Tubulovesicula lindbergi (Layman, 1930) is a species of hemiurid trematode with a wide range of fish hosts. Most records of this parasite are from the North Pacific, including the type locality. However, several reports indicate the presence of this species in the Atlantic. The aim of this study is the assessment of the conspecificity of T. lindbergi isolates from the Northwest Pacific and Southwest Atlantic. Specimens of T. lindbergi were collected from different fish species in the Sea of Okhotsk (Northwest Pacific) and characterised morphologically and molecularly. Comparison of our specimens with an available T. lindbergi isolate from the Southwest Atlantic does not support the hypothesis of their conspecificity. Thus, the current data cast doubt on the presence of T. lindbergi in the Southwest Atlantic.

Pink salmon, Oncorhynchus gorbuscha, introduced to the White Sea and expanded throughout the West Arctic and Northern Atlantics, represents a noteworthy example of a successful marine species introduction that has significantly impacted the local fish community. In our study, we analysed mitochondrial cytochrome b sequences from 635 individuals, representing both odd-year (n = 411) and even-year (n = 224) spawning lineages. These samples were collected from rivers in the White, Barents, Kara, and Northern seas, as well as from source populations in the Pacific. Fifteen cytb haplotypes were identified in the odd-year spawning lineage and twenty-six in the even-year lineage, with only ten haplotypes found in both lineages. The results demonstrated significant genetic differences between the native Pacific populations and the introduced Arctic populations in the odd-year lineage. In contrast, no genetic differentiation was found in the even-year lineage. This study describes the current haplotypic structure of European pink salmon in the Russian Arctic and provides insights into the genetic consequences of the species’ introductions.

Hyptiasmus Kossack, 1911 is a genus of cyclocoelid digeneans infecting the body cavity, air sacs, and lungs of mainly anseriform and gruiform birds. Some species of this genus have extensive ranges, spanning more than one continent. Therefore, this taxon is potentially of interest for studying patterns of genetic diversity and the associated problems of taxonomic delimitation of species. We focused on the examination of Hyptiasmus specimens initially identified as Hyptiasmus oculeus Kossack, 1911. These digeneans were collected from the nasal sinuses of specimens of Eurasian Coot Fulica atra Linnaeus, 1758 taken in both European and Asian parts of Russia. Phylogenetic analyses based on 28S rRNA, nd1, and cytb markers, in combination with morphological data, indicate that the newly collected digeneans are in fact members of H. oculeus complex represented by true H. oculeus and a putative new species Hyptiasmus sp. The morphological differences between these species appear to be the length of the postovarian space and the position of the genital pore. A detailed morphological study revealed the presence of rudimentary ventral and oral suckers in gravid specimens of H. oculeus complex. We did not find any geographic structuredness of the haplotype diversity in true H. oculeus. Our findings do not support the recent proposal to resurrect H. singilum Witenberg, 1923 and H. brumpti Dollfus, 1948. In addition, we hypothesise that Paracyclocoelum Vainutis, Andreev & Voronova, 2024 is a synonym of Cyclocoelum Brandes, 1892.

In August 2023, a comprehensive expedition “Crillon 2023” was performed, covering both terrestrial and marine biotopes in the southeastern part of the Crillon Peninsula (Sakhalin Island, Russia). A group of specialists carried out field work in ichthyology, invertebrate zoology, entomology, botany, lichenology, bryology, mycology, parasitology, microbiology and marine biology. In the scarcely studied territory of the southeastern part of the peninsula, a notable amount of data on species diversity was collected, including more than 200 species of plants, 101 species of lichens, 74 species of mosses, 117 species of basidial macromycetes were recorded. Marine coastal communities of the littoral and sublittoral zones were studied, and diving methods were exploited. As many as 119 species of invertebrates and 20 species of seaweed have been recorded. Information was collected on 20 species of fish, including data on helminth infestation. Parasitological studies included the search for microsporidia in all available animal hosts, as well as the search for root crayfish - parasites of arthropods. Among the animals and plants found, there are rare and endangered species included in the regional and federal Red Books, as well as those previously not recorded for Sakhalin. The preliminary results obtained indicate great potential for further study of the eastern part of the peninsula from the standpoint of biological sciences, as well as the prospects for establishment a biological station in this area for long-term research and development.

