Comparison of taxonomic and functional structures of macrobenthic communities in the eelgrass meadows of Zostera marina L. was conducted for the White Sea and Azov Sea regions. In the meadows, Z. marina in both regions formed almost monospecific beds interspersed in the White Sea with single specimens of other macrophytes. In each region, two groups of macrozoobenthos communities were identified: with the dominance of Mytilus spp. (Mytilus galloprovincialis in the Sea of Azov and Mytilus edulis in the White Sea) and with the dominance of other species. However, the role of Mytilus spp. was not key in eelgrass meadows: the variation in the macrozoobenthic species abundance did not depend on mussels' quantitative characteristics. No common macrofaunal species were found for the two regions; however the functional structure of macrozoobenthos across these two contrasting latitude zones showed similarities. Filter feeders and detritivores with similar life strategies dominated both regions (maximal body size ranged from 1 to 3 cm, epifaunal lifestyle, and high fecundity). Such functional convergence within the communities exceeded latitudinal differences between faunas, supporting the hypothesis that similar ecological mechanisms underlie the formation of macrozoobenthic communities in seagrass meadows even when fauna originates differently. In both areas, key community structure drivers included grain size composition of bottom sediments as well as abundance of Z. marina.

Recently, global climate change and environmental pollution led to changes in macrozoobenthic communities. Existing approaches to defining and mapping communities are extremely diverse. Consequently, assessment of both temporal changes and spatial variability depends greatly on the approach used. This paper proposes a three-level (ecological complex, biocenosis, subcenosis) hierarchical approach to classification of macrozoobenthos communities. Ecological complex is a species pool typical of a well-defined range of leading factors. Corresponding biocenoses are formed in specific biotopes from this pool; they have a characteristic species composition and a specific structure which result from joint actions of both biotic interactions and the habitat properties. Different subcenoses have a similar species composition and partially resembling quantitative structure; differences between biotopes inhabited by subcenoses of one biocenosis are minor or undetectable in practice. Proposed approach enhances combining and comparing datasets obtained by different methods. The paper focuses at the example of soft-bottom macrozoobenthos in the northeastern Black Sea shelf and slope. Two ecological complexes are identified in the study area. They are distinguished by temperature and salinity regimes in main water masses; their boundary coincides with the deepest position of the seasonal thermocline. Three biocenoses and eight subcenoses are described within these complexes. The differentiation of biocenoses and subcenoses within the ecological complexes is driven primarily by the sediment type and oxygen regime. The proposed approach is used to designate a base of level 5 in the EUNIS habitat mapping for the northeastern Black Sea shelf. Glossary Community and biocoenosis in ecological studies can be used by various authors as synonyms or as terms with different meanings. Various authors described benthic fauna divisions of the Black Sea and Sea of Azov as biocenoses (Vorobyov, 1949), communities (Sezgin et al., 2010), or mixing the concepts (Chikina and Kucheruk, 2005). In the present work, these terms are adopted as synonyms, denoting the biological component of biogeocenosis according to (Sukachev, 1949).

The morphology, biological traits, sex and size structure, and genetic diversity of the recent invader—the Asian date mussel, Arcuatula senhousia—in the Kerch Strait, Taman Bay, and the southern part of the Sea of Azov were described. The Sea of Azov population exhibits a high level of genetic diversity, comparable to that of other invasion regions. The surveyed population showed significant similarity to invasive populations from both Italy and the United States. Arcuatula senhousia specimens were found on soft bottoms, rocky substrates, and macrophytes as fouling. The mussels were observed attached to other mollusks and forming cocoons with byssus and filamentous algae on soft bottoms. The highest abundance in the study area on soft bottoms reached 190 ind/m2, which was an order of magnitude lower compared to the abundance in native regions and other invaded areas. Dense mats of A. senhousia were not observed. Specimens of varying sizes were found in the study areas from 2019 to 2021, with shell lengths ranging from 0.5 to 28 mm. The largest specimens were of a size comparable to adults from both invasive and native regions. The gonads of specimens with shell lengths of 7–21 mm were mature and in the pre-spawning phase, indicating the formation of a self-sustaining population in the Sea of Azov. The main limiting factors for the formation of a dense and widespread population were high wave action and predation pressure. Further expansion of the species could be expected in the Caspian basin.

