Biological cycle of Podocoryna exigua (Cnidaria : Hydrozoa) from a sandy bottom of the Ligurian Sea

The genus Podocoryna is characterized by species living on shells usually i nhabited by hermit crabs; but the mutual benefits of this association are n ot well understood. This study, besides describing the biological cycle of the hydroid Podocory na exigua, brings new information on the relationship between hydroid and t he hermit crab Diogenes pugilator, especially concerning their trophic stra tegies. The hydroid shows a seasonal cycle with a minimum density during summer and an increase during autumn and winter, reaching the highest value in spring . The production of medusae is almost constant during all seasons. The dwel ling behaviour of the hermit crab affects a particular sedimentivorous stra tegy of the polyps while a clepto-commensalistic strategy is adopted by the hermit crab towards the hydroid. Laboratory experiments show that the pres ence/absence of the hermit crab in the shell and also the seawater temperat ure strongly influence the number of spines, particularly structures develo ping from polyps. In addition, free-living polyps in the sediments may repr esent a strategy to colonize free shells.