ADAPTATIONS OF BIVALVES TO DIFFERENT BEACH TYPES

Burrowing ability, shape and density of 12 bivalve species from a wide range of beach types were compared as part of a general investigation of the adaptations of bivalves to the swash climates experienced on e xposed sandy beaches. The genera used were Donax, Mesodesma, Tivela, S iliqua, Atactodea, Paphies and Donacilla. Burrowing rates varied widel y and burrowing ability showed no relation to beach type along the ref lective/dissipative beach gradient, i.e. from beaches with low wave en ergy and coarse sand to beaches with high wave energy and fine sand. B urrowing rate indices (BRI's) ranged from 2 to 17 (rapid to very rapid ) and tended to be higher for juveniles of most species. Bivalve speci es from dissipative beaches varied in shape from almost blade-shaped t o almost spherical, whereas those from reflective beaches were more un iform generally wedge-shaped. Species with the most flattened shapes a nd (greatest height/width ratios) tended to burrow fastest. Striking i nterspecific differences were found in densities of whole intact bival ves, these ranging from 1.04 g.cm-3 to 2.10 g.cm-3. The highest densit ies were recorded in bivalves from reflective beaches and the lowest i n bivalves from dissipative beaches. Bivalve species typical of interm ediate and reflective beaches were successfully separated from those t ypical of dissipative beaches on the basis of their density, morpholog y and BRI, particularly their size and density, using discriminant ana lysis. It is concluded that small species with high density and stream lined shape are best adapted to the dynamic swash conditions that char acterise reflective beaches.