Enhanced Larval Settlement of the Hard Clam Mercenaria Mercenaria by the Gem Clam Gemma Gemma
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MARINE ECOLOGY PROGRESS SERIES Vol. 99: 51-59.1993 Published September 2 Mar. Ecol. Prog. Ser. 1 Enhanced larval settlement of the hard clam Mercenaria mercenaria by the gem clam Gemma gemma In-Young ~hn',Robert Malouf 2, Glenn ~opez~* 'Polar Research Center, Korea Ocean Research and Development Institute (KORDI), Ansan, PO Box 29, Seoul 425-600, Republic of Korea 'Oregon Sea Grant Institute, Oregon State University, Corvallis, Oregon 97331, USA 3~arineSciences Research Center, State University of New York, Stony Brook. New York 11794, USA ABSTRACT: The presence of dense assemblages of Gemrna gernma enhanced settlement of Mercenaria mercenaria larvae in laboratory experiments. There was a tendency toward increased set- tlement of hard clam larvae with increasing density of gem clams. Microscopic examination showed that increased settlement did not result from trapping or entanglement in sticky secretions from gem clams. Enhancedsettlement in the presence of gem clams was persistent in both gently aerated and re- circulated waters, and in both sand and muddy sand, suggesting that M. rnercenaria larvae could rec- ognize settlement cues associated with G. gemma in various habitats and hydrodynamic conditions. Preferential settlement in sediment previously exposed to gem clams indicates that gem clams alter sediment in some way to make it attractive for larval settlement. In addition, increased settlement in the presence of empty gem clam shells in sediment suggests that settlement cues are properties of the shell, although the nearly significant settlement with gravel particles suggests that physical cues, such as grain size and bottom roughness, may play a role. Thus, this study demonstrates dense assemblages of small suspension feeders can enhance larval settlement of another bivalve species. INTRODUCTION ers have reported sharp declines within a few months after settlement in the density of hard clams (Carriker Abundance and distribution of the hard clam 1959) and other infaunal species (Thorson 1966, Muus Mercenaria mercenaria have been extensively studied 1973, Shaffer 1978, Moller & Rosenberg 1983, Lucken- for the appropriate management of this commercially bach 1984, Jensen & Jensen 1985). These declines important species (Carriker 1961, Hibbert 1976, 1977, have been attributed to various biological and physical Walker & Tenore 1984).Most work, however, has been factors (reviewed by Thorson 1966). focused on juvenile and adult stages. Due primarily to Established adult infauna may affect recruitment difficulties in sampling, identification, and experimen- of hard clams and other infaunal species through tal manipulation, there have been relatively few stud- ingestion or filtration of larvae, sediment reworking, ies on settlement and early post-settlement stages competition and predation. Some studies have demon- (Peterson 1986, Butman et al. 1988, Wilson 1990). strated negative intra- and inter-specific adult-larval Larval recruitment of Mercenaria rnercenaria may interactions on settling or newly settled infaunal play an important role in determining spatial patterns species (Williams 1980, Wilson 1980, Brenchley 1981, and abundance of adult populations. Many research- Levin 1981, Peterson 1982, Gallagher et al. 1983, Luckenbach 1984, Tamaki 1985, Woodin 1985). Large suspension-feeding bivalves have been impli- Addressee for correspondenct: cated in the inhibition of recruitment of planktonic O Inter-Research 1993 5 2 Mar. Ecol. Prog. Ser 99: 51-59, 1993 larvae of benthic organisms. Woodin (1976) predic- MATERIALS AND METHODS ted that infaunal species would not attain their highest densities among densely packed sus- Four to five-day-old Mercenana mercenaria larvae pension-feeding bivalves. Field experiments tested were obtained from a hatchery company (Bluepoints Woodin's prediction, demonstrating negative effects Co., Inc.) and cultured in the laboratory until they be- of dense adult population on settlement of their spat, came competent to settle. Gemma gemma (2 to 5 mm but even in dense assemblages, larvae were not in shell length) were collected from the intertidal zone prevented from successful recruitment (Williams 1980, of Flax Pond, a Spartina salt marsh on the north shore Peterson 1982). In other cases, adults did not affect of Long Island, New York, USA. Both bivalve species larval settlement (Maurer 1983, Hunt et al. 1987, Black were acclimated to experimental conditions for at least & Peterson 1988). Dense assemblages of small (few 3 d. They were fed Isochrysis galbana (clone T-iso, mm) suspension feeders such as Gemma gemma have 4 pm) during acclimation and experimental periods. also been implicated in controlling larval recruit- Algal concentration as determined by Coulter Counter ment. The gem clam, which grows up to 5 mm in shell (Model TAII) was maintained at 0.5 to l X 105 cells length, is one of the most abundant infaunal species m[-'. in shallow estuarine waters (Bradley & Cooke 