A constant pattern of medusa release has been detected in the hydroid Eugymnanthea japonica commensal with a bivalve Mytilus edulis galloprovincialis. In near-natural conditions in Tanabe Bay, Shirahama, Japan mature medusae were released from the mantle cavity of the host for a few hours a day around sunset, strongly correlating with decrease in light intensity. Release time also coincided well with both a drop in water temperature in the evening and maximum shell opening of the host. Such synchronized mass medusa release may enhance cross-fertilization in this species and seems related to its short life-span.