The phylogenetic position of the Arctic Sculpin Myoxocephalus scorpioides from the East Arctic was determined using molecular genetics and karyological methods. This study presents the first data on the mitochondrial DNA genes of this species from the East Siberian Sea, whose haplotype contains 32 species-specific nucleotide substitutions. Moreover, it is the first to report the karyotype of the species, which is comprised of 42 chromosomes and 44 chromosome arms. Phylogenetic analysis revealed that the Arctic Sculpin is positioned in one of the two Myoxocephalus groups, together with Myoxocephalus quadricornis and Megalocottus platycephalus as well as with M. aenaeus and M. octodecemspinosus. The position of M. scorpioides confirms the non-monophyletic relationships of the genus Myoxocephalus. The results of this study indicate that the genera Myoxocephalus and Megalocottus need to be revised.

A sculpin Myoxocephalus sp. of uncertain taxonomic status from the East Arctic (Chaun Bay, East Siberian Sea) has been studied by molecular genetics, morphological, and karyological methods. This species has been identified as the warty sculpin, Myoxocephalus verrucosus. This study presents the first data on the mitochondrial DNA genes of M. verrucosus from the East Siberian Sea, whose haplotype contains seven species-specific nucleotide substitutions, as well as the first karyotype of the species, which is comprised of 36 chromosomes and 46 chromosome arms. A set of morphological, genetic, and karyological traits has been identified that allow reliable differentiation of M. verrucosus from other species of the genus Myoxocephalus.

For the first time, a karyological and molecular genetic ( COI , cytochrome b , 16S rRNA genes) study of the freshwater fish species Cottus kolymensis , recently described from the Dukcha and Kolyma rivers in northeast Asia, was carried out. A complex of karyological and molecular-genetic marker traits has been determined to identify the species. Genetic differences between the populations of C. kolymensis inhabiting the water bodies of the Kolyma River basins and the northern part of the Sea of Okhotsk were found. On their basis, the species is divided into groups in accordance with the habitat. The existence of a separate group Cottus poecilopus within the genus Cottus , which includes taxa of the species rank, including C. kolymensis , has been confirmed.

A molecular genetic and karyological study of the fourhorn sculpin Myoxocephalus quadricornis (Linnaeus, 1758) has been carried out. The karyotype was studied for the first time and the results of Ag-staining of the chromosomes of specimens of the species from the East Siberian Sea are presented. The karyotype of M. quadricornis is stable; it contains 32 (10 metacentric, 2 submetacentric, and 20 subtelocentric) chromosomes, while the number of chromosome arms is 44. Nucleolar organizers were found in the telomeric regions of the short arms of two homologous medium-sized subtelocentrics. Variability in the number of nucleolar-forming chromosomes and stained blocks of nucleolar organizers was found. Common features in the chromosome sets of M. quadricornis and Megalocottus platycephalus indicate their greater similarity compared to species of the genus Myoxocephalus . The results of the karyological analysis are consistent with the molecular genetic data about the sister relationships between M. quadricornis and Megalocottus platycephalus . A complex of karyological and molecular-genetic features has been identified that allow one to reliably distinguish M. quadricornis from species of the tribe Myoxocephalini and indicate the need for its removal from the genus Myoxocephalus .

A molecular genetic and karyological study of the frog sculpin Myoxocephalus stelleri Tilesius, 1811 was carried out on an extensive sample from a large area of the species’ range. A total of 42 specimens was sampled from the Sea of Japan, Sea of Okhotsk, and coastal waters off the southern Kuril Islands, which makes this sampling scheme the most comprehensive to date. The level of mtDNA polymorphism was found to be low. The haplotypes of the species formed three phylogenetic groups. The unique M. stelleri haplotype from the coast of Shikotan Island linked all the studied groups, indicating that it is likely ancestral. Robertsonian polymorphism was identified in the species. In all five cytotypes (I – 2n = 44, II – 2n = 43, III – 2n = 42, IV – 2n = 41, V – 2n = 40; NF = 44+2) were identified, all of which were present in the Sea of Japan. Only one (cytotype I) was found in the Sea of Okhotsk, which is probably the closest to the ancestral karyotype. The significant chromosomal polymorphism and the presence of common haplotypes in the studied samples indicate their recent origin from a common ancestor and/or relatively recent contacts within the range. The discrepancies between mtDNA and karyotypes in assigning the ancestral M. stelleri to the coastal waters off Shikotan Island (southern Kuril Islands) and the Sea of Okhotsk, respectively, can be explained by the different inheritance mechanisms and the rates of evolution of molecular genetic and karyological traits.

