Adaptations of bivalves to different beach types

Abstract

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 exposed sandy beaches. The genera used were Donax, Mesodesma, Tivela, Siliqua, Atactodea, Paphies and Donacilla. Burrowing rates varied widely and burrowing ability showed no relation to beach type along the reflective/dissipative beach gradient, i.e. from beaches with low wave energy and coarse sand to beaches with high wave energy and fine sand. Burrowing rate indices ( BRI's) ranged from 2 to 17 (rapid to very rapid) and tended to be higher for juveniles of most species. Bivalve species from dissipative beaches varied in shape from almost blade-shaped to almost spherical, whereas those from reflective beaches were more uniform generally wedge-shaped. Species with the most flattened shapes and (greatest height/width ratios) tended to burrow fastest. Striking interspecific differences were found in densities of whole intact bivalves, these ranging from 1.04 g · cm −3 to 2.10 g · cm −3. The highest densities were recorded in bivalves from reflective beaches and the lowest in bivalves from dissipative beaches. Bivalve species typical of intermediate and reflective beaches were successfully separated from those typical of dissipative beaches on the basis of their density, morphology and BRI, particularly their size and density, using discriminant analysis. It is concluded that small species with high density and streamlined shape are best adapted to the dynamic swash conditions that characterise reflective beaches.