ECOLOGY OF THE BIVALVE-INHABITING HYDROID EUGYMNANTHEA-INQUILINA IN THE COASTAL SOUNDS OF TARANTO (IONIAN SEA, SE ITALY)

The life history of the bivalve-inhabiting hydroid Eugymnanthea inquil ina was investigated in two different hosts, the mussel Mytilus gallop rovincialis (collected from 1989 - 1991 from the Ionian Sea, Italy) an d the clam Ruditapes decussatus (collected from 1991-1992, Ionian Sea, Italy). Hydroids living in mussels revealed a size selection for host s longer than 40 mm, being almost completely absent in mussels below t his size. This might be controlled by signals linked to the sexual mat urity of the bivalve. The proportion of molluscs inhabited showed a se asonal trend in mussels only, with a sharp decline at the onset of med usoid liberation. Production of medusoids was high in mussels, whereas medusoids were rare and often abortive in clams. This indicates a hig her degree of fitness of E. inquilina in mussels than in clams. The hy droid life cycle was not completed in clams, which therefore were pres umably supplied by planulae from medusoids produced by mussel-inhabiti ng hydroids. Mussels played a key role in the stability and persistenc e of E. inquilina populations in the studied area. Selective ingestion of trematode sporocysts by E. inquilina hydroids indicated a protecti ve role of the hydroid against mussel parasites, leading to reconsider ation of this symbiotic association as a possible mutualism rather tha n a simple inquilinism.