Role of dispersal ability in the phenotypic differentiation and plasticity of two marine gastropods : I. Shape.

Species with contrasting abilities for dispersal may adopt different strategies in response to wide ranges of environmental conditions. These strategies were investigated here by comparisons of phenotypic differentiation and plasticity in the gastropods Austrocochlea constricta and Bembicium vittatum, which coexist in a range of intertidal habitats on the Abrolhos Islands and Albany in Western Australia. They differ in their potential for larval dispersal, A. constricta having a short mobile planktonic stage and B. vittatum developing directly from benthic eggs. Levels of among-population variation in shell shape and maximum size did not differ between the species, or between locations subject to contrasting levels of gene flow in the case of A. constricta. Results of a common garden translocation experiment, coupled with those of previous breeding experiments, suggest that variation in shape is predominantly genetically determined in B. vittatum, but plastic in A. constricta. Plasticity of shape in A. constricta was most pronounced at Albany, where previous data suggested the widest gene flow, and was greater than in B. vittatum at both locations. Change of shape in A. constricta subsequent to translocation was related to growth rate, a trait which was also more plastic in this species than in B. vittatum. The results agree with the notion that wide dispersal enhances physiological flexibility, while restricted dispersal promotes local genetic adaptation, but disagree with the hypothesis that wide dispersal promotes phenotypic homogeneity.