The effect of hydrocarbon pollution on the fungal communities of littoral sediments of the cold-water White and Barents seas was investigated. The samples were collected at locations with different levels of pollution with oil products, from ports to relatively undisturbed areas. Using the diesel fuel-containing medium resulted in detection of hydrocarbon-degrading fungi in almost all studied samples, although in all cases they were less diverse than sugar-degrading fungi. In this relatively small group, Penicillium chrysogenum and Penicillium brevicompactum were the most common organisms. Fungal communities isolated on a sugar-containing medium exhibited higher diversity and abundance, with being the most common sugar degraders. The major factors affecting the structure of the fungal communities were the percentage of hydrocarbons in the total mass of organic carbon in the samples in the case of hydrocarbon-degrading fungi and location, for sugar degraders. In the experiment, the highest hydrocarbon-degrading activity was shown for Penicillium chrysogenum (the loss of residual hydrocarbons was 77.4%), Cadophora fastigiata (72%), and Tolypocladium inflatum (67.2%).

A recent molecular phylogenetic and biogeographic study on the krill genus Hansarsia revealed undescribed cryptic diversity in the Atlantic. Each of four species analysed encompassed robust molecular clades that were linked to dimorphic males in H. microps, H. atlantica and H. tenella. We tested the robustness and divergence of the observed clades using an integrative approach including (1) three independent species delimitation methods for the mitochondrial COI gene (ASAP, GMYC, bPTP), (2) variability of two nuclear genes (H3 and ITS1) and (3) morphological analysis (MDS and PCA) with a dataset of 22 characters scored for 131 specimens. Both molecular and morphological analyses resulted in at least six distinct clades within the Atlantic Hansarsia. The strongest divergence was revealed between the two clades of H. tenella, one of which we diagnosed as a new species. Two clades of H. megalops also showed significant divergence but in the absence of males, we were reluctant to designate new species. Different clades linked to male forms in H. microps and H. atlantica are suggested as an incipient species. We also hypothesise an unusual trend in the evolution of euphausiids, in which visual recognition enhances tactile interaction during mating. Our results show that analyses of ostensibly well studied groups may yet yield taxonomic surprises. ZooBank: urn:lsid:zoobank.org:pub:AE045636-50EF-450A-B9B3-9231E8B91522

The species of the genus Melita that was found in the Kerch Strait is morphologically similar to M. setiflagella and M. nitida.A comparative analysis of the morphological parameters of 14 body parts with those in the known melitid amphipods M. setiflagella and M. nitida, and also those in a species identified as M. cf.setiflagella recorded from the Kerch Strait waters in March 2019, has shown that such character as setation of antennae 2 pair cannot be diagnostic at the species level and is probably a response of individuals to variations in their habitat conditions.According to the results of a genetic analysis, the species M. cf.setiflagella is actually M. nitida.

