A GASTROPOD COLOR POLYMORPHISM: ONE ADAPTIVE STRATEGY OF PHENOTYPIC VARIATION

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Abstract
A color polymorphism in Crepidula convexa from Massachusetts, USA, is reported, and investigated to determine how it is maintained. Two populations are analyzed for color composition; chi-square analyses are performed on shell color vs. substrate color. The relative fitnesses of individuals of each color on each substrate are calculated by relating reproductive output to shell size and age. Application of the data to fitness set theory shows that the conditions necessary for the polymorphism to be balanced are met. Major predators of C. convexa prey by sight, and hence color matching of shell and substrate on the 2 major substrate types (dark Littorina littorea shells and white clam shells), coupled with limited adult movement and redistribution of the juveniles relative to the substrate each generation, may be responsible for maintaining the polymorphism. Ecological comparisons are made with other species of Credidula. C. adunea from California, USA which, like C. convexa, is small, relatively short-lived, intertidal and eaten by visual predators, is also color polymorphic. Species which are uniform in morphology such as C. plana are restricted by their behavior and life history to environments homogeneous in space and time. Species such as C. fornicata which display phenotypic plasticity are longer-lived, preyed by olfactory predators, and indiscriminate in choice of substrate. Polymorphism and phenotypic plasticity are alternate ways of increasing the niche size of a species.