Colour polymorphism in the intertidal snail Littorina rudis Maton

Abstract

The tictors affecting the variation of shell colour morph frequencies of the intertidal snail Littorina rudis Maton have been examined using ordination and correlation techniques. The major gradient underlying variation in shell colour is related to habitat type and structure. Dark, patterned shells are characteristic of cliff and salt marsh habitats whilst light, unpatterned shells reach high frequencies on boulder shores. Colour morphs do not differ significantly in shell thickness and between-shore variation in colour morph frequencies cannot be directly attributed to habitat dilterences in the risk of shell injury, but to other factors associated with habitat type and structure, e.g. parasitism. It is suggested that the maintenance of shell colour polymorphism in L. rudis is probably mediated by selection on pleiotropic characters rather than shell colour alone.