Some new and reconfirmed biological observations in two species of Eugymnanthea (Hydrozoa, Leptomedusae, Eirenidae) associated with bivalves

Abstract

New observations on the morphology and the relationship with the host of two species of bivalveinhabiting hydrozoans Eugymnanthea (Eirenidae) are reported. The mature medusae of Mediterranean Eugymnanthea inquilina and Japanese E. japonica were clearly distinguishable by their two diagnostic features (presence/absence of a manubrium and the number of statoliths per statocyst), and exhibited a different attachment manner of their hydroids to the gill of their common host Mytilus galloprovincialis. In order to determine if the differences in the diagnostic features are due to differences in salinity, E. japonica hydroids were raised under Mediterranean conditions (40‰), which are never experienced in Japan (30‰). However, the medusae released from these hydroids still exhibited the typical Japanese features, suggesting they may be under genetic, rather than environmental, control. Additional new observations, including the association of E. inquilina with two unrecorded species of bivalves, and the appearance of hermaphroditic medusae in other populations, including the one collected from the type locality, are reported.