1959, Medium-sized (250 to 500 pm) sand was used as sub- Sanders et al. 1962, Sellmer 1967, Green & Hobson stratum. This size range of sand was selected because 1970, Maurer et al. 1978, Woodin 1981, Thomson 1982, larval or newly settled hard clams (200 to 500 pm) can Botton 1984). Adult densities often reach 105 ind. m-2 be easily extracted with repeated vortex mixing and in summer months (Bradley & Cooke 1959, Sanders et decantation. Sand was dried at room temperature for al. 1962, Sellmer 1967, Green & Hobson 1970, several weeks and was soaked in filtered seawater for Thomson 1982). It shares with Mercenaria mercenana several days prior to use in experiments. much of the same habitat and geographic range Illumination (12L: 12D) was by fluorescent light and the 2 species frequently CO-occur, parti- (Daylight) equipped with a dimming unit to simulate cularly in sandy sediments (Sellmer 1967, Green & natural crepuscular periods. Dawn and dusk periods Hobson 1970, Greene 1981). It has very short siphons lasted 100 min each. Daytime light intensity was ap- and feeds mostly at the sediment-water interface proximately 30 yE m-2 S-', which was similar to sum- (Sellmer 1967).Bradley & Cooke (1959) and Sanders et mer intensity at 1 m depth of Great South Bay, Long al. (1962) observed the inverse relationship between Island (Monteleone 1988). Water temperature (20 "C) abundance of G. gemma and other suspension-feeding and salinity (26 %o) were kept constant throughout the bivalves, and suggested that dense G. gemma popula- experimental period. Unless otherwise stated, sea- tions inhibit the recruitment of other suspension-feed- water was filtered through a 0.22 pm filter. ing bivalve species by outcompeting the tiny spat for Upon completion of an experiment, sediment con- food at the time of settlement. Therefore, C. gemma taining clams was collected by siphoning, preserved was chosen in this study as a representative infaunal with buffered 5 % formalin, and stained with rose ben- species that may affect the early recruitment of hard gal. Gem clams were sieved out with a 1 mm sieve and clams. hard clams were extracted by repeated high-speed Competitive exclusion of hard clam larvae by vortex mixing (5 times, 15 S each time) followed by de- densely packed gem clams could conceivably be ex- cantation. No more than 5 cm3 of sediment was pro- pected during settlement. Spawning of Mercenana cessed each time in a 50 m1 plastic centrifuge tube. A mercenaria occurs during summer months (in mid- and capture efficiency experiment determined that 195 % North Atlantic), when Gemma gemma density reaches of hard clams were extracted with this method. Clams the highest values in temperate waters. The planktonic preserved alive and those preserved dead were larval stage lasts 6 d to several wk depending on water counted separately with the aid of a dissecting micro- temperature. A M. mercenaria larva is approximately scope; only the former were considered for statistical 200 ,pm in shell length at the time of settlement analysis. ANOVA and multiple comparisons among (Loosanoff 1959). means (Sokal & Rohlf 1981) were used to test for statis- In this study, the effect of Gemma gemma on larval tical significance of each experimental factor settlement of Mercenaria mercenaria was determined Assumptions for ANOVA were tested and transforma- in various experimental conditions which would be ex- tions were performed when necessary. perienced by hard clams in the natural environment, Larval settlement experiments. The purpose of these and an example of a positive interspecific adult-larval experiments was to determine whether Gemma interaction was demonstrated. Furthermore, settle- gemma affects Mercenana mercenaria larval settle- ment-inducing factors associated with G. gemma were ment. Experiments were conducted in a 7.8 1 Plexiglas examined.. aquarium (27.5 x 19 X 15 cm) filled with seawater to a Ahn et a].. Enhanced larval settlement of Mercenaria mercenaria depth of 13 cm. The experimental unit was a small well mll added to aerated water. Three replicate wells (3.6 cm in diameter, 1 cm depth) of a 6-well culture were run for each of the 4 treatments. dish (12.7 X 8.5 X 1.7 cm) (Falcon). Expt 3: effectin sand and muddy sand: This experi- Effect of Gemma gemma on Mercenaria mercenaria ment was conducted to determine the separate and was studied for a wide range of G. gemma densities. combined effects of Gemma gemma and sediment type Because of seasonal variation in body size, numbers of on Mercenaria mercenaria larval settlement. We com- G. gemma for low (27 to 50 clams per 3.6 cm diameter pared clean sand and muddy sand, 2 sediment types in well) and high (110 to 200 clams well1)densities were which both hard clams and gem clams commonly determined on the basis of total wet weight (-25 mg occur. Muddy sand (containing approximately 10 % cm" for low and -100 mg cm2 for high density). mud and 1 %organic matter on a dry weight basis) was Densities of gem clams were within the range occur- prepared by mixing clean sand with mud (< 63 \im) col- ring in nature (Bradley & Cooke 1959, Sanders et al.