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.

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.

Chuatienpupa megacanalis gen. et sp. nov. is described from Hoa Binh province, northern Vietnam. Chuatienpupa gen. nov. is characterized by a small, pupoid-shaped, smooth, glossy shell. Peristome with two canals; anterior canal twisted funnel-shaped, appearing as a very wide groove, opening on the basal side; posterior canal continues vertically upward and forming a wide groove, which is divided by the upper palatal plica. The operculum bowl-shaped, strongly convex, its outer surface smooth, glossy, and covered by a glaze. Both shell and opercular characters distinguish this new genus from other Southeast Asian pupinid genera. General shell morphology of Chuatienpupa gen. nov. is most similar to Pupina Vignard, 1829. However, the new genus differs in having two palatal and basal plicae, a twisted anterior canal, opening on the basal side, and a solid, bowl-shaped operculum, with outer surface dome-shaped and completely smooth.

Here we present detailed morphological and molecular comparison of the Tonicia species and ecotypes along Southeastern Pacific coast. Several specimens were examined by scanning electron microscopy and based on plates morphology we propose a new taxonomic key for identification of species and ecotypes of Tonicia. A total of 566 specimens were measured to explore morphological differences between ecotypes and ecoregions by means of multivariate analyses. Genetic distances and a maximum likelihood (ML) phylogeny were estimated using Cytochrome Oxidase I (cox1) to compare species and ecotypes. Morphometry and multivariate analyses revealed morphological differences between ecotypes in each species. However, cox1 genetic distances were low between ecotypes in both species (T. calbucensis and T. chilensis) and the ML phylogeny revealed an absence of monophyletic relationships between ecotypes in each species clades. The new key will be useful to identify Tonicia species by morphology without molecular analysis.

The work is based on a study of long-term collections of molluscs of the family Bithyniidae from the waterbodies of the Tyumen region (Western Siberia), located within the world's largest Ob-Irtysh natural focus of opisthorchiasis. Difficulties with species identifications of bithyniid snails, the first intermediate hosts of Opisthorchis, necessitate the publication of photographic images of Bithyniidae shells and an identification key based on discrete morphological characters. It has been established that 14 species of Bithyniidae belonging to five genera inhabit the Tyumen region, an annotated list of species is given, and a brief zoogeographical description is given. The most abundant species are Digyrcidum starobogatovi and Boreoelona sibirica, whereas Paraelona milachevitchi were found solitary.

A new camaenid Aegistohadra baii sp. nov., close to Aegistohadra delavayana but having smaller and more conical shell and anatomically different, was described from Yunnan, southwest China. Helix seraphinica Heude, 1889 was confirmed to be an Aegistohadra species based on structure of genital organs. Cryptozona yunlongensis Chen Y., Chen D. et Zhang, 1997 is transferred to Aegistohadra based on shell and genital characters.

Described is a new species of the genus Stenosemus from the Gulf of Alaska. It differs from the other species of this genus from the Northeast Pacific by having longitudinal undulating folds in the pleural areas of the intermediate valves, flattened, smooth dorsal calcareous corpuscules on perinotum and rectangular central teeth of radula.

On the basis of the collections of the R/V Vityaz in the Banda Sea, Indonesia, a review of the chiton fauna of the genera Belknapchiton and Leptochiton was carried out. Leptochiton commandorensis, until now known only from the northern Pacific near the Commander Islands, is here added to the four known Indonesian deep sea species. This species turned out to be with juveniles in the pallial groove. A scanning electron microscope study of the collected specimens of B. giganteus made a corrected, supplemented description of this rare species possible. The age variability of the valves shape and shell sculpture of the two collected species was studied. Particular attention is paid to the age variability of the number of micraesthetes on the tegmentum grains and the position of the mucro.

In terrestrial pulmonate gastropods Cochlodina laminata, Monachoides incarnata, Helicigona lapicida, Arianta arbustorum, Cepaea hortensis, Trochulus hispidus and Succinea putris, the morphological properties of the pupil of camera-like eyes were studied for the first time: location, spatial orientation, change in spatial orientation, structure, shape, shape change, relative size, change in relative size. Several methods were used in the work: making of preparations of isolated eyes, making of histological preparations of eyes, light microscopy, morphometric analysis, statistical analysis, calculations. All the studied morphological properties of the pupil of the camera-like eyes of C. laminata, M. incarnata, H. lapicida, A. arbustorum, C. hortensis and T. hispidus are similar to the analogous morphological properties of the pupil of the camera-like eyes of other species of terrestrial pulmonates. One of the studied morphological properties of the pupil of the camera-like eye of S. putris – the relative size – differs from the analogous morphological property of the pupil of the camera-like eyes of other species of terrestrial pulmonates.

The article reports the finding of a population of freshwater snails morphologically and genetically identified as Ladislavella cf. terebra (Westerlund, 1885), in a reservoir situated in Penza City. This is the first reliable record of representatives of the genus Ladislavella on the territory of European Russia, at a considerable distance from the previously known boundaries of its range. Molecular data suggest that the examined population belongs to the previously unknown species of Ladislavella. The morphoanatomic and ecological characteristics of the studied population and some information about parasites and commensals of mollusсs are given.

This study examines and provides illustrations of the diagnostic shell characteristics of juvenile and adult of Crimean clausiliids Balea perversa. The study demonstrates that the distinctive sculpture and shape of the shells make it possible to accurately differentiate Balea perversa. Additionally, the study includes information on previous references in the literature and recent findings of Balea perversa in the Crimea region, including a discovery on the Southern Demerdzhi Yayla in 2002. The study confirms the reliability of at least the last finding.

Chicomurex kozlovi sp. nov., collected from the Saya de Malha Bank in the Mascarene Plateau, Western Indian Ocean, is described and illustrated. The holotype was sampled at a depth of 200 m during the expedition of R/V Gordy in 1989, while paratypes 1 and 2 were trawled from a depth of 100-200 m by research-fishery boats during the eighties. Chicomurex kozlovi sp. nov. is compared with morphologically similar congeneric species.

The taxonomic and biogeographical composition of gastropod molluscs in the antique part of the fund collection of the State Historical and Archaeological museum-preserve Tauric Chersonese was diagnosed and analyzed. The complex of molluscs includes species from the Black Sea, Mediterranean, and Indo-Pacific groups. The shells of the European horn of Triton Charonia seguenzae were found only in Chersonese. To establish probable trade links between Chersonese and the Scythian-Sarmatian and Goth-Alanian worlds, the studied mollusc complex has been compared with archaeological finds in the Crimea and beyond. The biogeographical composition of mollusc complexes from the archaeological layers of Chersonese and "barbarian" necropolises has been shown to be identical, indicating the unity of the sources and the import route of shells.