Life history of Perarella schneideri (Hydrozoa, Cytaeididae) in the Ligurian Sea

Perarella schneideri is a cytaeidid hydroid living in association with the bryozoan Schizoporella longirostris. In this study we have distinguished me presence of two kinds of polyps in its colonies: large gastrozooids and ve ry extensible filiform polyps with 4 short tentacles. A study of the relati onships between the hydroid and its host indicates a parasitic behaviour of P. schneideri on bryozoan adult colonies, and predation on bryozoan larvae . Study of hydroid stomach contents indicates that the trophic strategy of P. schneideri depends on two food sources: ii) a benthic one, mainly consti tuted of nematodes and polychaetes and (ii) a bryozoan source, which is per formed in various ways. The Perarella schneideri colonies are, in rum, a fo od source for halacarids which perforate the hydranth wall and, penetrating the gastrovascular cavity, suck the semi-digested prey without evident dam age to the polyps.