The influence of various factors on the fish body is reflected in changes in metabolic processes, including an imbalance of redox processes. The study of the level of oxidative processes and antioxidant protection in the tissues of aquatic organisms is often used in ecological and ecologicaltoxicological studies to assess the environmental conditions. Obtaining such data when conducting comprehensive ichthyological studies makes it possible to assess the features of adaptation to various habitat conditions, as well as the health status of fish. Mature pink salmon Onchorhynchus gorbuscha and chum salmon O. keta were caught in the summer of 2018 in the open waters of the northwestern Pacific Ocean (east of the Kuril Ridge) using a mid-water trawl during a trawl survey. Immediately after catching, liver, kidney and spleen samples were taken from the fish. Test tubes with organ tissue samples were frozen at –20 °С. Homogenates were prepared from thawed organ samples under laboratory conditions. The level of lipid peroxidation products and antioxidant activity was evaluated. As a result, intertissue and interspecific similarities and differences in the studied parameters of pink salmon and chum salmon during the marine period of life were established. The detected differences in indicators are presumably related to the structural and functional organization of the tissues and organs studied, as well as species characteristics. The results can be used to monitor population health and compare with closely related species.

In August 2023, the combined expedition “Crillon 2023” was accomplished that exploring the terrestrial, aquatic and marine biotopes in the southeastern part of the Crillon Peninsula (Sakhalin Island, Russia). A group of specialists carried out field work in ichthyology, invertebrate zoology, entomology, botany, lichenology, bryology, mycology, parasitology, microbiology, and marine biology. In the previously underexplored territory of the southeastern part of the peninsula, an appreciable amount of data on species diversity was collected, including more than 200 species of plants, 101 species of lichens, 127 species of mosses, and 117 species of basidial macromycetes. Marine coastal communities of the littoral and sublittoral zones were examined, including those achieved by scientific diving techniques. As many as 119 species of invertebrates and 20 species of seaweed were recorded. Information was collected on 20 species of fish, including data on helminth infection. Parasitological studies included the search for microsporidia in all available animal hosts, as well as search for rhizocephalans: parasites of arthropods. Rare and endangered animal, plant and fungal species were found that are included in the Regional and Federal Red Books as well as those not previously recorded from Sakhalin. The preliminary results indicate great potential for further study of the eastern part of the peninsula from the point of view of biological sciences as well as the prospects for establishment of a biological station in this area for long-term research and development.

Pink salmon is currently the main species for the anadromous fish fishery in the north of the European part of Russia. Upward trends were observed in this species returns and catches in odd years of the 2013–2021 in the regions of North-West Russia (Murmansk Region, Republic of Karelia, Arkhangelsk Region, and Nenets Autonomous Region) that allowed to increase the annual catch limit to 1,619 t in 2023. However, despite this optimistic forecast, the factual total landing of pink salmon in this part of Russia in 2023 was 203.26 t by all fishing gears (12.55 % of the quo- ta). Oceanographic conditions (SST) along the migration routes of pink salmon were rather favorable both in the White Sea and North Atlantic, so reasons of weak returns are vague. Unpredictability of pink salmon returns is well known for the main habitat of the species — the North-West Pacific, so this phenomenon has occurred in the European waters, too, in 2023.

