Habitat requirements for the hard clam, Mercenaria mercenaria, in Chesapeake Bay

The hard clam, Mercenaria mercenaria, is found along the eastern coast of North America from the Gulf of St. Lawrence to Texas. In Chesapeake Bay the hard clam is restricted to salinities above approximately 12 ppt. The abundances and distribution patterns of hard clams in Chesapeake Bay are based on studies performed nearly 20 years ago - a more extensive survey of hard clam resources is due. Statements concerning long term trends in populations are not feasible. The basic anatomy of hard clams conform to that of venerid bivalves. Hard clams grow to a maximum shell length (anterior-posterior dimension) of about 120 mm. There are few documented cases of diseases in wild hard clam populations. Parasitic infestations are also slight. The life cycle of the hard clam is typical of other venerid bivalves, and includes a pelagic larval phase and a relatively sedentary benthic juvenile and adult phase. In Chesapeake Bay, ripe gametes can be found between May and October, and spawning commences when temperatures rise above 20-23°C. The larvae are planktotrophic (feeding). Metamorphosis usually commences at a shell length of 200-210um. Predation on new recruits is very high, dense aggregations of hard clams were found in the absence of predators. Aside from predation and fishing pressure, the natural mortality of larger clams appears very low. Hard clams are important members of the suspension-feeding infauna. As such they are important in benthic-pelagic coupling, grazing of primary production, transfer of carbon and nitrogen to benthic food chains and, through excretion, rapid recycling of particulate nitrogen as ammonia. The major food source for hard clams is planktonic microalgae. In Chesapeake Bay growth occurs in spring and fall, when optimum water temperatures coincide with abundant food. Clams are capable of living in a variety of sediment types. Field surveys have often found higher abundances of clams in sandy rather than muddy sediments. A heterogeneous substrate mixture of sand or mud Hith gravel or shell often show high relative abundances of clams. Hard clam stocks are susceptible to overfishing. Recruitment rates are poorly understood, as are possible reestablishment periods if areas are depleted of clam populations through commercial harvesting. Larval settlement rates and annual recruitment, and the factors that influence these processes, are poorly understood. Hard clam mariculture is well established and could easily be expanded into sites within the bay. Given the ability of clams to bioaccumulate toxic substances, an adequate monitoring system showed be maintained. The sublethal effects of toxic material readily found in the lower James River should be examined