Effect of commensal hydroids on hermit crab competition in the littoral zone of Texas.

In the winter in Galveston Bay, Clibanarius vittatus competes with the Pagurus longicarpus and P. pollicaris for the gastropod shells. C. vittatus is agressive and able to outcompete Pagurus even if exceeded in size. About 20%-30% of Pagurus occupy shells bearing hydroid colonies, while C. vittatus is rarely found in such shells. Pagurus is immune to the hydroid sting, but C. vittatus is not. C. vittatus chooses shells without hydroid colonies, and Pagurus prefers hydroid covered shells. Pagurus sp. avoid exposure by migrating to deeper waters as the tide recedes, but C. bittatus withstands intermittant exposure. Since the hydroids cannot survive exposure to the air a case of mutualism exists: Pagurus benifits from the protection, and the hydroids benefit by mobility to deeper water.