Most likely, the alien snow crab Chionoecetes opilio entered the Kara Sea from the Barents Sea, both due to the migration of adults and with currents at the larval stage. Currently, all bottom stages, including mature individuals and a large number of pelagic larvae, are present in the Kara Sea. However, the origin of larvae has not yet been clarified. The larvae hatched in the Kara Sea should be at an earlier stage of development compared with the larvae arrived here from the Barents Sea due to later development of phytoplankton and, accordingly, later hatching. The larvae of the snow crab Chionoecetes opilio and the spider crab Hyas araneus were collected in the central and southwestern Kara Sea in July–early August 2019 by a Bongo zooplankton net with a diameter of 60 cm. It was established that the larvae were unevenly distributed across the Kara Sea. The most populus region was the border with the Barents Sea in the St. Anna Trough (up to 860 ind./m2), and relatively high concentrations of larvae were recorded in the southwestern part, where their abundance at stations varied from 18 to 302 ind./m2. In the zone of the Ob–Yenisei plume, crab larvae were absent or their abundance was minimal. Using molecular genetic methods, the species identity of 361 larvae (344 C. opilio and 17 H. araneus) was reliably determined, and measurements of a number of morphological structures were made for 401 larvae. Significant differences in size were found at the zoea II stage between C. opilio and H. araneus. In July 2019, in most of the Kara Sea, in zooplankton samples, crab larvae were represented by zoea I of C. opilio with rare specimens of zoea I of H. araneus of the Kara Sea origin. Only in the southwestern part, at the boundary with the Barents Sea, was the presence of zoea II of C. opilio and H. araneus observed in samples with an increased share of the latter species in catches, which probably originated from the Barents Sea.

Hansarsia megalops and Thysanoessa gregaria are two abundant krill species in the warm-temperate belts in the North and South Atlantic. This pattern of disjunct distribution, often referred to as anti-tropical (or bipolar), is a regular phenomenon among marine organisms that can appear at different taxonomic levels. Analysis of spatial genetic diversity based on variation of the mitochondrial cytochrome oxidase I (COI) gene was carried out to understand whether these broadly sympatric krill species have similar patterns in population structure and demographic history. The results suggested that the anti-tropical distribution of both species was driven by trans-equatorial migrations during different historical periods. We observed contrasting differences in genetic diversity between species coupled with similar patterns of genetic diversity and structure in disjunct populations on both sides of the equator. We also found additional genetically distinctive populations of T. gregaria in the South Atlantic in Subantarctic waters and showed that the Southern Subtropical Frontal Zone acts as a strong boundary between two genetically distinctive populations of T. gregaria. The results suggest that both species-specific habitat preferences and interactions between closely related species (congeners) may shape differences in population genetic diversity and structure among species.

The first record of a species from the order Zoantharia in the Black Sea is given. Zoantharians were successfully reared from planktonic larvae collected in a single planktonic net haul conducted in shallow coastal waters of the Golubaya Bay, Gelendzhik area, Caucasus. The larvae were subsequently settled in a small glass container and resulting colonies were maintained in an aquarium with a salinity level of 18 psu for approximately nine months, but in June 2018 all colonies died due to uncontrolled bloom of filamentous algae. The presence of symbiotic algae of the family Symbiodiniacea in tissues of colonies is shown. Phylogenetic analysis using mitochondrial (COI) markers revealed a high degree of similarity between the Black Sea zoantharian and Isozoanthus sulcatus (Gosse 1860) from European seas.

We inventoried all nine species of the 'Acanthephyra purpurea' complex, one of the most abundant and cosmopolitan group of mesopelagic shrimps. We used 119 specimens at hand and genetic data for 124 specimens from GenBank and BOLD. Phylogenetic analysis of four genes (COI, 16S, NaK, and enolase) showed that the 'Acanthephyra purpurea' complex is polyphyletic and encompasses two species groups, 'A. purpurea' (mostly Atlantic) and 'A. smithi' (Indo-West Pacific). The 'A. purpurea' species group consists of two major molecular clades A. pelagica and A. kingsleyi - A. purpurea - A. quadrispinosa. Molecular data suggest that hitherto accepted species A. acanthitelsonis, A. pelagica, and A. sica should be considered as synonyms. The Atlantic is inhabited by at least two cryptic genetic lineages of A. pelagica and A. quadrispinosa. Morphological analyses of qualitative and quantitative (900 measurements) characters resulted in a tabular key to species and in a finding of four evolutionary traits. Atlantic species showed various scenarios of diversification visible on mitochondrial gene level, nuclear gene level, and morphological level. We recorded and discussed similar phylogeographic trends in diversification and in distribution of genetic lineages within two different clades: A. pelagica and A. kingsleyi - A. purpurea - A. quadrispinosa.