Parasitic fauna of the White Sea cod, Gadus morhua marisalbi ; the navaga, Eleginus nawaga ; and the shorthorn sculpin, Myoxocephalus scorpius , in the White Sea was repeatedly studied, but no large-scale parasitological surveys have been made in the recent three decades. To fill this gap, we conducted a survey of the helminths of these three fish species at the White Sea Biological Station (Karelia, Russia) of the Lomonosov Moscow State University in August 2021. The navaga (50 specimens studied) was found to be infected with 13 species of helminths; the White Sea cod (50 specimens), with 12 species; and the shorthorn sculpin (21 specimens), with 13 species. Plerocercoids of Diphyllobothrium schistochilus and third-stage juveniles of Pseudoterranova bulbosa were recorded in the White Sea for the first time. The helminth infracommunities of the navaga and the White Sea cod were closer in structure to each other than to those of the shorthorn sculpin. In general, the levels of helminth infection of the White Sea cod, the navaga, and shorthorn sculpin have been consistently high over 85 years of observations in the White Sea, but long-term trends in the abundance of some helminth species were multidirectional.

Atlantic cod Gadus morhua and American plaice Hippoglossoides platessoides are two of the most commercially valuable species in the Barents Sea (FAO Area 27). They are considered as an important but neglected source of zoonotic risk associated with nematodes from the genus Phocanema. The abundance of Phocanema spp. in a fish host individual in the Barents Sea may be quite high, which is convenient for studying the genetic structure of its populations. A total of 69 third-stage juveniles of Phocanema spp. were isolated from the liver, the mesentery, and the musculature of G. morhua and H. platessoides and genotyped by the mtDNA Cox2 gene. Almost all these juveniles (68) were molecularly identified as P. bulbosum. The mtDNA Cox2 gene was also used to reveal the haplotype diversity and the genetic structure of P. bulbosum. A comparison of the specimens examined in this study with each other and with the haplotypes previously identified by us in the White Sea showed that there were no significant differences between the groups from different hosts and from different catch areas.

Onchobothrium malakhovin. sp. was found in the spiral valve of the softnose skate Bathyraja (Arctoraja) sexoculata off the Simushir Island (Kuril Islands, Russia). The new species has bothridia with three loculi and no additional suckers on bothridia, single-toothed hooks unconnected by their bases, no spines at the bases of the hooks, dense matrix around the hook bases shaped as an unpaired butterfly wing, and a short and wide ovary. Onchobothrium malakhovin. sp. differs from O. antarcticum and O. magnum in having a smaller total length, cirrus sac and ovary, smaller testes and eggs. Additionally, the new species differs from O. antarcticum by the absence of a vaginal sphincter and shorter bothridia; differs from O. magnum in having fewer proglottids and smaller vitelline follicles. It differs from O. farmeri, O. convolutum, and O. pseudouncinatum, by the absence of a small spine at the base of the hooks and the absence of accessory suckers on bothridia; from O. pseudouncinatum, additionally, by unconnected hooks; from O. schizacanthium, by the number of testes and by the presence of a postvaginal group of testes. Onchobothrium malakhovin. sp. was placed among other members of the Onchoproteocephalidea with a high support based on the sequence data for the D1-D3 region of the 28S rDNA and cox1 gene. The phylogenetic position of the genus Onchobothrium sensu lato remains ambiguous. We suggest that Onchobothrium sensu lato is a complex genus containing at least two morphologically different groups of species. Onchobothrium farmer, O. convolutum, O. schizacanthium, and O. pseudouncinatum, for which there are no molecular genetic data, are considerably different morphologically from O. malakhovin. sp., O. antarcticum, and O. magnum. A new genus might have to be established for the latter three species after the accumulation of genetic data.

The cellular composition of the peripheral blood of the Baikal seal Phoca sibirica (Gmelin, 1788) has been studied. Various cells have been found in the blood smears: large and small lymphocytes; monocytes; and neutro-, eosino-, and basophils. Analysis has shown a high percentage of band neutrophils, eosinophils, and lymphocytes in the leukogram of young seal individuals when compared to adult individuals. Similar forms of leukocytes have been recorded during the study of some marine representatives of true seals (Phocidae). However, the proportion of band neutrophils is significantly lower in the leukograms of adult gray, harp, and ringed seals. The differences may be determined by physiological features and living conditions.

