Re: Okenia distincta from eastern Russia

February 6, 2008
From: Alexander Martynov

Concerning message #21293:

Okenia distincta Baba, 1940 in Peter the Great Bay, Japan Sea

Okenia distincta is not a poorly known species and has a quite long history of study as well as misidentifications in Peter the Great Bay. This species here was firstly discovered by Minichev in 1960s, and further shortly described and figured by him (Minichev, 1976) under the name of a North American species Okenia angelensis Lance, 1966. In frames of my thorough study of the Peter the Great Bay opisthobranchs in second part of 80s and in 90s, I rediscovered this species and found out that it differs considerably from Okenia angelensis in external features as well as in the radula structure. Further checking of the Japanese literature, revealed that it was in fact Baba's species, Okenia distincta, described from Northern Honshu. External characters, i.e. ca. 10 subcylindrical or club-shaped lateral notal processes, off-white colour with brown patches on the dorsal side, and especially morphology of the radular teeth (second laterals are distinctly bicuspidate with conspicuous inner fold), described in the first description of Okenia distincta well agreed with the features of the Peter the Great Bay specimens and thus it was listed for the first time for this region under the name Okenia distincta in Martynov (1997, 1998). Morphology of the rhinophores is somewhat variable in Peter the Great Bay specimens of Okenia distincta - for instance I have found specimens which possess one entirely smooth rhinophore and another one with very weak lamellation. These variations likely refer to the vestigial nature of the rhinophoral lamellae in Okenia distincta. The characteristic cup-shaped rhinophoral structures as in, for instance, Okenia zoobotryon (Smallwood, 1910) and Okenia mija Burn, 1967 are absent in Okenia distincta. Unfortunately, publication data of Okenia distincta in these two lists were mistakenly recorded as Baba, 1941. However, this error was further improved in the recently published first comprehensive illustrated catalogue of all Russian nudibranchs (Martynov, 2006).

• Baba K. 1940. Some additions to the nudibranch fauna of the northern part of Japan. Bulletin of the Biogeographical Society of Japan, 10(6): 103-111.
• Martynov A.V. 1997. Subclassis Opisthobranchia. [In:] Kussakin O.G, Ivanova M.B., Zurpalo A.P. et al., Eds. The list of animals, plants and fungi of the intertidal of Far-Eastern Seas of Russia. Vladivostok, Dalnauka: 77-80.
• Martynov A.V. 1998. Subclassis Opisthobranchia. [In:] Adrianov A.V., Kussakin O.G, Eds. Taxonomic catalogue of biota of Peter the Great Bay, Japan Sea. Vladivostok, Dalnauka: 204-209.
• Martynov A.V. 2006. Nudibranchia. [In book:] Yu.I. Kantor, A.V. Sysoev. Marine and brackish water Gastropoda of Russia and adjacent countries: an illustrated catalogue. Moscow: KMK Scientific Press Ltd., pp. 268-294, plates 131-138.
• Minichev Yu.S. 1976. Subclass Opisthobranchia. [In:] A.V. Zhirmunskiy, Ed. Animals and plants of Peter the Great Bay. Leningrad-Moscow, Nauka: 92-95.

Alexander Martynov

sasha_martynov@inbox.ru

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