Spongiactis japonica, a new genus and species of Actiniaria, an obligate symbiont of hexactinellid sponge is described from Sagami Bay, Japan, Pacific Ocean. Numerous small specimens of this sea anemone live in the caves in a superficial layer of the sponge Hyalonema sieboldi and probably reproduce asexually. The species has no acontia and it is taxonomically related to Actinoscyphiidae (which members are believed have lost acontia) and to Hormathiidae (comprising species possessing acontia) but cannot be accommodated in either family. It is therefore assigned to the new family Spongiactinidae fam.n. The two related families, Actinoscyphiidae and Hormathiidae may well be united. How to cite this article:

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