A comparative study of the composition of peripheral blood leukocytes of plunderfish and grey rockcod or squama caught in the Ross Sea has been carried out. Blood smears revealed cells of different morphofunctional characteristics and structure: lymphocytes, monocytes, neutro- and eosinophils, and blast cells. An analysis of the leukogram showed that the white blood of the studied species has a lymphoid character. In the rockcod leukogram, the proportion of lymphocytes is significantly higher, while that of monocytes and blast forms is lower. In the plunderfish, in comparison with the grey rockcod, the leukocyte cells of the same name are usually larger.

A comparative study of some parameters of nonspecific immunity in sexually mature individuals of pink salmon and chum salmon caught on June 6–July 1, 2018 by an epipelagic trawl in the open waters of the northwestern part of the Pacific Ocean was carried out. The level of antimicrobial properties of blood serum and nonspecific immune complexes in the tissues of the kidney, spleen and liver and the proportion of immunodeficient individuals were studied. Significant differences in the antimicrobial properties of blood serum and immunodeficient individuals were found. These differences are probably due to species characteristics, as well as the pre-spawning period of the annual cycle in pink salmon.

The marine amphipod Caprella septentrionalis was examined for helminth infection at four localities in the Velikaya Salma Strait (Kandalaksha Bay, White Sea).A non-zero infection with helminths was found in skeleton shrimps collected at three sampling sites located closest to the White Sea Biological Station (WSBS) of the Lomonosov Moscow State University.The larvae of four parasitic species were found: third-stage juveniles of the nematodes Hysterothylacium aduncum and Pseudoterranova sp., cystacanths of the acanthocephalan Echinorhynchus gadi, and progenetic metacercariae of the digenean Progonus muelleri.Skeleton shrimps collected at a distance from WSBS were uninfected.This striking difference seems to be associated with an unusual hydrological regime near WSBS.A high turbulence during tidal motion results in an abundant supply of oxygen and biogenic substances, promoting a high species diversity, biomass and abundance of hosts.Comparison with earlier studies showed a significant reduction in infection rates of skeleton shrimps since the middle of the 20th century.

During the parasitological survey on the research vessel Professor Kaganovsky, the western part of the Bering Sea and the southern part of the Sea of Okhotsk was investigated for fish infection by parasites.In total, 1083 specimens of 17 species of teleosts and salmon shark Lamna ditropis were dissected, in which 32 species of parasites were found.The plerocercoids Pelichnibothrium speciosum s. lato prevailed in the Bering Sea.In the Sea of Okhotsk, the maximum intensity and prevalence of infection were noted for P. specioum s. lato, Anisakis sp. and Echinorhynchus gadi s. lato.The difference in the number of studied fish species in the Bering and Okhotsk Seas (14 vs 9) significantly affected the species diversity of noted parasites (28 vs 18).Teleosts caught in the coastal area and the ones performing vertical migrations to some degree differed in terms of infection pattern, primarily due to digeneans component.Macroparasites of the crested sculpin Blepsias bilobus were studied for the first time, and 20 new records were made on the infection of previously studied fish species.Parasite fauna of fish in the epipelagic zone of the western part of the Bering Sea and the southern part of the Sea of Okhotsk is characterized by significant intensity, however, relatively poor species diversity.

The cellular composition of leukocytes in the peripheral blood, head kidney and spleen was studied in individuals of three species of chars of the genus Salvelinus, white char S. albus, long–headed char S. kronocius and long-nosed char S. schmidti, as well as two forms, small-mouth and large-mouth chars living in Lake Kronotskoe. In the studied individuals, the proportion of lymphocytes of leukocytes in the peripheral blood was significant. The forms (species) of chars differed in the proportion of cell types, the size of leukocytes, as well as the intensity of leukopoiesis. The most significant differences in the proportion of different forms of leukocytes were found in the large-mouth char compared to other species and small–mouth char: high content of lymphocytes in blood smears, low content of monocytes in all tissues and organs, blast forms in the blood and neutrophils in the pronephros.