The ice-associated nematode Theristus melnikovi has a long-distance dispersal throughout the Arctic. However, this species’ taxonomic position and phylogenetic relationships remain unresolved. We used an integrative approach of single-specimen barcoding of morphologically identified specimens of T. melnikovi with note on the phylogeny of two new ice-associated species of the family Xyalidae. DNA barcodes (18S and 28S gene markers) were obtained from 13 specimens of T. melnikovi collected from sea ice in the White and Kara Seas. Analysis of the D2D3 region found three genetically distinct lineages of T. melnikovi from the White Sea ice, suggesting that T. melnikovi specimens belong to the cryptic species, with genetic differentiation attributed to limited ice exchange between remote semi-isolated areas and open Arctic seas. The Kara Sea specimens belong to a single species. Our results highlight that using 18S rDNA data alone underestimates species diversity within sympagic nematodes. The barcoded specimens of T. melnikovi could not be distinguished morphologically, as the majority of the sequenced specimens were juveniles (92.3

A new leptostracan species, Nebalia piipensis sp. nov., collected during the 82nd cruise of the RV Akademik M.A. Lavrentyev on the submarine Piip Volcano (the southwestern Bering Sea) at depths of 400-472 m, has been described. These leptostracans were abundant at bacterial mats and in assemblages of chemosymbiotrophic bivalves Calyptogena pacifica (Pliocardiinae). Nebalia piipensis sp. nov. differs from the known Nebalia species by a set of features including a distally tapering eye-stalk mostly covered by an ommatidial part; a rostrum with the length 2.5 times the width; an article 2 of mandibular palps bearing three thin plumose and one thin simple seta; an article 1 of the second maxilla endopod 1.8 times longer than an article 2; furcal rami longer than the combined length of the pleonite 7 and telson; rounded denticles of pleonites 6 and 7; an indistinct anal-plates 'shoulder'. Nebalia piipensis sp. nov. is the second species of the genus Nebalia reported from hydrothermal vent fields after N. tagiri, described from hydrothermal vents in Kagoshima Bay, at 200 m. The sequences of COI barcoding region, 16S and Histon H3 loci for the new species were obtained. The phylogenetic analysis based on COI sequence showed that all the northwestern Pacific species, including N. tagiri and N. piipensis sp. nov. are clustering together. However, N. piipensis sp. nov. is genetically closer to N. dolsandoensis from regular habitats than to the vent species N. tagiri. A preliminary examination of the body surface of the new leptostracan species using scanning electronic microscopy revealed a variety of epibiotic bacteria.

The species composition and the structure of the macrophytobenthos, macrozoobenthos, and bottom sediments of Taman Bay are studied. This inlet is inhabited by seagrass Zostera marina L. which forms vast underwater meadows here. Samples were collected in 2008–2009, before the ongoing rise of salinity in the Sea of Azov. Three main biotopes with different bottom sediment composition (sands, sands with shells, and silts) were identified. They were inhabited by four main macrobenthic biocenoses (unvegetated near-shore biocenosis, mosaic macrophyte vegetation outside the surf zone, pure eelgrass’s meadows belt, and the communities, dominated by the mobile macrozoobenthos species with low abundance of macrophytes occupying the central part of the bay). The main environmental factor associated with this distribution of the vegetation was the silt content (grain size <0.001 mm). The spatial structure of the macrozoobenthos correlated with the projective cover of Z . marina . Possible reasons for the revealed pattern in the distribution of communities are discussed.