The material for research was a collected fish in the littoral and sublittoral zones of the Islands of Nedorazumeniya and Zavyalov coasts (Northern part of the sea of Okhotsk) in June 2018 and August 2019.The authors’ research identified 22 and 25 species of fish that live in the summer in the littoral and sublittoral zones of the sea coast of Zavyalov and Nedorazumeniya Islands, respectively. Of these the most numerous are the Blue rockfish Sebastes glaucus (24.5-49.0%), and White spotted greenling Hexagrammos stelleri (13.5-35.7%). Most species are demersal and benthic ichthyocoen. 41% of the species are characterized by the broad-boreal near-Asian type of range, 28% - mainly boreal, 13% - Arctic-boreal, 9% - high-boreal near-Asian and broad-boreal Pacific. For the development of recreational fishing, 15 species of fish can be promising, for which size and weight indicators are given.

A comprehensive study based on molecular marker and karyotype analyses has provided evidence for the monophyly of the subfamily Myoxocephalinae, which includes the genera Myoxocephalus, Megalocottus, Microcottus, Porocottus, Enophrys and Argyrocottus. In addition, the karyotype of the threadfoot sculpin Argyrocottus zanderi Herzenstein 1892 has been studied for the first time. Marker traits of karyotypes identified 13 species among six cottid genera. As the molecular genetic results confirmed, the subfamily is divided into two groups corresponding to Enophrys and Myoxocephalus. The molecular genetic data did not support the formation of tribes within the subfamily Myoxocephalinae, as proposed earlier based on morphological characters. Moreover, the genera Trichocottus and Taurocottus should be excluded from the Myoxocephalinae. The evolutionary transformations of karyotypes in cottid fish tended towards a reduction in the number of chromosomes and chromosome arms.

Data on the first records of Pholis nea Peden et Hughes, 1984 from Russian waters of the Sea of Japan (Tatar Strait) and a description of the species morphology are presented. The genetic identification is based on an analysis of the COI gene region of mitochondrial DNA (mtDNA). A new common English name is here proposed for Pholis nea, the Lesser Gunnel.

Karyological and molecular genetic study of the sculpins of the genusMyoxocephalusis conducted. Molecular genetic analysis demonstrates that the genus is heterogeneous in the contemporary composition and is divided into Pacific and Arctic-Atlantic groups. In the Pacific group, a close phylogenetic relationship is found between the speciesM. stelleri,M. polyacanthocephalus, andM. jaok. The representative of the Arctic-Atlantic group,M. quadricornis, is monotypic and sister toMegalocottus platycephalus. Close relations ofM. scorpiusand the sculpins of the Pacific group are revealed on the basis of the molecular genetic and karyological data. Analysis ofM. brandtii,M. ochotensis, andM. jaokfrom different parts of their habitats shows the absence of karyological variability. The degree and direction of the divergence of the karyotypes in Pacific speciesM. brandtii,M. ochotensis,M. jaok,M. polyacanthocephalus,M. stelleri, andM. scorpiusare established.

A comparative molecular-genetic study of the belligerent sculpin Megalocottus platycephalus (Pallas, 1814) from the Sea of Japan and the Sea of Okhotsk has been carried out for the first time. The heterogeneity of the species, which was established based on an analysis of nucleotide sequences of mitochondrial DNA genes, is associated with its geographical distribution. It has been shown that the samples of M. platycephalus from the different seas form separate groups, whose differentiation does not exceed the intraspecific variation. The studied geographical groups of the belligerent sculpin are assumed to belong to the southern M. platycephalus taeniopterus (Kner, 1868) and northern M. platycephalus platycephalus (Pallas, 1814) subspecies.

