The main goal of the present paper is to revise the diversity of the nudibranch genus Ziminella and update the geographic ranges of its species, based on historical museum material and newly collected specimens of Ziminella japonica (Volodchenko, 1941) from the Sea of Japan. We provide morphological examinations, a molecular phylogenetic analysis based on four markers, and species delimitation analyses of Ziminella. Our results indicate that Z. salmonacea (Couthouy, 1838) and Z. circapolaris Korshunova et al., 2017 represent a single species but display some intraspecific variation in the COI marker (up to 2.54% between individuals). The species delimitation analyses cannot explicitly support the distinct status of Z. japonica and Z. vrijenhoeki Valdés et al., 2018, but in this case the minimal interspecific p-distance in COI comprised 7.95%. Moreover, several differences in the coloration and external morphology are found in specimens from the type localities of these two species. We also show that Z. japonica displays high morphological diversity across its putative geographical range. Since this variation could be either intraspecific or interspecific, we suggest that specimens collected outside the type localities and lack molecular data, cannot be precisely identified. The molecular phylogenetic analysis does not support close relationships of the shelf species Z. japonica and the abyssal species Z. abyssa Korshunova et al., 2017, thus a hypothesis of a shelf origin of the latter species cannot be confirmed by the presented data. Finally, we provide novel evidence on the hidden diversity of flabellinids in the deep-sea waters of the Sea of Japan.
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