Interspecific competition for space by hydroids and a sea anemone living on gastropod shells inhabited by hermit crabs

Abstract

The sea anemone Calliactis tricolor (Lesueur) and hydroids of the genera Hydractinia and Podocoryne rarely occur together on gastropod shells inhabited by hermit crabs. The hermit crab Pagurus pollicaris Say does not lose its anemone-transferring behavior while in hydroid-colonized shells In choice experiments, however, C. tricolor (which is capable of transferring itself to shells unaided) settled much more frequently on bare shells than on shells bearing hydroids. Additionally. both the sea anemones and hydroids retracted their tentacles and polyps, respectively, after contacting each other This study indicates that the negative association between these sea anemones and hydroids on gastropod shells is the result of direct avoidance of each other, presumably caused by discharged nematocysts.