Microevolution and phenotypic plasticity in Donax serra Roeding (Bivalvia: Donacidae) on high energy sandy beaches

Morphological differences between populations of the wedge clam Donax serra inhabiting two different coasts and biogeographic regions of South Africa were investigated. Both adults and juveniles occupied different positions along the beach gradient depending on the coast: on the southeast coast adults occurred in the mid intertidal and juveniles and recruits were low intertidal to subtidal; on the west coast the zonation pattern was reversed. Not only adults but also juvenile clams had shapes differing significantly between the two coasts; west coast clams were thinner, rounder and had a higher body density than the southeast ones recruits were less dense in the former coast. Differences in shell shape between coasts are probably the result of directional selection on the adults with the microevolutionary changes being maintained by geographical isolation. Shell density, on the other hand, seems to be environmentally determined through physiological control of shell calcification, i.e. more mobile intertidal clams having lower shell density than less mobile subtidal clams. Ontogenetic changes in shape and density are presumably adaptive and appear to be related to mobility, i.e. the larger, heavier and denser adults being more stable in the substrate, and the smaller, thinner and less dense juveniles being more mobile and dispersive. Phenotypic plasticity in present D. serra populations is an important factor that enabled this species to occupy different habitats and biogeographic regions and to survive 5 million years of environmental changes.