Abstract
Hydroids symbiotic with octocorals in the Bunaken Marine Park and in the Lembeh Strait (North Sulawesi, Indonesia) were studied. Four species,Hydrichthella epigorgia,Ralpharia neira,Pteroclava krempfiandZanclea timidasp. nov. were recorded and are described. The new speciesZ. timidais the only one of the genus associated with an octocoral. It is characterized by a naked hydrorhiza producing nematocyst knobs and by polyps which are able to retract into their own rigid, cup-shaped, basal region. The relationship between epibiontic hydroids and their octocoral hosts affects the morphology of both partners. The hydrorhiza ofHydrichthella epigorgiais naked when associated withAnthoplexaura dimorpha, but perisarc-covered when growing on other gorgonian host species. Vice versa, the hydroid is also able to affect the host morphology:Ralpharia neirainducesEllisellasp. to develop new branches, with the inner skeleton enveloping the hydroid stem.
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