The new species, Buccinum kosyanae sp. nov. is described from the Middle Kurile Islands. Previously the species was included in multigene molecular phylogeny of the genus [Kantor et al., 2022b] as B. cf. kobyakovae. The new name, B. fedosovi nom. nov. is proposed for Buccinum solidum Golikov et Sirenko, 1988, non Buccinum solidum Philippi, 1887.

The paper deals with two species of chitons of the genus Leptoplax, L. unica and L duongae sp. nov. collected in the South China Sea off the Spratly Islands. The new species differs from morphologically similar species of the genus by the sculpture of the jugum, absence of notch on the anterior margin of apophyses of the tail valve, rounded shape of pustules of tegmentum and distinctly ribbed dorsal girdle spicules.

Regenerative capabilities vary among different groups of invertebrates and despite being a highly abundant and diverse group of invertebrates with significant commercial and scientific value, gastropods remain relatively understudied in this respect. This work presents the first investigation of post–traumatic regeneration in the nudibranch mollusc Doridina, specifically focusing on Onchidoris muricata. Dorids have unique subepidermal calcite spicules that form a complex network inside the body. However, their capacity for complete or partial recovery, as well as the impact on regeneration of organs containing these spicules, has never been studied. We examined the regeneration of chemosensory organs (rhinophores) and dorsal body outgrowths (tubercle), both containing spicules and having different innervation. Our investigation explores three models of rhinophore regeneration: 1) after the removal of the apex and three lamellae of the rhinophore, 2) when the entire metameric lammelae part is removed, and 3) when the rhinophore is entirely excised. Additionally, two series of experiments were conducted to examine tubercle regeneration in the peribranchial region and in the rhinophore region. The study reveals varying regenerative abilities of these organs, likely linked to their different innervation patterns. Notably, we observed that the presence of the apex and spicules de novo synthesis influence the formation of the first rhinophore lamellae. The search for new patterns and mechanisms underlying the restoration of elements in the nervous system, muscular system, and solid skeleton can significantly contribute to our understanding of regenerative biology. This research expands our knowledge of nudibranch molluscs regeneration and the unique restoration of the subepidermal spicule complex. Furthermore, the regeneration of spicule-containing organs can be a model for studying the formation and structure of biomineralized structures, including their organic component.

Information on distribution of protected freshwater bivalve Amuranodonta kijaensis (Bivalvia, Unionidae, Anodontinae) is being clarified. For the Amur Region, three new locations of the species are indicated: Yatsenkovo and Peresheechnoye lakes on the territory of the Khingan Nature Reserve and Zeya Reservoir at the Beregovoy settlement. First record of A. kijaensis near Chnyrrakh village (Nikolaevsky District, Khabarovsk Territory) confirms the presence of the species in the Lower Amur basin. Data on morphometric analysis of collected shells of adult mollusks is presented. Using scanning electron microscopy, morphology of glochidial shells with special focus on microsculpture of the outer valves structure is investigated.

We provide a morphological examination and a barcoding study to investigate the species identity and variation limits within the Cadlina laevis species complex. Our molecular analysis based on the COI marker revealed seven new clades in the North-West Pacific Cadlina diversity. The distances between these clades are low in some cases (2.08-7.51% overall), and the species delimitation tests gave controversial results (1, 2, 13, 14 groups, depending on the analysis method). This does not allow to conclusively classify this diversity as interspecific or intraspecific. Morphological analysis showed a significant similarity of all examined groups, with minor differences found in the morphology of the central tooth of the radula and the reproductive system. However, these variations fit into the morphological variability of the North Atlantic species Cadlina laevis s.str. and cannot serve as evidence of the isolation of these identified groups. The discovered diversity may represent both a complex of at least 11 very close and cryptic species with not well-established species boundaries, or be a part of a single amphiboreal species Cadlina laevis s.l. This indicates an extremely complex evolutionary history of Cadlina laevis species complex, making this group is an interesting model object for studying speciation in boreal and Arctic communities.

