Janolus fuscus O’Donoghue, 1924 (Gastropoda: Proctonotidae), a species of a nudibranch family that is new to the marine fauna of Russia

The seaslug Janolus fuscus of the family Proctonotidae, which is new to the Russian fauna, was found in the Sea of Japan for the first time. A small population of this mollusk was found at the northwestern coast on a bank occupying approximately 3000 m2 of the sea bottom south of Rudnaya Bay. Data on the distribution, ecology, radula morphology, and feeding of this mollusk on the bryozoan Microporina articulata are presented. The differences between J. fuscus and similar species of the genus Janolus are discussed. The partial sequences (614 bp) of the mitochondrial COI gene differed in two nucleotide substitutions, suggesting that the population that is found in the Sea of Japan is conspecific to the North American J. fuscus.