Vertical migration of the clam Mercenaria mercenaria (L.) (Mollusca:Bivalvia): environmental correlates and ecological significance
In a field study of marked individuals, the clam Mercenaria mercenaria (L.) migrated vertically in a rhythm that was synchronized with the local tidal cycle. Clams were shallowest (5 mm) at high tide and deepest (25 mm) at low tide. Clam depth was independent of clam size. M. mercenaria migrated vertically in sand, mud, and oyster rubble habitats but not in Spartina alterniflora (Loisel) grass beds. When relocated to a sand habitat, clams from S. alterniflora beds also migrated vertically in concert with tidal phase. Clams exhibited relatively little horizontal movement. Laboratory experiments demonstrated that an increase in pressure caused upward movement of M. mercenaria. Field experiments restricting vertical movements of M. mercenaria showed that clams buried 20 mm or deeper at low tide had reduced risk of gull predation. Predation by avian predators is suggested as a selective pressure in the evolution of the tidally based vertical migration of M. mercenaria. © 1989.
Roberts, D; Rittschof, D; Gerhart, DJ; Schmidt, AR; Hill, LG
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