BIOLOGIYA MORYA, 2016, Vol. 42, No. 6, pp. 449-457

Species identity of a nudibranch mollusk of the genus Cuthona Alder et Hancock, 1855 (Gastropoda: Tergipedidae) associated with hermit crabs in the Sea of Japan

© 2016 A. Yu. Chichvarkhin^{1, 2}, I. A. Ekimova^{2, 3}, E. L. Egorova², O. V. Chichvarkhina¹

¹A.V. Zhirmunsky Institute of Marine Biology, Far Eastern Branch, Russian Academy of Sciences, Vladivostok 690041;
²Far Eastern Federal University, Vladivostok 690950;
³M.V. Lomonosov Moscow State University, Moscow 119899

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A confirmed occurrence of the nudibranch mollusk Cuthona nana is recorded in Russian and Pacific waters for the first time. Two populations of this mollusk were discovered in Rudnaya and Kievka bays at the northwestern shore of the Sea of Japan. Observations on the distribution, ecology, and radula morphology of C. nana are presented. This species feeds on hydractiniid hydrozoans and lives symbiotically with Pagurus hermit crabs occupying their shells. Morphologically, it is clearly distinct from sympatric Tergipedidae and other Aeolidina. A small number of nucleotide substitutions in the COI gene fragment of the mtDNA, along with morphological similarity, confirm the conspecific identity of C. nana populations from the Sea of Japan with the nominative form of this species from the Atlantic and the Barents Sea and with the eastern Pacific Cuthona divae. Cuthona hermitophila described from the Sea of Japan is probably a junior synonym of C. nana.

Key words: Cuthona nana, Cuthona hermitophila, Opisthobranchia, Tergipedidae, Pagurus, Precuthona chrysanthema, Sea of Japan.