The genus Abyssopathes Opresko, 2002, comprises deep-sea black corals known almost exclusively from lower bathyal and abyssal depths, mainly from seamounts covered by cobalt-rich crusts and areas of polymetallic nodules. The taxonomical position of the genus and its placement in the family Schizopathidae has been repeatedly questioned, but fruitlessly. Known only in extremely deep habitats, these corals have rarely been collected in a state suitable for morphological or molecular studies that could help to clarify their status. Recently, increasing attention has been paid to the study of fauna associated with deep-sea minerals. Using material of Abyssopathes lyra (Brook, 1889) sampled during these studies, we transfer the genus Abyssopathes from the family Schizopathidae to the family Cladopathidae based on morphological and molecular data. Morphological data includes six mesenteries in the polyps, a unique pinnulation pattern found only in genera within the Cladopathidae, and relatively short polyp tentacles, a feature typical of some cladopathids. Sequencing data, consisting of 626 bp from the mitochondrial cox1 gene, showed that Abyssopathes is 99% identical to Chrysopathes Opresko, 2003, Cladopathes Brook, 1889, Heteropathes Opresko, 2011, and Trissopathes Opresko, 2003 (all Cladopathidae), in this gene region.

Subclass Ceriantharia is a well-defined and probably ancient group of marine benthic organisms renowned for their bilateral symmetry, which is reflected in the arrangement of tentacles and mesenteries. Four species of Ceriantharia have been reported in the Arctic, including Cerianthus lloydii Gosse, 1859, also known from the Northern Atlantic and Northern Pacific. The integrity of this species was questioned in the literature, so we performed a molecular study of C. lloydii from several geographically distant locations using 18S and COI genes. The phylogenetic reconstructions show that specimens of C. lloydii form a single group with high support (>0.98), subdivided into distinctive clades: (1) specimens from Northern Europe, the Black and Barents seas, and (2) specimens from the White, Kara, Laptev, and Bering seas and also the Canadian Arctic and the Labrador Sea available via the BOLD database. There are several BOLD COI sequences of Pachycerianthus borealis (Verrill, 1873), which form a third clade of the C. lloydii group, sister to the European and Arctic clades. Based on low similarity (COI 86–87%) between C. lloydii and the type species of the genus Cerianthus Delle Chiaje, 1841—C. membranaceus (Gmelin, 1791), we propose a new status for the genus Synarachnactis Carlgren, 1924, and a new family Synarachnactidae to accommodate C. lloydii.

Deep-sea colonial Scleractinia (Anthozoa: Hexacorallia) are indicator species of vulnerable marine ecosystems known as deep-sea coral reefs. The number of species of deep-sea community-forming colonial Scleractinia is small and specimens of good preservation, especially those collected in a live state, can be easily identified to the species level. Identification of fragments, especially those of the basal part of colonies, can be difficult. We have shown that X-ray examination can effectively distinguish colony fragments of the widespread deep-sea community-forming scleractinian corals Desmophyllum pertusum and Solenosmilia variabilis (Caryophylliidae). The skeleton of S. variabilis is considerably denser and less X-ray permeable compared to D. pertusum. The calyx cavities of the corallites of S. variabilis are visible only at the periphery of the fragment and practically do not extend to the outside. In D. pertusum, the cavities of the corallites can be deeply traced in the fragment body, they have a conical shape and considerably expand to the periphery. The proposed method of identification is inexpensive, does not require special sample preparation, and allows rapid identification of bulk material.

In the exploration of the meiofauna associated with sponges and corals in the shallows of Cuba, we investigated nine species of sponges (Demospongia), wherein 26 nematode species were revealed. Most nematode specimens (50–95% of all individuals) in all sponge samples belonged to the family Desmodoridae (order Desmodorida), followed by the family Chromadoridae (order Chromadorida). A major part of Desmodoridae is constituted by the genus Acanthopharynx. A statistical morphometric analysis (principal component analysis and multidimensional scaling with testing via analysis of similarities) revealed two close cohorts that differed in size and pharynx shape. Molecular genetic analyses (COI, 18S, and 28S) also distinguished two groups of specimens that corresponded to morphometric cohorts. Based on the morphometry and molecular genetics, the larger-sized group was defined as Acanthopharynx micans (Eberth, 1873), while the smaller-sized group was considered A. parva sp. n. In light of the taxonomic review of the Acanthopharynx, emended generic diagnosis, and the annotated list of ten valid species, A. parva sp. n. differed from other Acanthopharynx species by its peculiar shape of the pharynx (gradually widened to cardia), smaller body size, and pattern of precloacal organs.

