Nella osyoro (Baba, 1940)

Description

A small aeolid nudibranch, typically a few centimetres in body length. The digestive gland running through each ceras is dark brown and shows clearly through the translucent body wall. A diagnostic white line runs along the dorsal surface of each ceras, and an orange to red ring-shaped marking is present near the ceratal tips. Background colour and the intensity of these markings vary considerably between individuals.

Distribution

The type locality is Oshoro Bay on the western coast of Hokkaido, Japan. Recorded from the northern Sea of Japan and the coasts of Hokkaido, possibly extending to the southern Kuril Islands.

Etymology

The specific epithet osyoro refers to the type locality, Oshoro Bay in Otaru City, Hokkaido. "Osyoro" is an older romanisation of the place name now usually written "Oshoro"; Kikutaro Baba adopted the older form when he described the species in 1940.

Remarks

Originally described by Baba 1940 as Cuthona osyoro and later treated under Cuthonella. A 2025 revision reorganised the North Pacific aeolids and transferred the species to the newly erected genus Nella. Nella osyoro closely resembles its congener Nella soboli but is distinguished externally by the conspicuous orange to red ring near the ceratal tips, which is absent in N. soboli.

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References

• Cuthona osyoro Baba Osyoro-minoumiushi (n. n.), Baba, K. 1957. A revised list of the species of Opisthobranchia from the northern part of Japan, with some additional descriptions. J. Fac. Sci., Hokkaido Univ.,ser. 6, Zool. 13(1-4):8-14.
• Marcus E. (1964). Additions to the nudibranch fauna of the east Pacific and the Gulf of California. Transactions of the San Diego Society of Natural History. 13: 377-396. https://doi.org/10.5962/bhl.part.9602
• オショロミノウミウシ属の1種 2, Cuthona cf. punicea, 中野理枝. (2004). 本州のウミウシ. ラトルズ.
• Kantor Yu.I. & Sysoev A.V. (2006) Marine and brackish water Gastropoda of Russia and adjacent countries: an illustrated catalogue. Moscow: KMK Scientific Press. 372 pp. + 140 pls.
• Cuthonella soboli, Chichvarkhin A. (2016). Shallow water sea slugs (Gastropoda: Heterobranchia) from the northwestern coast of the Sea of Japan, north of Peter the Great Bay, Russia. PeerJ. 4: e2774. https://doi.org/10.7717/peerj.2774
• ソボルイミノウミウシ（新称）, 中野理枝. (2018). 日本のウミウシ. 文一総合出版.
• Cuthonella osyoro, Korshunova T.A., Sanamyan N.P., Sanamyan K.E., Bakken T., Lundin K., Fletcher K. & Martynov A.V. (2020). Biodiversity hotspot in cold waters: a review of the genus Cuthonella with descriptions of seven new species (Mollusca, Nudibranchia). Contributions to Zoology. 90(2): 216-283. https://doi.org/10.1163/18759866-bja10017
• Nella osyoro (Baba, 1940) comb. nov., Martynov, A. & Korshunova, T. (2025). Hidden diversity of the North Pacific prompts reorganization of the taxonomic system. Moscow: Neptune. ISBN 978-5-9905149-3-5. 48 pp.