Colour polymorphic prey ( Littorina saxatilis Olivi) and predatory effects of a crab population ( Carcinus maenas L.)

Abstract

Many marine animal species are genetically polymorphic in colour. Visual selective predation is commonly suggested as a mechanism maintaining colour polymorphism in prey species and may, for example, result in correlations between colour frequencies and the background colour. Such correlations are commonly observed as, for example, in the polymorphic gastropod Littorina saxatilis. On Swedish shores the colour polymorphism of this snail is related to habitat. On boulder shores, with dense populations of predatory crabs ( Carcinus maenas) the snails are mostly cryptically brownish. In contrast, on exposed rocky shore sites where crabs are absent, Littorina saxatilis have white, banded and tessellated forms. In a field experiment on Swedish boulder shores, natural crab populations were allowed to choose between conspicuous and cryptic morphs of L. saxatilis. White and dark snail shells were glued to greyish granite stones and to black diabase stones, which were put at the mid-tide level on two shores. The white shells were cryptic on the granite stones but conspicuous on diabase, while the opposite was true for dark shells. When half of the shells had been crushed, which took only 1 or 2 days, there was no difference in the number of cryptic and conspicuous shells in any of the replicate experiments. Thus crabs on these shores do not heavily influence the colour frequencies of L. saxatilis, despite the strong correlation between presence of crabs and low colour variation, and even though crabs are the most important predators.