In this research, the species composition and structure of macrophytobenthos, macrozoobenthos, and bottom sediments of the Taman Bay, one of the areas of the Sea of Azov where the seagrass Zostera marina L. forms underwater meadows. The material was collected in 2008–2009 before of the Sea of Azov salinization. Three main zones with different types of bottom sediments (sands, sands with shells and silts) were identified, within which four main macrobenthic biocenoses were located (unvegetated coastal biocenosis, mosaic macrophyte communities outside the surf zone, Z. marina underwater meadows and the central regions’ biocenosis with the dominance of mobile forms of macrozoobenthos and a low abundance of macrophytes). The main environmental factor associated with this distribution of macrophytobenthos was the content of silt (with a granule size of less than 0.001 mm). The spatial structure of the macrozoobenthos correlated with the projective cover of Z. marina. Possible reasons for the revealed regularities in the distribution of communities are discussed.

We report the presence of Vermiliopsis striaticeps (Grube, 1862) at the northeastern coast of the Black Sea. Provided descriptions of the studied material match descriptions of Mediterranean V. striaticeps. Identification based on morphological characters is verified with molecular tools. However, there are marked minor differences from Mediterranean specimens, including a striped red–white branchial crown and a higher number of radioles. In the Black Sea V. striaticeps forms dense settlements on rocks at depths of 0.5–7 m, but single specimens may be detected deeper, up to 37–41 m. Its wide distribution across the studied part of the coast, its size structure, and the presence of specific fauna inhabiting the biotope formed by the V. striaticeps tubes allow us to suggest that this species could have been present in the northeastern coast of the Black sea for a long time without being noticed by scientists. Possible origins of the northeastern Black Sea population of V. striaticeps are discussed.

The state of macrozoobenthos and meiobenthos of the Gelendzhik Bay of the Black Sea was assessed in 2020–2021 based on three bottom grab surveys. Five types of biotopes were identified based on the particle size distribution of the bottom sediments, the redox potential of pore water, the content of organic carbon, and the type and presence of macrophytobenthos. A study of the oxygen content in the water column did not reveal hypoxia. Four types of macrozoobenthos communities were noted in the bay. The main dominants were bivalves and gastropods. In the central part of the bay, the communities were similar to typical Black Sea communities from the venus sand belt. The spatial structure of macrozoobenthos was associated with the type of biotope identified on the basis of abiotic characteristics and benthic vegetation. Macrozoobenthos was represented mainly by mobile burrowing infauna, belonging to the type of food filter feeders with a bivalve shell (bivalves). The meiobenthos of the bay was represented mainly by nematodes, which dominated both coastal areas and deeper waters. The spatial structure of the meiobenthos was determined by the granulometric composition of the bottom sediments, and the dependence on the type of vegetation and the redox potential of pore water was unreliable. In the coastal area, the reducing conditions were found in the surface layer of the bottom sediments, and also the low abundance and biomass of macrozoobenthos. The station with reducing conditions was distinguished by the dominance of larger (up to 5 cm) worm-like segmented polyphages and predators and the absence of juvenile forms of macrozoobenthos. At the station with reducing conditions, the lowest species diversity of meiobenthos was noted, and the nematode-copepod index reached 620:1, which is an order of magnitude higher than at other stations in the bay. A comparison of the current state of communities with historical data (1979, 1990) was made. It is shown that at the present stage the biodiversity of the bay has increased, while the abundance of macrozoobenthos has decreased. There were no stable areas of hypoxia or freezes in the bottom layer of waters.