Hard clam, continued 



and large particles are rejected. The rejected material 

 is voided as pseudofeces through the inhalant siphon. 

 The size range of particles ingested changes as the 

 hard clam grows (Riisgard 1988). Food items include: 

 marine diatoms, naked flagellates and other phytoplank- 

 ton, protozoans, micro-crustaceans, larvae of other 

 mollusks, rotifers, bacteria, and other zooplankton 

 (Belding 1912, Loosanoff and Davis 1963, Eversole 

 1987). 



Biological Interactions 



Predation : Predation is an important natural control of 

 hard clam populations, and its impact is felt by all size 

 classes (Killam et al. 1992). Blue crabs are a major 

 predator of hard clams (Craig et al. 1988). Arnold 

 (1 984) demonstrated the effects of blue crab predation 

 in different substrates, with predation rates being higher 

 in sand and sand/mud substrates. Clams greaterthan 

 40 mm SL were not consumed, even by large crabs. 

 Other predators include gastropods (oyster drills (Thais 

 sp.), moon snails (Polinices duplicatus and Lunatia 

 heros), and whelks (Busycon sp.)), starfish, stone 

 crabs and other xanthid crabs, skates and rays, various 

 bony fishes (sciaenids, puffers, flounders), and birds 

 (Craig and Bright 1 986, Craig et al. 1 988, Bisker et al. 

 1989, Killam et al. 1992). The fish species feed on 

 juvenile seed clams, and in localized areas, skates and 

 rays may be important predators (Killam et al. 1992). 

 The importance of fish predation is minor, however, 

 when compared with that of invertebrate predators. 

 Starfish prey on both juvenile and adult hard clams. 

 Small clams are attacked by individual starfish, but 

 larger clams (>50 mm shell length) are usually at- 

 tacked by several starfish. Several species of shore- 

 birds prey on clams and other bivalves, however, their 

 influence is restricted to hard clams exposed in the 

 intertidal area. Herring gulls have been observed 

 capturing hard clams, flying them up, and dropping 

 them onto hard surfaces to break them open. Grass 

 beds may serve as refuges from predation (Craig and 

 Bright 1986, Coen and Heck 1991), although it has 

 been suggested these areas can have higher preda- 

 tion rates than bare areas (Coen and Heck 1991). 



Factors Influencing Populations : Recruitment success 

 and predation are two of the factors most limiting to 

 large populations in the Gulf of Mexico. The sub-lethal 

 effects of siphon nipping by predators is known to 

 impact growth (Coen et al. 1994). The oyster toadfish 

 (Opsanus tau) reduces predation on juvenile hard 

 clams from xanthid and portunid crabs by preying on 

 these species in field experiments (Bisker et al. 1 989). 

 Natural mortality decreases as clams reach sizes 

 greater than 50 mm in length; however, fishing mortal- 

 ity can become significant at this point (Eversole 1987). 

 It has been noted that the settlement and survival of 

 juveniles is enhanced in beds where abundance of 



large clams is low due to fishing pressure (Rice et al. 

 1989). Possible reasons for this are the removal of 

 competition and larviphagy from adults, and the distur- 

 bance of sediment from fishing activities forming a 

 more suitable substrate for settlement. A parasitic 

 copepod, Ostrincola gracilis, occurs in the mantle 

 cavity of the hard clam (Humes 1 953), but probably has 

 little adverse impact on its host. Changes in the 

 environment due to storm events can have either 

 positive or negative effects on hard clam population 

 (MacKenzie 1989). Storms can widen inlets that can 

 lead to improved water circulation which can increase 

 clam populations by increasing the water salinity. 

 However, in some cases, wider inlets can cause swifter 

 currents that sweep clam larvae out to sea or alter the 

 sediment to a coaser less favorable texture. In the 

 Indian River Lagoon of east central Florida, M. 

 mercenaria x M. campechiensis hybrid clams have a 

 high incidence of gonadal neoplasia, which may act as 

 a barrier to gene flow, and reinforce reproductive 

 isolation between the two species (Bert et al. 1993, 

 Arnold pers. comm.). 



Personal Communications 



Arnold, W.S. Florida Marine Research Institute, St. 

 Petersburg, FL. 



Dillon, Robert T. College of Charleston, Dept. Biology, 

 Charleston, SC. 



Marelli, D. Florida Marine Research Institute, St. 

 Petersburg, FL. 



References 



Abbott, R.T. 1974. American seashells, 2nd ed. D. 

 Van Nostrand Co., Inc. New York, 663 p. 



Andrews, J. 1979. Texas Shells. Univ. Texas Press, 

 Austin, TX, 179 p. 



Ansell, A.D. 1967. Egg production of Mercenaria 

 mercenaria. Limnol. Oceanogr. 12:172-176. 



Arnold, W.S. 1 984. The effects of prey size, predator 

 size, and sediment composition on the rate of preda- 

 tion of the blue crab, Callinectes sapidus Rathbun, on 

 the hard clam, Mercenaria mercenaria. J. Exp. Mar. 

 Biol. Ecol. 80:207-219. 



Arnold, W.S., T.M. Bert, D.C. Marelli, H. Cruz-Lopez, 

 P. A. Gill. 1996. Genotype-specific growth of hard 

 clams (genus Mercenaria) in a hybrid zone: Variation 

 among habitats. Mar. Biol. 125:129-139. 



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