benthic species.

Problem

Recruitment of benthic marine invertebrates includes such variables as the reproductive condition of adult populations, physical oceanographic processes, geologic oceanographic processes, predator-prey relations, and non-predator biological interactions. This study focuses on the latter variable.

According to SCHELTEMA (1974) biological interactions are far more subtle and important than non-biological ones in determining settlement of attached and benthic marine invertebrates. This view is espoused in recent research dealing with animal-animal interactions. WOODIN (1976) stated that adult-larval interactions maintained shifts in infaunal abundance patterns. She proposed that sharp boundaries observed among dense assemblages of infaunal organisms are due to interactions among established infaunal individuals and settling larvae. WOODIN identified three types of infaunal assemblages: 1. deposit feeders, 2. suspension feeders, and 3. tube builders. The studies of WILSON (1980) and LEVIN (1981) on the effect of adult deposit feeding polychaetes on larval survival of other species of polychaetes and of LEVINTON & STEWART (1982) on the effect of deposit feeding gastropods on an oligochaete species support the hypothesis of WOODIN.

* Southern California Ocean Studies Consortium Contribution No. 20.

U.S. Copyright Clearance Center Code Statement: 0173-9565/83/04034263$02.50/0

264 MAURER

A corollary of WOODIN'S hypothesis was that larval settling was excluded in dense infaunal assemblages of filter feeders. WILLIAMS (1980) concluded that dense assemblages of adult Manila clams (Tapes japonica [DESHAYES]), did not prevent the settlement of clam larvae. In contrast BEST (1978) found that increased densities of the suspension feeder Mercenaria mercenaria (L.) resulted in decreased recruitment of other molluscan species with pelagic larvae.

My observations in estuarine areas containing M. mercenaria, showed that such assemblages were not markedly reduced in density or numbers of species (MAURER, 1977). Instead it seems that burrowing, feces production, and shell accumulation by M. .rnercenaria actually enhances larval settling and coloniza- tion producing a diverse and abundant fauna. Based on this impression, I decided to test the hypothesis that colonization of bottom dwelling macroin- vertebrates is more successful in low density than in high density populations of infaunal suspension feeders. Benthic recruitment was assessed by calculating the mean number of species, mean number of individuals, mean wet weight biomass, mean species richness, and mean dominance index per sample recorded through a sampling period in various densities of the hard clam Mercenaria mercenaria.

Material and Methods

1. Size and tank conditions of microcosm The study was conducted from May to October, 1978, in a greenhouse off the Broadkill Rivcr, Delaware, USA (38"47'30"N and 75"95'5XW), a secondary tributary emptying into Delaware Bay. Quartz and feldspar dune sand (median sediment size 0.5U-0.75 mm) was collected and placed in four, wooden, epoxy lined boxes (1 18 cm long, 102cm wide) locatcd in a concrete tank (3.35 m long, 2.751~1 wide, 0.86m deep). Two boxes were I6cm deep and two were 27cm deep. Dune sand was evenly distributed throughout the four boxes to a depth of 15cm. Dune sand has been used previously as defaunated sediment in other experimental studies (MCINTYRE, 1977).

One hundred hatchery reared clams (ic length 2.2cm k SD 0.61) arranged in 10 rows and 10 columns were placed in one half of a box yielding a density of 330 and 50 clams arranged in 10 rows and 5 columns were placed in the other half of the box (165 rn-'). This arrangement was repeated in a second box. Seventy-five clams arranged in 10 rows and 7 columns were placed in one half of the third box (247.5 m-l) and 25 clams arranged in 5 rows and 5 columns were placed in the other half of the box (82.5 m-'). This arrangement was repeated in the fourth box. To obtain some impression of natural densities of hard clams in the Delaware Bay region, 1-2 adult clams m-? were estimated for Rchoboth Bay, Delaware, a nearby lagoon (MAURER. unpiihlished data).

drawn from rhc Broadkill River and pumped unfiltered dircctly into one end of the concrete tank. Boxes were situated so that at least 50 9'0 of the densities were exposed to initial flow of seawater and 50 % of all densities were distributed to the two different box depths. Water ran continuously for the duration of the experiment (May &October 9) with interruptions for mainte- nance of the system (two times) or sampling (five times). The water ran the length of the tank and was drained through an overflow pipe. Within 10 minutes of being covered by seawater, the clams burrowed into the sediment and extended their siphons. The boxes in the tank were normally covered by 85 cm of running seawater except during maintenance and sampling.

