Bivalves (Tellinacea: Donacidae) on a North Carolina Beach: Contrasting Population Size Structures and Tidal Migrations

Abstract

Populations of the beach clams Donax parvula and D. variabilis overlap spatially in North Carolina (USA). Previous studies may have lumped these species, so that differential patterns of growth and potential differences in tidal migrations may have been obscured. Intertidal populations of Donax spp. were sampled monthly during 1976 and 1977 with a core tube and a hand dredge along transects perpendicular to shore. Juveniles of 2 Donax spp. colonized the beach in late winter. D. variabilis individuals grew more rapidly than D. parvula, and these 2 populations were easily distinguished during summer by size differences. Tidal migrations of both species ceased in late summer. D. parvula moved offshore, but D. variabilis remained on the beach, stranded during low tides. These populations appear to undergo seasonal migrations between beach and subtidal habitats. Emigrations from the beach may be related to temperature declines and reproductive requirements. Summer stranding of D. variabilis might be an adaptation to fish and crab predation, or an avoidance reaction to crowding stress.