The effects of hermit crabs on hydractiniid hydroids

Hermit crab shells are often encrusted with diverse communities of epibiont s. To explore the effects of hermit crabs on the interactions of these encr usting species, two species of hydractiniid hydroids, Hydractinia symbiolon gicarpus and Podocoryna carnea, were examined in the presence and absence o f Pagurus longicarpus hermit crabs. Colonies growing on Littorina littorea snail shells with hermit crab hosts were compared to those growing on shell s without crabs. These experiments suggest that hermit crabs variably affec t colony polyp number, colony morphology and the outcome of interspecific c ompetition, according to the size and number of crabs; other factors includ e water temperature, food availability and hydroid clonal genotype. In orde r to allow image analysis of perturbations of colony morphology, hydroids w ere grown on glass surfaces both exposed and unexposed to hermit crabs. Rel ative to colonies growing on unexposed surfaces, colonies growing on expose d surfaces exhibited larger within-colony patches, thus decreasing the tota l encrusted surface area and polyp number of colonies. Further, on glass su rfaces, hermit crabs accelerated the time to the resolution of interspecifi c competition (i.e., the overgrowth of one colony by the other). While a nu mber of factors may contribute to the relative abundance of these encrustin g species, under most circumstances P. longicarpus have large effects on hy dractiniid hydroids. Mechanisms underlying these effects likely include hyd rodynamics and mechanical disturbance.