Karyological and molecular genetic research on fringed sculpins of the genus Porocottus has been conducted for the first time. Karyotypes of P. japonicus (2n = 42, NF = 42), P. allisi , and P. minutus (2n = 40, NF = 42) have been studied. An analysis of mitochondrial DNA shows high levels of species differentiation, as well as significant distinctions of P. camtschaticus from P. minutus and P. japonicus . On the basis of a comprehensive study, the characters that allow reliable identification of P. japonicus , P. allisi , P. minutus , and P. camtschaticus have been found.

The suggestion that the fish specimens caught in the littoral zone off Nedorazumeniya Island, Taui Bay, Sea of Okhotsk, belong to the eelpout species Magadanichthys skopetsi [23] and Hadropareia middendorffii Schmidt, 1904 has been confirmed by the analysis of their molecular-genetic and morphological characters. Their karyotypes have been studied for the first time: M. skopetsi , 2 n = 48 (2 meta-, 26 subtelo-, and 20 acrocentric chromosomes), NF = 50; H. middendorffii , 2 n = 48 (2 meta-, 36 subtelo-, and 10 acrocentric chromosomes), NF = 50. As a result of a comprehensive study, the levels of interspecific differentiation of these species has been established, the degree of intraspecific variability has been determined, and the lack of the population-geographical subdivision of H. middendorffii has been shown.

Molecular genetic and karyological analyses of antlered sculpin, Enophrys diceraus, from the Sea of Japan and the Sea of Okhotsk were carried out. The karyotype of this species was studied for the first time. On the basis of karyological analysis, it was established that E. diceraus from the Sea of Japan and the Sea of Okhotsk was polymorphic in terms of the number of chromosomes and their morphology (2n = 36, 35, and 37, NF = 40). It was suggested that the karyotype with 35 chromosomes could have been produced as a result of Robertsonian translocation; the karyotype with 37 chromosomes could have been produced by crossing of individuals with different number of chromosomes. On the basis of the molecular genetic analysis of the mitochondrial COI and 16S rRNA genes, considerable differences between E. diceraus from the Sea of Japan and the Sea of Okhotsk corresponding to the level of interspecies genetic variability were established. It is concluded that E. diceraus from the Sea of Japan belongs to another species, most likely, E. namiyei.

A complex study, including genetic and karyological approaches, as well as an analysis of the ultrastructural organization of spermatozoa of the Nozawa’s prickleback Stichaeus nozawae (Jordan et Snyder, 1902) and Grigorev’s prickleback S. grigorievi (Herzenstein, 1890) from Vostok Bay, Sea of Japan, is considered in the present work. The species exhibit differences in the spermatozoon head width and flagellum length. These characters are species-specific and, along with morphological traits, can be used as a proof of the validity of the species under study. The similarity between S. nozawae and S. grigorievi, as established based on genetic and karyological analyses (2n = 48, NF = 70, localization of nucleolar organizers), does not correspond to inter-species differences in morphological and biological characters. The lack of variations in DNA and the chromosome set that are suitable for species differentiation, between Nozawa’s and Grigorev’s pricklebacks is an evidence for the evolutionary adolescence of these species.

Based on the analysis of variability of mitochondrial and nuclear DNA and calibration of the molecular clock using the paleontological data, the divergence time of the eelpouts is assessed. The temporal frames of the evolution of the suborder Zoarcoidei are restricted to the Late Oligocene and the middle of the Late Miocene, within a diapason of approximately 12 Ma. The families Bathymasteridae and Cebidichthyidae were the first to separate from the common ancestor, 14.7–22.5 and 13.1–19.1 Ma, respectively. The differentiation of other families occurred between the Middle and the Late Miocene approximately 10–15 Ma. The period from the Late Miocene to the Mid-Pliocene (3.6–7.8 Ma), most likely, was the time of the appearance and distribution of the present species of the suborder. Assessment of the DNA divergence rate in the eelpouts conducted in this study is in accordance with known evolutionary scenarios and with key steps of geological development of the World Ocean.