抄録
Two invasive hydromedusae Maeotias marginata and Blackfordia virginica, both distributed worldwide, were recorded for the first time in Japan, from the river-mouth of Ariake Bay. Maeotias marginata resembles superficially another limnomedusa, Eperetmus typus, but has more numerous tentacles than E. typus at the same level of the bell margin. Maeotias marginata is distinguished from another cosmopolitan freshwater limnomedusa, Craspedacusta sowerbyi, by the presence of numerous centripetal canals. Blackfordia virginica is distinguished from the Campanulariidae by the character that the endodermal core of tentacles extends inwards from the bell margin into the mesoglea. Both M. marginata and B. virginica have been known to occur in the bay by local fishermen for at least several decades. The introduction of edible bivalves from China and Korea, and transport in the ballast of freighters are considered as possible mechanisms for the introduction of these species.
