Hard Clam Mercenaria mercenaria

The hard clam 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. An extensive survey of hard clam resources is overdue. Statements concerning long term trends in populations are not feasible. Hard clams ·grow to a maximum shell length 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 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 200-210 mm. Predation on new recruits is very high; dense aggregations of hard clams have been 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 suspension-feeding infauna, thus they are important in 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, but higher abundances are found in coarse-grained sediments. Hard clam stocks are susceptible to overfishing. Recruitment rates are poorly understood, as are possible reestablishment periods if areas are depleted through commercial harvesting, and factors influencing larval settlement rates. 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, adequate monitoring should be maintained. The sub lethal effects of toxic material readily found in the lower James River should be examined