Orchistoma integrale sp. nov. (Hydrozoa, Orchistomatidae), a new species of hydromedusa with the first record of the genus from the Northwest Pacific

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Four species belonging to the genus Orchistoma in the monotypic family Orchistomatidae are currently recognized from several localities around the world, but none of these have been reported from the Northwest Pacific, including Japan. In this study, juvenile medusae collected in Tanabe Bay, Wakayama Prefecture were raised to full maturity and used to describe a new species, Orchistoma integrale sp. nov., based on integrated analysis of morphological and molecular data. Orchistoma integrale can be distinguished from its congeners by several morphological features, including prominently curled gonads and scattered nematocysts on the umbrella. Our molecular characterization of this species, connected to its type material, will allow the species to be detected through emerging genetic techniques involving eDNA derived from environmental samples. This is the first record of this family in Japan. We also describe medusa development from juvenile to adult.