The new species, Vallonia yakutica sp. nov. is described from the central part of the Republic of Sakha (Yakutia). The population lives on the edge of the forest of larch (Lárix sibírica ldb.), and birch (Betula pendula Roth.) trees, with various herbs. The presence of herbs is necessary for the habitat of Vallonia yakutica sp. nov. which feeds on their fallen leaves. The maturity of the snail is achieved in two years. The differences between the new species from the morphologically most close Vallonia kamtschatica Likharev, 1963 are discussed.

The response of mollusk communities to radioactive contamination in natural freshwater ecosystems remains understudied. We have analyzed the data on species composition of bivalve and gastropod mollusks inhabiting the «Mayak» Production Association (PA) (Russia, Chelyabinsk Region) special industrial reservoirs (R-3, R-4, R-10, R-11, R-17). These waterbodies have the status of facilities of atomic energy industry (the “atomic heritage”) and are used as storages of liquid radioactive waste. The Shershnevskoye Reservoir (Chelyabinsk City, Russia), a waterbody with background activity concentration of anthropogenic radionuclides, has been studied as a control site. Radioactive contamination has led to chronic exposure of freshwater mollusks. The dose rate ranges were as follows: 6–14 mGy/day for reservoir R-11; 16–40 mGy/day for reservoir R-10; 37–73 mGy/day for reservoir R-4; 49–72 mGy/day for reservoir R-3; 1,270–2,370 mGy/day for reservoir R-17. It has been found that chronic radiation exposure leads to a decrease of mollusk species diversity. 21 species of bivalves and 35 species of gastropods were registered in the control waterbody. In radioactively contaminated sites, a sharp decrease in species diversity has been observed. For example, in reservoir R-11, 7-fold decrease in the number of bivalve mollusk species and 2-fold decrease in the number of gastropod species as compared with the control reservoir have been recorded. In R-10 and R-4, the number of bivalve species was reduced 20-fold and the number of gastropod species was reduced 5- and 9-fold, respectively. In R-3, no bivalves were registered, and the number of gastropod species was reduced 6-fold. No mollusks were detected in R-17. Among all hydrobionts, pelophilous bivalves of the family Sphaeriidae, as well as gill-breathing snails (Gastropoda, Caenogastropoda), whose life cycle is associated with bottom sediments, are the most vulnerable to the effect of chronic radiation exposure.

Age-related variability of the shell valves and radula in two species of the genus Callistochiton and one species of the genus Ischnochiton is discussed based on the photographs obtained with a scanning electron microscope. Particular attention is given to the shape and sculpture of the valves, as well as the shape and number of teeth in the transverse row of the radula. It is shown that even such a key feature of the genus Callistochiton as radial ribs on the head, intermediate and tail valves is absent at the early stages of development and appears only when the body length reaches 4–5 mm. The results of studying age-related variability are of great importance for the taxonomy of both recent and fossil mollusks.

A description of Turanena gissarika sp. nov. from the Gissar Ringe is presented. Conchologically, the new species is most similar to T. cognata, but anatomically it differs in having a tubercle-like swelling on the opposite side of the caecum.

The morphological features and ultrastructure of the hinge margin of the giant oyster Magallana gigas larvae, which were obtained from a nursery with the Black Sea broodstock and grown under optimal conditions, were studied. The paper presents scanning electron micrographs of disarticulated larval valves at different stages of development. In larvae at the D-veliger stage (2 days old, shell dimensions: 72×62 μm), as well as at the veliger stage (shell dimensions: 84×96 μm), the provinculum of the right and left valves consists of two anterior and two posterior teeth. We noted 2 to 3 teeth on the provinculum in both ”uneyed“ (H: 140–182 μm) and ”eyed“ velikonch larvae (H: 200–230 μm) on the anterior and posterior edges of the right and left valves. In pediveligers (H: 338–358 μm) there are two anterior teeth in the hinge margin of the right and left valves; the posterior teeth are reduced. We also determined logarithmic dependences of the hinge margin length (l, μm) on the length (L, μm) of the shell [l=32.518•ln(L) – 96.35; R2=0.904] and the maximum provinculum width on the shell height [c=5.8937•ln(H) – 17.994; R2=0.8993] of larvae ranging in size from 72×62 to 322×358 μm. The morphological characteristics and provinculum ultrastructure of the reared larvae were compared with those of M. gigas larvae of Atlantic and Pacific origin. The article discusses the potential hybrid nature of the Black Sea broodstock.

The taxonomic status of Indoartemon vietnamensis Thach, 2020 is reassessed. The morphology of I. vietnamensis falls within the intraspecific variation of I. huberi (Thach, 2016). The similarities suggest that I. vietnamensis should be treated as a junior subjective synonym of I. huberi. Furthermore, a new subspecies of the genus Indoartemon is described from Chua Chan Mountain, Dong Nai Province, Vietnam. Indoartemon huberi chuachanensis subsp. nov., has oblique-heliciform shell with axially deflected last whorl, narrow and deep umbilicus, aperture with one strong parietal lamella and one small palatal lamella, slightly enlarged columellar fold along the columellar margin.

Gastropod mollusk Jujubinus exasperatus is recorded for the first time in the Black Sea off the coast of Crimea. A brief species diagnosis, photographs of the shells and live specimen in situ are provided. Previously, the species was listed for the fauna of Russia and the Black Sea based on literary sources, the reliability of the information discussed in this paper. The possibility of naturalization of J. exasperatus in the Black Sea ecosystem is considered. J. exasperatus is recognized as an accidental element of the Black Sea benthos.

