The digenean parasites of Littorina neritoides (L.) in relation to microhabitat on the Plymouth breakwater

Abstract

The Plymouth breakwater affords an artificial rocky shore environment with an east–west increasing exposure gradient, a uniquely uniform profile, and a highly restricted splash zone. A population of Littorina neritoides, elsewhere characteristic of splash zones, is found on the breakwater divided between two contrasting niches. Some live submerged in small, deep, permanent pools along the top of the breakwater, while others cluster in splash zone crevices on the walls of a few isolated shelter block structures. The occurrence in submerged snails of both Microphallus similis cercariae and the cysts of an unidentified metacercaria shows an inverse correlation with the degree of exposure to wave action at any place along the breakwater. Splash zone snails have very few infections with M. similis, but marked differences in infection rates with the metacercarial cysts can be correlated with the relative moisture-retaining properties of the stonework faces of each shelter block.