A bivalve-inhabiting hydrozoan Eugymnanthea japonica KUBOTA, 1979 (Thecata, Leptomedusae, Eirenidae), commensal with the blue mussel Mytilus edulis galloprovincialis LAMARCK collected from 20m in depth, 200m off Nishikigaura, Shizuoka Prefecture, Japan on December 1, 1990, is the deepest recorded occurrence of this species. The association rate with this host (60%, N=40) is as high as that with the same host species located in the intertidal region at Atami, Shizuoka Prefecture, Japan. Thus the present species also occurs in subtidal waters, as is the case with the related species Eutima japonica UCHIDA, 1925 distributed in Japan. The polyp bearing more than two stalks of medusa buds is an uncommon and previously unreported type of zooid for this species, and is illustrated. Eighty medusae 1 day old and 1 medusa 2 days old, which were liberated as mature medusae from polyps taken from a total of 13 hosts, were examined within 8 days after collection of the hosts. The morphology of the mature medusae is not different from that of medusae released from polyps found in the intertidal region or just below it in Japanese waters, except for the most modified individual, which had only one radial canal and one gonad.
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