Reproductive Season and Some Biological Notes on a Bivalve-Inhabiting Hydrozoan Eugymnanthea japonica (Thecata-Leptomedusae : Eirenidae) at Shirahama, Tanabe Bay, Japan, with Comparison of Related Species

Abstract

At Shirahama, Wakayama Prefecture, japan Eugymnantheajaponica very actively produce medusa buds with mature gonads (in 77-100% of larger hosts examined) from June to November during which water temperature was high (21.3-27.1°C in nearly monthly average). The hydroids were found on every soft body portion of Mytilus edulis galloprovincialis attached to rafts, buoys, and ropes within I m depth. The association rate of the hydroids of E. japonica within larger specimens of this bivalve was very high in every month, maximally 100% (N = 78) and 96.7% in yearly average (N = 931). The hydroids were simultaneously found in almost all of the hosts harboring a commensal crab Pinnotheres sinensis (58 out of 59 ones). Differing from two related species, Eugymnanthea inquilina and Eutima japonica, the association rate of the present hydroids in smaller specimens of Mytilus edulis galloprovincialis was high.