Seaw,ltci

2. Samples and sampling During sampling (June 2, June 30, August 4, September 7 , October Y), water was shut off and carefully drained to the sediment-water interface so as not to disturb surface features. Four samples were taken in each half of each box per sampling period with a PVC core (18cm long and 5.1 cm

Effect of Mercenaria mercenaria on benthic recruitment 265

diameter) yielding a total of 160 samples. Samples were collected several cm apart in a regular linear sequence to restrict sediment disruption from slumping to a small area. This approach made it possible to sample fresh surfaces for the duration of the experiment. No cores were taken from a surface previously sampled. Samples were placed in labelled bottles and were fixed immediately with buffered 10 % formalin. Differential preservation precluded using all 32 cores per sampling period in analyses. Accordingly the degrees of freedom in the A N O V A vary with sampling period.

In the laboratory the samples were elutriated several times and then sieved through a 0.5 mm mesh sieve. A smaller sieve size would probably yield a greater proportion of early stages but this would also increase taxonomic problems. The 0.5 mm mesh was deemed the best compromise between retention of small species and early stages and taxonomic facility. The residue on the screen was flushed with tap water to remove excess formalin and then stained with rose bengal. Samples were transformed to 70 % isopropanol prior to processing.

Samples were sorted into major taxonomic groups (polychaetes, molluscs, amphipods. other crustaceans, miscellaneous) under a dissecting microscope. A n estimate of wet weight biomass (8) was obtained by blotting samples dry and measuring them on a balance sensitive to 0.0001. Planted hard clams were not weighed. Animals were identified to species whenever possible and were counted. Species composition, the number of species, density, wet weight biomass, species richness and dominance were determined as measures of community structure. Species richness (SR) was measured by SR = - - I ; where S = number of species and N = total number of individuals per sample. The dominance index (DI) according to MCNAUCHTON (1967) was computed by DI = A

second most abundant species, N = total number of individuals per sample.

In N

-. Bq where A = number of individuals of most abundant species, B = number of individuals of

3. Environmental setting During the experiment surface and bottom temperatures, salinity, dissolved oxygen, Eh , and p H were monitored. Since there were occasional differences between surface and bottom, the range of bottom valucs will be brietly described. From May to October bottom measurements of the environmental variables ranged from 15 to 26"C, 17 to 30%, 2.4 to 7.0ppm. + 120 to + 200mV, and 6.5 to 8.9. These values were very similar to those in ambient water of the Broadkill River. Since suspendcd matter is heavy in the Broadkill, detritus (marsh grass and benthic diatoms) accumulated on the defaunated dune sand. Suspended mutter contained 70 to 100 % silt-clay. Between 2-4cm of fine grain sediment was deposited on the boxes during the experiment.

Results

1. Species composition

Approximately 63 species were identified. This is a conservative estimate because some organisms were identified at the phylum (Nemertea) and class (Ofigochaeta) level and probably contained several species each. Moreover, the Polychaera contained some specimens identified only to family (e. g . Cir- ratulidae and Paraonidae) which also probably contained several species.

Annelida contained 46.1 % (29 spp.) of the total number of species, Arthro- poda 28.5 % (18 spp.), Mollusca 15.8 % (10 spp.), and miscellaneous 9.6 % (6 species). Annelida were represented by polychaetous and oligochaetous annelids, arthropods by amphipod crustaceans, and molluscs by pelecypods. The category miscellaneous contains Cnidaria, Nemertea, and Chordata. The cnidarian Nematostella vectensis (CROWELL), the pelecypods Mytilus edulis (L.) and Tellina agilis (STIMPSON), the polychaetes Eteone heteropoda (HARTMAN), Polydora ligni (WEBSTER), Spiophunes b0mby.u (CLAPAREDE), oligochaetes, and the crustaceans Corophirim tuberciilatum (SHOEMAKER), Crangon septemspinosa (SAY) and Neomysis americana (SMITH) were the first colonizers.

266 MAURER

In addition to the early colonizers, the polychaetes Capitella capitata (FA- BRICIUS), Cistena gouldii (VERRILL), Mediomastw ambiseta (HARTMAN), and Streblospio bendicti (WEBSTER) occurred frequently throughout the remaining collecting period. The number of species per collection date ranged from 14 to 50. There was a marked increase in number of species between September (29) and October (50).

