A tubulariid hydroid associated with anthozoan corals in the Mediterranean Sea

A solitary hydroid of the genus Ectopleura was described associated with several Mediterranean anthozoans: the black coral Antipathella subpinnata and three gorgonian species, Eunicella cavolinii, Paramuricea clavata and Paramuricea macrospina. This find represents the first record of a hydroid species epibiont of an antipatharian coral and also the first record of such association with Mediterranean gorgonians. Hydroids arise from the branches of the hosting corals and are enveloped by a thin sheet of their skeleton and by a layer of living coenenchyme up to the neck of the hydranths. The relationship causes no apparent damage to the host, while the epibiotic habitus allows the hydroid to avoid siltation and to gain in defence and support. It is hypothesised that actinulae of Ectopleura sp. are able to settle on the corals (both skeleton or tissue) and, during their growth, to be enveloped by the skeletons of their host.