First description of larvae of Radulinopsis derzhavini Soldatov et Lindberg, 1930 (Scorpaeniformes: Psychrolutidae) with remarks on melanin colouration and relationships with related species.

The Derzhavin's sculpin (Radulinopsis derzhavini Soldatov et Lindberg, 1930) is a psychrolutids species that leads a cryptic life in coastal waters of the Japan Sea and the southern Okhotsk Sea. To date, larvae of this species have remained unknown; therefore, their biology is poorly understood. In the present study, the early developmental stages of R. derzhavini are described for the first time. In Peter the Great Bay, Japan Sea, this species is characterized by a spring spawning season (May) and a short pelagic period of larval development, usually from mid-May to the last 10 days of June. Species identification of the described larvae was confirmed by incubation, rearing of larvae in captivity, and genotyping of a fragment of the mitochondrial cytochrome c oxidase I (COI) gene. In addition, the development of pigmentation in larvae of this and related species was compared. Morphological analysis of both adults and larvae, together with complementary molecular genetics, confirms the previously obtained conclusions that the species of the genera Radulinopsis, Radulinus, Asemichthys, and Astrocottus form a natural group with monophyly by all types of data.