Description and ecology of Cytaeis capitata n. sp. (Hydrozoa, Cytaeididae) from Bunaken Marine Park (North Sulawesi, Indonesia)

Abstract

A new hydroid species, Cytaeis capitata n. sp. (family Cytaeididae), is described. This species, an epibiont of gastropod shells living in the shallow water of the Bunaken National Marine Park, North Sulawesi, Indonesia, is characterised by specialised polyps with an elongate tentacle, armoured by a capitation of peculiar nematocysts and lacking in the normal polyps. The medusae arise from gonophores enveloped in a layer of perisarc. Each has four oral tentacles, four radial canals, and four marginal tentacles. The diet of the polyps is related to the behaviour of gastropods: when the snails remain on the substratum, polyps are predators, whereas when the snails sink in the sediment, the polyps become sedimentivorous. The importance of the living mollusc for the hydroid is experimentally documented: the polyps living on shell fragments decrease in number by 50-60% in a couple of days and completely disappear in 8 days.