A GASTROPOD COLOR POLYMORPHISM: ONE ADAPTIVE STRATEGY OF PHENOTYPIC VARIATION

Abstract

A case of shell color ploymorphism in the shallow water mesogastropod Crepidula convexa is described. Polymorphism of color in C. convexa appears to be based on disruptive selection, which occurs due to the availability of two contrasting shades of substrate, heterogeneous in time and space, combined with mortality from visual predators.Data on the relative survival of the two phentoypes of C. convexa in two patches of a heterogeneous environment are applied to the fitness set model developed by Levins (1968). The necessary and sufficient criteria for a stable polymorphism are met, but reasons are set forth for believing that this is, rather, a case of unstable polymorphism.Only one other species of the genus, C. adunca, shows a similar color polymorphism. Ecological factors uniquely shared by the two species are: visibility of inherited shell color, a major part of the population living in the intertidal zone, and an impact by visual predators. Color polymorphism is correlated with visual predation on a heterogenous substrate, and with low adult mobility. In general, the strategy of polymorphism is advantageous to some species of Crepidula and not others, depending on heterogeneity of the environment in time and space, with respect to its use by the organisms.Other patterns of phenotypic variation found in Crepidula are phenotypic plasticity and phenotypic uniformity. Uniformity occurs when a species' behavior pattern restricts it to homogeneous environments. Plasticity, like polymorphism, increases niche size in heterogeneous environments; plasticity is favored when each organism must cope with environmental change within its lifetime. These patterns of natural selection for phenotypic variation probably apply to a wide variety of organisms.