Table 1. Abundance of macrofauna in the samples from June through October.

June June Aug. Sept. Oct . Total Top ten species 2 30 4 7 9

Capitella capitara Coropliiurn

rubercirlarum Mediomastus

ambiseta Oligochaeta Edoka triloba Gummarus

fnucronalus Slreblospio benedicri Tellina agilis Polydora ligni Myti1u.r edulis

66

8 3

58 1 1

5

29 4 12 s 3 21

5

26

26 11

4 3

10 5

394

2228

696 23 72

116 50 37 52

2175

269

87 I 32 1

74

2 63 44 34 5

2534

165 1 357 15 1

118 117 117 116 34

Total number of

top ten species individuals among 28 191 90 3668 386 1 7838

Total number of individuals among 50 225 106 3752 4234 8367

all species

Relative abundance of top ten species (% of total) 56.0% 84.8% 84.9% 97.7% 91.1 % 93.6%

Based on abundance through all sampling periods the top ten taxa comprised 93.6% of the total number of individuals (Table 1). The top ten included Capitella capitata, Corophium tuberculatum, Mediomastus ambiseta, Oligochaeta, Edotea triloba (SAY), Gammarus mucronatus (SAY), Streblospio benedicti, Tellina agilis, Polydora ligni, and Mytilus edulis. These species comprised 56.0% of the total number of individuals at the beginning of the study but 97.7 % the second to last month. Based on wet weight biomass M. edulis, T. agilis, C. capitnta, M . ambiseta, C. tuberculatum and Molgula manhat- tensis (DEKAY) were dominant species.

Effect of Mercenuriu mercenaria on benthic recruitment 267

2. Quantitative analysis

The mean number of species, mean number of individuals, mean wet weight biomass, mean species richness and mean dominance index per sample in relation to the densities of hard clams were determined from June to October (Table 2).

Table 2. Analysis of variance of mean number of species ( S ) , individuals (N), wet weight biomass (8) (BM), species richness (SR), and dominance index (DI) of macrobenthic invertebrates per sample in relation to the numher of hard clams per mz.

Clam Density 82.5/mL 165/m2 247.5/m2 330/m2 F Values

June 2

S 1.5 1.2 2.0 2.4 F (3.16) = 2.89 N 2.0 1.6 3.0 3.14 F (3.16) = 0.95 BM 0.004 0.004 0.012 0.007 F (3.16) = 0.44 SR 0.36 0.29 0.67 1.19 F (3.16) = 1.91

June 30

s 2.4 1.57 1.28 3.5 F (3.23) = 2.97 N 26.8 2.14 2.28 7.5 F (3.23) = 1.3 B 61 0.030 0.030 0.005 0.270 F (3.23) = 1.3 SR 0.78 0.43 0.11 1.29 F (3.23) = 2.39 Di 0.18 0.071 0 0.45 F (3.23) = 3.59*

August 4

s 2.8 2.57 2.6 2.57 F (3.20) = 0.036 N 3.6 3.57 6.0 4.71 F (3.20) = 0.46 BM 0.045 0.101 0.068 0.680 F (3.20) = 2.54 SR 1.43 1.07 1.08 0.92 F (3.20) = 0.44 Di 0.38 0.25 0.40 0.27 F (3.20) = 0.32

September 7

s 8.87 5.75 9.14 7.87 F (3.27) = 0.32 N 117.1 99.0 210.2 68.8 F (3.27) = 3.48' BM 0.172 0.312 0.32.5 0.274 F (3.27) = 0.57 SR 1.67 1.72 1.57 1.69 F (3.27) = 0.11 DI 0.52 0.82 0.84 0.74 F (3.27) = 2.07

