A new cryptic species of Clione (Gastropoda) in deep waters of the Japan Sea (East Sea) and shallow waters of the southern Okhotsk Sea

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We describe a new cryptic species of Clione from intermediate and deep waters of the Japan Sea (East Sea) and from shallow coastal waters of the southern Okhotsk Sea. Diagnosis is based on a suite of stable morphological traits—particularly laterally extended foot lobes and a broader central radular tooth—with COI data providing supplementary support. ML analyses of the COI dataset resolved two sister clades: specimens from the Japan Sea (East Sea) occurred only in the clade that also contains the new species, whereas specimens from the southern Okhotsk Sea occurred in both. In contrast, ABGD applied to the same alignment recovered a single MOTU, consistent with shallow divergence and the lack of a clear barcode gap. As COI distances from C. okhotensis Yamazaki & Kuwahara, 2017 are modest, we rely primarily on morphology and reciprocal monophyly for species recognition, with COI data playing a supplementary role. We hypothesize that hydrographic structure and Late Quaternary isolation of the Japan Sea (East Sea) shaped this lineage, and that its occurrence in the Okhotsk Sea reflects transport by regional water masses. The new species highlights hidden diversity in pelagic pteropods and the role of semi-enclosed seas in fostering regional differentiation.