SPECIFICITY IN THE ASSOCIATION BETWEEN HYDRACTINIA ECHINATA AND SYMPATRIC SPECIES OF HERMIT CRABS

¹ Department of Biology, The Pennsylvania State University, Ogonts Campus, Abington, Pennsylvania 19001
² Department of Zoology, North Carolina State University, Raleigh, North Carolina 27650

1. A distinct pattern in the occurrence of Hydractinia on shells occupied by four sympatric species of hermit crabs was identified. Hydractinia occurred most frequently on shells occupied by Pagurus longicarpus and P. pollicaris and rarely on shells occupied by P. annulipes and P. brevidactylus. The abundance of Hydractinia fluctuated seasonally, concurrent with changes in the abundance of P. longicarpus and P. pollicaris.

2. Species of shell appeared to be a less significant factor than species of crab in determining the pattern of association between Hydractinia and hermit crabs.

3. In laboratory studies, naked P. annulipes rejected Hydractinia-covered shells even when offered only such shells to enter, while P. longicarpus and P. pollicaris readily accepted Hydractinia-covered shells. None of the three species showed any preference for or against shells supporting only the perisarcal crust of dead Hydractinia colonies.

4. Our results indicate that occurrence of Hydractinia on gastropod shells acts to partition this resource and thereby tends to reduce competition among the four sympatric species of hermit crab in Bogue Sound, N. C.