Podocotyle is a genus of marine opecoelid digeneans that parasitize a wide variety of fish as adults. We present the first phylogenetic analysis of several Podocotyle isolates using nuclear 28S rDNA and mitochondrial cox1 DNA regions. New sequences were obtained for Podocotyle specimens from fish caught in the Sea of Okhotsk and the White Sea. Based on morphological and molecular data, eight Podocotyle lineages of species rank were revealed. However, this diversity is poorly formalized within the current taxonomic model of the genus. As a result, we identified Podocotyle cf. angulata, Podocotyle cf. atomon, Podocotyle cf. reflexa, Podocotyle atomon of Sokolov et al., 2019, Podocotyle sp. of Denisova et al., 2023, Podocotyle sp. 1, Podocotyle sp. 2 and Podocotyle sp. 3. We also highlight the unresolved question of the life cycles of representatives of Podocotyle whose intramolluscan stages parasitize the intertidal snails Littorina spp.

Adult digeneans of Maritrema afanassjewi Belopol'skaya, 1952 parasitize birds and mammals along the North Pacific coasts and were previously studied in terms of morphology. However, morphological features not supported by molecular characterization are considered an insufficient tool for the identification of evolutionary lineages of microphallid digeneans. Here we provide the first data on the phylogenetic position of this species inferred from sequences of the 28S rRNA gene and the ITS2 locus of the nuclear DNA, as well as additional morphometric data. The trematodes were isolated from the intestine of the Ussuri brown bear ( Ursus arctos lasiotus Gray, 1867) on Iturup Island (the southern Kuril Islands). This is the second record of M. afanassjewi in the brown bear. The phylogeny based on the 28S rRNA gene was poorly resolved for this parasite species. The analyses based on the ITS2 locus dataset showed that M. afanassjewi was a low-supported sister to Maritrema subdolum Jägerskiöld, 1909.

We have conducted a comparative study of some of the parameters of the nonspecific immunity in mature individuals of pink salmon and chum salmon caught during June–July 2018 with an epipelagic trawl in the open waters of the northwestern Pacific Ocean. The levels of the antimicrobial properties of blood serum and nonspecific immune complexes in the tissues of the kidney, spleen, and liver were studied. The percentage of immunodeficient individuals was determined. The study showed significant differences in the antimicrobial properties of blood serum in chum salmon and pink salmon, as well as different proportions of immunodeficient individuals. These differences are probably due to species characteristics, as well as the short life cycle of pink salmon.

Parasitic 'turbellarians' are known from various animals such as echinoderms, crustaceans, annelids, bivalve and gastropod molluscs. So far, however, no 'turbellarians' have been reported from cephalopods. In this paper we report a parasitic 'turbellarian' from the giant Antarctic octopus, Megaleledone setebos. We dissected two specimens of M. setebos caught in the Ross Sea (Antarctica) and found numerous worms in their intestine and liver. The worms were spherical or oblong and had two morphologically different poles. The frontal pole bears a small conical protrusion containing large elongated pear-shaped frontal glands and large polygonal cells. The ducts of the frontal glands open terminally to form the frontal organ. The caudal pole has an opening shaped as a folded tube connected by the genital pore with a common genital atrium, which continues into a canal with a muscular sheath. The worms were identified as 'turbellarians' from the family Notenteridae (Fecampiida). This family contains only one species, Notentera ivanovi, reported from the gut of a polychaete at the White Sea. The worms that we found in the gastrointestinal tract of the octopuses were morphologically similar to N. ivanovi but differed from it in several important respects. Phylogenetic analysis based on 28S rDNA gene showed that the newly found worm clustered together with other fecampiids in a highly supported clade and was closely related to N. ivanovi. On the basis of these morphological and molecular data, we described a new species, Octopoxenus antarcticus gen. nov., sp. nov. (Fecampiida: Notenteridae), establishing a new genus to accommodate it and provided an updated diagnosis of the family Notenteridae. This is the first report of a parasitic 'turbellarian' from a cephalopod mollusc.