In this paper we investigate a collection of nudibranch molluscs from Sakhalin Is. by means of integrative taxonomy, including morphological analyses, and molecular data from the cytochrome c oxidase subunit I, 16S rRNA, Histone H3 and 28S rRNA markers. Material used in this study was collected during two independent research programs: the expedition of the R/V “Akademic Oparin” (Russia) to the Sea of Okhotsk, July 2019 at depths of 38–282 m, and the survey of Cape Crillon biodiversity in August 2023, at depths of 0.5–20 m. Our integrative results revealed clear cases of the unknown diversity within some groups of the nudibranch molluscs. Among the dorid nudibranchs, the new species Adalaria boussoleana sp. nov. is described and differs in morphological and molecular characters from other species of the genus. Also, among specimens of the genus Cadlina we have detected two genetically distinct groups, one is close to C. umiushi and another group likely represents a new species for science. Their morphology is similar to the diagnosis of C. umiushi and may represent a case of true cryptic diversity. Among the cladobranch molluscs our results revealed a new species of the genus Cuthonella described herein under the name Cuthonella anastasia sp. nov. Four species are recorded for Sakhalin Is. waters for the first time: Cadlina umiushi, Onchimira cavifera, Eubranchus rupium and E. alexeii. After this paper, the total nudibranch fauna of Sakhalin Is. includes 21 species. Although we have limited numbers of nudibranch species in Sakhalin waters due to low sampling effort, a comparison of the biogeographic affinities of species from different regions is consistent with the presence of a biogeographic boundary between northeastern and southwestern Sakhalin.

Juvenile and subadult individuals of the Painter’s mussel Unio pictorum (Linnaeus, 1788) were recorded in dredge samples from the Om’ River in the Omsk City (southwestern part of Western Siberia). Information on the size-weight characteristics of U. pictorum from the site, its abundance and biomass are given. Possible consequences of the introduction of this neobiotic species into aquatic ecosystems of the Irtysh River basin are discussed.

This study investigates the shell variability of Phenacovolva rosea, a symbiotic gastropod inhabiting octocorals, in relation to geographical location and host coral species. We analyzed shells from 53 adult specimens collected at three locations in the north of Vietnam and one in the vicinities of Nha Trang City (southern Vietnam).

To describe the shells' outlines, elliptical Fourier descriptors were applied. Based on the shell outline data, principal component analysis was performed. Additionally, height and width of P. rosea shells were measured. Only the first two principal components were effective, with the first principal component accounting for 92.8% of the variation. It closely correlated with width-to-height ratio of shells. According to dispersion analysis (ANOVA), shells in Nha Trang were significantly slimmer with longer terminal processes compared to those from the three northern locations. The coral host species also impacted shell shape when considering the entire dataset. However, this difference may have stemmed from shell variations between locations, as coral composition is not independent of location. When regional samples were analyzed separately, no significant impact of the host was detected.

The factors that differed between northern locations and Nha Trang were then discussed, along with the mechanisms underlying their influence. We suggest that predation risk, influenced by water turbidity, is greater in the clearer waters of Nha Trang. This increased predator pressure may explain the presence of long terminals on P. rosea shells found in Nha Trang, potentially providing protection against some fish predators. Furthermore, the shorter and wider shells found in northern locations may be advantageous in waters with higher current speeds, whereas the long-teminal shells with narrow apertures observed in Nha Trang may be better suited for slower currents. The climate disparity, encompassing temperature and salinity mediated through precipitation, could also potentially influence shell morphology. However, the underlying mechanism remains unclear due to the complex nature of these interactions and the scarcity of literature data.

Isolating host-specific shell shape traits from other factors proved challenging, given the close association between host species composition and location. Laboratory experiments modeling each factor independently may provide a solution to this issue.

The present study reports the first observation of Euglesa globularis peaclams attached to dragonfly nymph Aeshna juncea, and E. parvula attached to the leg of the water boatmen nymph Callicorixa sp. from the Kolyma River basin in eastern Siberia (Asiatic Russia). It has been shown that the aquatic insects may play a potential role in the dispersal of bivalves both locally within one and between different water bodies.

For the first time carnivorous bivalve mollusc Cuspidaria trosaetes Dall, 1925 was recorded in the north-western Sea of Japan. There was only one species of Cuspidaria, C. ascoldica, previously known in this region. Alive specimens of C. trosaetes were collected in the Tatarsky Strait on the continental slope on muddy sediment at the depths of 531–589 m. Stomach contents of C. trosaetes consist of foraminifers, copepods and rissoid gastropods. Cuspidaria trosaetes was selected as a type species of a monotypic subgenus C. (Nordoneaera) Okutani, 1985, which is now considered a separate genus, based on the presence of a thick shell without sculpture and with thick periostracum, as well as a short rostrum. The first obtained data on anatomy of C. trosaetes suggest its close affinity with the type species of the genus Cuspidaria, C. cuspidata. Cuspidaria trosaetes and C. cuspidata share such features as the presence of 5 pairs of septal pores, thin elongated posterior labial palps and the presence of two rows of papillae on the rim of the common siphonal sheath. Taking into account new data we propose to synonymize the monotypic genus Nordoneaera with the genus Cuspidaria.