October 9

s 8.5 10.5 11.2 14.0 F (3.12) = 1.81 N 86.7 244.2 86.0 198 F (3.12) = 5.09" BM 0.091 0.731 0.157 3.109 F (3.12) = 2.72 SR 1.86 1.80 2.35 2.48 F (3.12) = 1.15 Oi 0.71 0.81 0.60 0.67 F (3.12) = 1.43

~~~

Multiply number of individuals and wet weight biomass (9) by 490 to obtain number of individuals m-' and wet weight biomass n r 2 . * significant at 0.05

268 MAUIWI

The mean number of individuals ranged from 1.6 (784m-') in June to 244.2 (119,658m-2) in October (Table 2). The increase in numbers was primarly due to the amphipod Corophium tuberculatum and the polychaetes Capitella capitata and Mediomastus ambiseta. Based on ANOVA, the mean number of individuals in September and October differed significantly at different densities of hard clams. Based on a posteriori test - TUKEY'S w - procedure (SOKAL & ROHLF, 1969) the mean number of individuals in September in clam density 165 m-? (E = 99) was significantly lower than in treatment 247.5m-' (i = 210.2). The same test applied to the data of October yielded no significant differences.

Mean wet weight biomass ranged from 0.0046 g (2.28 g m-') in June to 3,109 g (1,523.57g rn-2) in October (Table 2). Increase in wet weight biomass was primarily due to amphipods, polychaetes, the bivalve Mytilus edufis, and the tunicate Molgula manhattensis. Based on ANOVA there were no significant differences in mean wet weight biomass between the different densities of hard clams within a sampling period.

Mean species richness ranged from 0.29 in June to 2.48 in October (Table 2). Based on ANOVA there were no significant differences in mean species richness between density of hard clams within a sampling period.

Mean dominance index ranged from 0 in June to 0.84 in September (Table 2). Mean dominance for each treatment on June 2 was not computed because dominance was 0 in so many replicates. Based on ANOVA mean dominance in June 30 was significantly different between densities of hard clams.

In summary there were few significant differences in mean measures of recruitment between density of hard clams per sampling period. When signifi- cant differences occurred (mean number of individuals in September and October), the highest mean numbers were not necessarily related to the lowest density of hard clams.

Discussion

1. Number of species The experiment was generally comparable to similar studies and natural condi- tions of Naragansett Bay macrobenthos. From mid-March to mid-September, 35 and 36 species, respectively, were recorded at the termination of the experiments (GRASSLE et nl., 1981) and 22 species were recruited to defaunated boxes of mud placed in Long Island Sound between July 1972 and June 1973 (MCCALL, 1977). WATLINC (1975) identified 56 species in a seasonal study from a mud and detritus community in Rehoboth Bay, Delaware and WHITLATCH (1977) reported 26 species from muddy-sand stations in Barnstable Harbor. The number of species recorded in this experiment (63) is comparable to those of other microcosm studies and natural environments with similar temperature, salinity, depth and substratum.

2. Species composition The present experiment produced species assemblages very similar to natural estuarine systems (Table 1). These species have been collected frequently and

Effect of Mercenaria mercenaria on benthic recruitment 269

abundantly in natural local conditions (WATLING, 1975; MAURER et al., 1978, 1979; HAINES & MAURER, 1980; BURBANCK, 1981), north (WHITLATCH, 1977; SANDERS et al., 1980) and south of the Delaware Bay area (MARSH, 1973; ORTH, i973), in defaunated sediment (MCCALL, 1977; GRASSLE & GRASSLE, 1974) and microcosms (OVIATT et ul., 1977; GRASSLE et al., 1981).

3. Density

Maximum density recorded here was 264,110 m-’ in September. Estimates of maximum mean density per month reached 75,313m-’ in October (Table 2). These values, can be compared with estimates (0.30 and 0.50mm mesh) of 45,00Om-’ in a Naragansett Bay microcosm (GRASSLE et al., 19Sl), 470,115 m-l (0.297mm mesh) for defaunated sediment placed in Long Island Sound (MCCALL, 1977) and greater than 400,00Om-’ (0.297mm mesh) for azoic sediment placed in Wild Harbor, Massachusetts (GRASSLE & GRASSLE, 1974). In Rehoboth Bay, Delaware maximum density estimates of 270,360 m-’ (0.250 mm mesh) were obtained from a mud-detritus bottom (WATLING, 1975) and 106,871 m” (1.0mm mesh) from mixed soft bottoms (sand-mud) (MAURER, 1977). Density estimates in Long Island Sound (0.297 mm mesh), Charlestown Pound, Rhode Island (0.50 mm mesh), and the Pocasset River, Massachusetts (0.297 mm mesh) were 57,457 rn-’, 30,000 m-:, and 67,00Om-?, respectively (MCCALL, 1977; PHELPS, 1964; SANDERS et ul., 1965) and portions of Chesapeake Bay in eel grass beds and in muddy-sand yielded densities of i1,00Om-’ (1.Omm mesh) and 21,94Om-’ (1.0mm mesh) (ORTH, 1973; MOUNT- FORD et 01.. 1977). These data indicate that the density estimates recorded in the present experiment were well within the range of values recorded from other microcosms and natural environments.

