Interactions between two marine snails, Littorina rudis Maton and Littorina nigrolineata Gray, a predator, Carcinus maenas (L.), and a parasite, Microphallus similis Jägerskiold

Abstract

In the laboratory, the shore crab Carcinus maenas (L.) preferred the smallest size classes of Littorina rudis Maton and Littorina nigrolineata Gray available, but preyed more heavily on L. rudis. L. rudis has a greater prey value (energy yielded per unit handling time) for crabs than L. nigrolineata which has a thicker shell. The energy content of both snails is a function of their size, but if attack success rate is taken into account with handling time, smaller snails become more valuable than larger snails. Comparisons with Carcinus predation on mussels, Mytilus edulis L., and dogwhelks, Nucella lapillus L., substantiate the hypothesis that foraging behaviour is a consequence of prey dispersion patterns. Although crab predation may account for intra-shore differences in shell thickness and aperture size between Littorina rudis and L. nigrolineata, inter-shore differences cannot be entirely attributed to crab predation, at least in L. rudis. It is suggested that differences between L. nigrolineata and L. rudis in tolerance to physical factors and vulnerability to crab predation allow them to exploit different shore levels. In addition, interactions between the two snails are perhaps affected by their different susceptibilities to trematode. Microphallus similis Jägerskiold, infection, which in turn may be related to their prey value to shore crabs.