Sequentially appearing in ontogenesis, the morphometric features of mussel Mytilus galloprovincialis larvae can serve as a basis for their species identification among the pool of larvae of  other bivalve mollusc species found in the Black Sea plankton. The study presents the photographs of live mussel larvae and scanning electron microscope (SEM) images of the larvae hinge at different developmental stages: D-veliger, veliger, veliconcha and pediveliger. There was shown the sequence of morphological changes in provinculum, as well as the shell’s size and shape changes, from the earliest straight-hinge stage of veliger up to metamorphosis. The correlation between shell height (H, μm) and shell length (L, μm) [H = 1.0022∙L–29.374; R2 = 0.9879], as well as the correlation between hinge edge length (l, μm) and shell length (L, μm) [l = 0.0009∙L2 – 0.018∙L +77,78; R2 = 0.9872] were determined for mussel larvae ranging in size from 98 to 350 µm. By employing energy dispersive X-ray spectroscopy EDS(x), the elemental composition of the larvae shell’s hinge edge at the stages of veliger, veliconcha and pediveliger was determined; and it showed the presence of calcium, carbon, oxygen, sodium and chlorine. Magnesium was detected in pediveligers’ hinge edge only.

Mytilisepta virgata was first collected as empty shells attached to plastic debris in south-western part of Peter the Great Bay (Sea of Japan), near Russian-Korean border. This species may be considered as a potentially non-indigenous species in Russian waters due to coastal warming, influence of warm-water currents and its presence in North Korean waters. A description, illustrations, data on distribution and ecology and taxonomic comments are provided.

By sequencing ISSR DNA fragments of the Austrian snail (Caucasotachea vindobonensis), cloned into the pAL2-T vector, sequences of microsatellite loci (STR) and their flanking regions were obtained. For the nine identified STR loci with 2, 3 and 4 repeats, the design of primers adapted for fragment analysis was carried out. Subsequent genotyping of 100 individuals of the Austrian snail from various populations of Eastern Europe to estimate the volume allelic diversity of the species based on detected STR loci. All loci yielded unambiguous genotypes. Eight STR loci turn out to be polymorphic and have from 2 to 19 alleles.The data obtained can be used to assess the genetic diversity and phylogenetic analysis of populations of the Austrian snail, listed in the Red Book of the Belgorod region of Russia and the protection lists of Europe.

The genus Caspiohydrobia belongs to the less-known taxa of the family Hydrobiidae. According to a conchologically-based system, developed in the 1970-1980s by Yaroslav I. Starobogatov and his co-authors, the genus includes around 30 living species. However, the most recent data based on molecular evidence indicate that most (if not all) of these morpho-species should be considered junior synonyms of a widely distributed species, Ecrobia grimmi. However, the vast majority of nominal species described in the genus Caspiohydrobia have never been reassessed by molecular taxonomic methods. In this work, the genetic diversity of Caspiohydrobia spp. was studied based on a large sample taken in Lake Elanach in the Kurgan region (Trans-Urals, Russia). Conchological analysis made it possible to identify 7 nominal species of the genus Caspiohydrobia in the sample, whereas genetically all the studied individuals were the same, sharing a single COI haplotype. A phylogenetic tree of Ecrobia spp. has shown that all studied genetically specimens of Caspiohydrobia (including newly sequenced ones) forms a separate cluster on a cladogram, which can be identified as Ecrobia grimmi s. lato. However, a higher genetic diversity was revealed in a population of Caspiohydrobia spp., from a lake adjacent to the Caspian Sea (West Kazakhstan). Distinct morphotypes in the structure of the protoconch were found in samples of Caspiohydrobia spp. from the Caspian and Aral Seas. Although the characteristics of the protoconch (size, sculpture) do not give grounds to distinguish ‘good’ species within the group, the genetic and morphological diversity described in this work makes it possible to assume the ongoing microevolutionary processes within the discussed group and, possibly, the existence of young (incipient) species in Central Asia. Based on the study of the morphology of the protoconch, it has been suggested that the development of molluscs of the genus Caspiohydrobia (= Ecrobia, partim) can include the free-swimming larval stage; this hypothesis is, however, not supported by other evidence and needs to be carefully verified.

