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 inhabited by hermit crabs; but the mutual benefits of this association are not well understood. This study, besides describing the biological cycle of the hydroid Podocoryna exigua, brings new information on the relationship between hydroid and the hermit crab Diogenes pugilator, especially concerning their trophic strategies. 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 dwelling behaviour of the hermit crab affects a particular sedimentivorous strategy of the polyps while a clepto-commensalistic strategy is adopted by the hermit crab towards the hydroid. Laboratory experiments show that the presence/absence of the hermit crab in the shell and also the seawater temperature strongly influence the number of spines, particularly structures developing from polyps. In addition, free-living polyps in the sediments may represent a strategy to colonize free shells.