4. Biomass

The maximum estimate of wet weight in the present study was recorded in October (3,410g m-?), and was due to the presence of several tunicates (iMolgz~iu manhattensis). Maximum wet weight biomass (mean of a11 samples) recorded here was also in October (1523g m-?) (Table 2). On sand, mud and muddy-sand in Rehoboth Bay, Delaware estimates of wet weight biomass were 5.09g m-?, 31.7g m-? and 70.4g m-l, respectively (MAURER, 1977). Near the mouth of Delaware Bay in muddy sand an estimate of 1.84 g m-2 was obtained (MAURER el al., 1973). Species composition and density recorded in the present experiment were comparable to those for natural environments. However, biomass values in the microcosm were generally higher due to tunicates and blue mussels. These organisms are not considered characteristic members of infaunal communities. When their biomass was accounted for, the remaining biomass values were comparable to those from other natural infaunal assemblages. Exclusive of major predators it is concluded that the conditions in the present study were similar enough to natural conditions to extrapolate the results of the experiment to natural settings.

270 MAURER

5. Hypothesis

The relative efficiency of suspension feeding organisms to filter particulates from the water column is well documented (JBRGENSEN, 1966) and was meas- ured for Mercenaria mercenaria by RICE & SMITH (1958). The effect of filtering on the survival of settling larvae has been reported for epifaunal (KORRINGA, 1941) and infaunal organisms (KRrsTENsEN, 1957). MACKENZIE (1981) showed experinlentally that even when larvae were ingested by benthic invertebrates they could easily escape loosely aggregated pseudofaeces. When larvae pass through the gut of another organism, some mortality can be expected by entanglement in mucus and faeces (MILEIKOVSKY, 1974; MACKENZIE, 1981).

A mass of clam siphons would seem to be a hostile site for successful settlement of larvae (SINGARAJAH, 1969). However, intensive gregarious settling of larvae of estuarine and marine pelecypods, polychaetes, and barnacles can occur on their adult populations (KNIGHT-JONES, 1951, 1953; HIDU, 1969). Problems for settling larvae to avoid being ingested by filter feeding resident infauna must be equally hazardous (WILSON, 1980).

According to WILLIAMS (1980) significantly more clam larvae to Tapes japonica settled in areas with no or moderate abundances of adults than in areas with high adult clam abundances. However, even in dense assemblages of adults, larvae were not prevented from settling. WILL~AMS rejected WOODIN’S (1976) hypothesis.

MACKENZIE (1981) conducted laboratory tests to determine whether various invertebrates, abundant on beds during the settling season of oyster larvae, could consume or otherwise destroy pediveliger larvae (300 microns avg.). Adult Mercenaria rnercenaria ingested a small proportion of the larvae taken into their mantle cavitites, while most larvae were ejected. MACKENZIE (1981) concluded that the feeding of 11 species of benthic invertebrates, including the oyster, was only a minor factor in limiting the settling density of oyster spat on beds along the Connecticut coast.

BEST (1978) proposed that suspension feeding by Mercenaria mercenaria would necessarily be directed towards pelagic larvae and deposit feeders. In a series of field experiments in which the densities of adult hard clams were artificially increased, BEST found that pelecypod species that had a planktonic development were significantly reduced but polychaetes (excepting Heteromas- tus filiformis CLAPAREDE) and molluscan brooders were unaffected by the presence of M . mercenaria. Negative interactions by adult M . mercenaria via suspension feeding on settling larvae were not felt by the entire reproductive guild of planktonic larvae, but were selectively affecting each taxonomic group. In the present study two of the top ten taxa (Table 1) Tellina agilis and Myfilus edulis display planktonic development. Number per square meter of these species was relatively even among the four densities of hard clams. Three of the top ten species were brooders Edotea triloba, Gammurus mucronatus and Corophium tuberculatum and there was no reduction in their density with increased bivalve density. The remaining five taxa, four polychaetes (Capitella capifata, Mediomastus ambiseta, Streblospio benedicti, Polydora ligni) and oligochaetes again showed no relationship to clam density; some instances of high densities of these taxa were associated with high densities of clams and in