The available data about the recent distribution of slugs of the family Arionidae on the territory of Belarus are summarised. The findings of a species formally new to the malacofauna of Belarus, A. fasciatus, are discussed. The material was collected in the period from 2018 to 2024 on the territory of the Gomel region (South-Eastern Belarus). The original illustrations of the habitus and anatomical features of the reproductive system are given.

A new genus Barbadoschiton gen. nov. [type species B. bromleyi (Ferreira, 1985)] of the family Ischnochitonidae from shelf near Barbados and Martinique Islands is described. The genus possesses unique features of the tegmentum and girdle. The central area of the intermediate valves and antemucronal area of the tail valve show teardrop-shaped granules arranged in quincunx and have only one pore of aesthete. The marginal fringe of girdle is beset with five kinds of elements, among these long, fine needles, flattened, obtuse pointed spicules with three ribs on one side; spicules with longitudinal groove on one side and with 7 longitudinal ribs, on the other side; numerous, small spicules with 5 longitudinal ribs and one longitudinal groove; flat, obtuse spicules with two warts on upper half. The ventral spicules are of two kinds: main flattened, rectangular spicules with distal appendix and three weak longitudinal ribs on external part; and small distally pointed, smooth on the outer side and with two longitudinal grooves on the opposite side of the spicules. The latter are attached in transverse rows with their proximal part between the main ventral spicules. The major lateral teeth of the radula have a tridentate head, with the middle denticle being longer than others.

The development of the larval shell and hinge of the bivalve mollusk Macoma balthica in the White Sea is described. Macoma spawning occurs at the end of June. Larvae at the D-stage of development have a shell length of 120-140 µm. The shell is oval, the hinge edge is straight with a noticeable striation. During development, the shell becomes more elongated with a pointed anterior end. A characteristic feature is the expressed concentric striation of prodissoconch II. The umbos are low and rounded. Hinge edge is straight, with small teeth of provinculum. The ligament occupies the central position. At later stages, a pair of large hook-shaped teeth is formed in the anterior part of the hinge. Metamorphosis occurs at shell length of 300-310 µm.

In 1951, the Swedish malacologist Bengt Hubendick published a catalog of species-group names in the family Lymnaeidae proposed since 1758 [Hubendick, 1951]. This catalog included 1151 entries, with information in their type localities, bibliographic references, and the position of each taxon in the then used system. Extinct taxa were not listed. This publication is an expanded and substantially revised version of Hubendick's catalog, taking into account both all species-group names proposed for the pond snails, from 1951 to 2024, as well as a number of names once overlooked by Hubendick. The present Nomenclator contains 1557 names, for each of which a reference to the original description, an indication of the type locality, the position of the taxon in the current system, and an indication of a later publication in which a photographic image of the type is given (if such a publication occurred). In some cases, taxonomic and nomenclatural remarks are provided regarding the status of particular taxa.

A new species belonging to the Fionidae (sensu Cella et al., 2016), Eubranchus flexus sp. nov. is described based on specimens collected in Nha Trang, Vietnam, South China Sea, Indo-West Pacific, from the upper sublittoral depth. An integrative analysis was conducted, including a molecular phylogenetic analysis based on three markers (COI, 16S, H3), and an analysis of the external and internal morphology using light and scanning electron microscopy. The distinctiveness of Eubranchus flexus sp. nov. is well established both morphologically and genetically. Phylogenetically Eubranchus flexus sp. nov. represents a derived branch in the phylogeny of Eubranchus, but its relationships with other Eubranchus species are unresolved likely due to low taxon sampling. Eubranchus flexus sp. nov. is the first representative of the genus Eubranchus for the nudibranch fauna of Vietnam and for now its range is restricted only to the type locality.

Using light and scanning electron microscopy the first data on morphology of mature glochidia of freshwater bivalve Beringiana beringiana from Dalneye Lake in Kamchatka were received. Based on literature data a comparison with glochidia from other populations of this species is given. Among the Russian Far East populations of the discussed species, glochidia from the Dalneye Lake were the smallest, with shell sizes up to 287 µm and with a hook that does not exceed 1/3 of the glochidium height. Microsculpture of the outer surface of glochidia was tight-looped all over the whole valves.