The benthic eirenid polypoid phases of species in Eugymnanthea and Eutima inhabit the mantle cavity of bivalve molluscs. Whereas the polyps of two known species of Eugymnanthea live in this habitat, only a subset of Eutima species (three species) is associated with bivalves in the Eirenidae. Using 16S rRNA gene sequence data, we conducted phylogenetic analysis of eirenid and its allied hydrozoans (Eutonina and Blackfordia), and found that all of the bivalve-inhabiting species presently analyzed (four distinct species) form a well-supported clade in Eirenidae. This implies that this unique life habit evolved in the most recent common ancestor (MRCA) of Eugymnanthea japonica, Eugymananthea inquilina, Eutima saphinoa and Eutima japonica.