Effect of Mercenarin mercenaria on benthic recruitment 27 1

others low clam density. Thus in contrast to BEST (1978) my study indicated no particular taxonomic selectively from interaction with M. mercenaria.

WOODIN (1976) focused her attention on adult-larval relations. In the present experiment, young hard clams (6-9 month old) were used to serve as infaunal suspension feeders. The initial size of the clams averaged 2.2cm. The data showed that relatively high densities of infaunal suspension feeders, in this case 247.5 to 330 hard clams m+, did not preclude extensive recruitment of benthic invertebrates. The highest number of individuals was 539 (264,110 m-2) recorded in September in a density of 247.5 clams m-’. The results of this study and others (WILLIAMS, 1980; MACKENZIE, 1981) do not support the hypothesis posed at the outset. BEST (1978) used adult hard clams capable of generating higher filtering capacity than the juveniles used here. Under certain conditions her work supports the hypothesis of significant reductions in specific taxa. WILSON (1981) demonstrated that the relative and/or absolute sizes of the interacting species plays a crucial role in determining the outcome of inter- specific or inter-functional group interactions. It was concluded that feeding behaviour of dense populations of infaunal suspension feeders did not preclude significant recruitment of a diverse (63 species) and dense (2 = 784 to 119,658 m-?) benthic fauna. This relationship persisted at least five months after initial settlement and development of colonizers. It is recognized that as each species is added to the assemblage the permutation of potential biological interactions increases. As a result, the relatively straightforward effect of a single species of suspension feeders on benthic recruitment becomes consider- ably complex with the addition of diverse suspension feeders, deposit feeders and carnivores.

If dense infaunal assemblages of filter feeding bivalves, specifically Mer- cenariu mercenaria, do not initially exert strong negative interactions on settling larvae, what are the effects on shifts of infaunal abundance patterns? According to WooDrN (1976) dense populations of suspension feeders should be dominated by specific age classes due to failure of settlement once a large adult population has been attained. The size class or age class phenomenon observed in natural populations is evidence for this position. WOODIN’S view also implies a gradual reduction in the size of the dominant species population until the next big settlement and concomitantly a reduction in abundance of all the benthic populations.

The sampling methods used to study year classes of commercial species such as Mercenaria mercenaria, Spisula solidissirnu (DILLWYN), and Artica islandica (L.) are not always designed for efficiently assessing associated infauna. One’s view of the effect of dominant suspension feeding bivalves on larval settlement in these species is consequently biased and primarily restricted to conspecific larvae not reflecting total meroplankton. Quantitative examination of commer- cial beds of M . mercenuria in an enclosed bay (MAURER, 1977) and beds of S. solidissirnu and A . islandica on the Continental Shelf (BOESCH et al., 1977; MAURER & LEATHEM, 1981) has shown an abundant and diverse fauna. Certainly other factors ( e .g . marine geology and currents) have contributed to these assemblages but the year class of suspension feeding bivalves has not precluded their formation, development and maintenance.

272 MAURER

Summary

Four densities of juvenile M. mercenaria were established in a microcosm to determine their effect on benthic recruitment. During the experimental period (May-October) sixty-three species were identified from the microcosm. Mean density and biomass ranged from 786119,658 m-’ and from 0.004-1,523.57 g m-2, respectively. Based on species composition, density and biomass conditions in the microcosm were similar enough to natural conditions to extrapolate the results of the experiment to natural settings. It was concluded that feeding behaviour of dense populations of M. mercenaria did not preclude successful recruitment of other benthic species.

Acknowledgements

Mr. WAYNE LEATHEM. Mrs. RENEE MCGINNIS, Ms. CINDY LUSCHOUS, Mrs. KAREN PENNACHI and Ms. EDNA MILLER processed the samples upon which the study was based. Thc author is gratcful to Drs. CHARLES PETERSON, ROBERT VIRNSTEIN, SARAH WOODIN, ROBERT HESSLER and an anonymous reviewer who critiqued earlier drafts. Ms. BARBARA BEST generously sharcd unpublished data.

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