Three new species have been added to the formerly monotypic genus Liobaicalia Martens, 1876. We employed a combination of morphological, anatomical, and molecular approaches using mitochondrial COI gene, nuclear ATP-synthase alpha subunit gene intron (ATPase), and ITS1 spacer to delineate species boundaries between Liobaicalia stiedae (W. Dybowski, 1875), L. sidelevae sp. nov., L. karabanovi sp. nov., and L. riedeli sp. nov. The Liobaicalia species inhabit burrowing into sandy substrates in disparate regions of the ancient and/or recent deltas of the inflows to the south and central Baikal basins near tectonic faults where gases discharge happens. The genus Liobaicalia was not recovered as monophyletic based on current molecular-phylogenetic analyses based on mitochondrial COI data. We discussed discordance between morpho-anatomical and genetic traits, the possible origin of the Liobaicalia species, and the occurrence of scalariform shells in Baikal and non-Baikal gastropods belonging to different families.

The generic name Cryptobranchia Middendorff, 1851 is a junior objective synonym of Lepeta J.E. Gray, 1842. Designation of Patella caeca var. concentrica Middendorff, 1848 as a type of Cryptobranchia is invalid. For two species previously assigned to the genus Cryptobranchia [Lepeta concentrica (Middendorff, 1848) and Acmaea kuragiensis Yokoyama, 1920], the new generic name Cryptolepeta gen. nov. is proposed.

The taxonomic identity of Iberus alonensis has traditionally been surrounded by great controversy because under this name a large number of similarly-shaped populations distributed over a vast geographical area and probably belonging to different species have been subsumed. The publication of the first phylogenetic study on the genus Iberus began to unravel the alonensis complex, defining other likely new lineages. One of them is I. alonensis-like 02, a provisional name assigned to the traditional populations of I. alonensis from Andalusia (southern Spain) but lacks sufficient biogeographic support to be formally defined as a new species. In this study, the potential distribution area of I. alonensis-like 02 is thoroughly explored through systematic sampling. The resulting geographical mapping together with a morphometric study and new molecular evidence allow us to confirm the historical name I. alonensis rhodopeplus as a valid taxonomic denomination which we divide into two cryptic subspecies named I. rhodopeplus rhodopeplus and I. rhodopeplus bastetanus ssp. nov. Besides, the subspecies I. alonensis labiatus has to be renamed. Our findings contribute to revealing the species complex involved under the classic name I. alonensis and to improving our knowledge on the cryptic nature of numerous taxa within the genus Iberus.

Several rostroconch specimens of the family Bransoniidae are discovered in the Pennsylvanian of the Russian Platform for the first time. All of them are assigned to the species Hoarepora parrishi (Worthen, 1890), which is the type species of Hoarepora Mapes in Mazaev, 2022. Until now, the distribution of this species has been restricted to the Morrowan – Missourian interval in the North American Midcontinent Basin. All studied specimens come from Moscovian and Gzhelian stages of two localities on the Russian Platform. Both shell morphology and taphonomy of this species are examined in detail. H. parrishi is redescribed, since the previous description lacks many specific characters. It is shown that Conocardium missouriensis Roundy in Girty, 1915 is a junior synonym of C. parrishi Worthen, 1890. The new occurrences significantly expands the geographical range of this species and supports the existence of biogeographical connections between the paleobasins of North America and the Russian Platform in the Pennsylvanian, and also increases the known geochronological range of H. parrishi by approximately two million years.

Buccinum osagawai is a valuable commercially exploited gastropod species inhabiting the northern Pacific. An accurate and reliable method for estimation the mollusk’s individual age is needed to understand the population dynamics and size-age structure of B. osagawai stock. The most commonly used method of estimation of the individual age of mollusks of the genus Buccinum is counting the increments on the operculum. However, this method has disadvantages, for example, since the operculum is an external recording structure, its readability is relatively low. To develop a more accurate method for determining individual age, the microstructure of statoliths of the gastropod B. osagawai and its features were studied and the possibility of using B. osagawai statoliths to determine the individual age of an animal by counting increments on thin sections of statoliths was tested. The relationship between the number of statolith increments and the number of marks on the operculum surface was studied. A comparative analysis of these two methods of age determination is provided.

This paper described the second species of the genus Calybium Morlet, 1892, Calybium plicatus sp. nov. was collected in Son Doong Cave, Phong Nha–Ke Bang National Park, Quang Binh Province, Central Vietnam. Calybium plicatus sp. nov. has a similar shell shape to Calybium massiei Morlet, 1892 but differs in having a smaller shell size, the parietal wall with six evenly spaced parietal lamellae. This discovery represents the first report of the genus Calybium in Vietnam.

A new shallow-water species Lepidochitona cubensis sp. nov. collected in the Caribbean Sea off Cuba in sea grass Thalassia testudinum is described. This chiton is closest to the species L. bullocki with which it forms a separate group within the genus, differing from other species of the genus by the presence of small square grooves on tegmentum of valves.

Based on specimens collected during the field trips in 2019–2020, 2023 in the Tho Chu Islands and Spratly Islands (Truong Sa Islands), Vietnam, we have identified five species of the genus Viriola. Two of them, namely Viriola thochuensis n. sp., Viriola namyitensis n. sp. are new to science. Other three species, namely, V. abbotti, V. bayani, V. intergranosa are the first records for Vietnam. The diagnostic characters and distribution of Viriola species in Vietnam are presented.

This study reports the first record of the operculate micro land snail from the Alycaeinae, Chamalycaeus aduncus Jirapatrasilp sp. nov., also the first new species of the genus in Cambodia, based on a recent survey in Battambang. This new species is similar to Chamalycaeus excisus (Möllendorff, 1887) and C. excisus sublimus Páll-Gergely et Auffenberg, 2019 from the Philippines, in shell size and shape, and the strongly downwards coiling of R3, but differs in having a shorter R2 without any blunt keel, a round aperture without any incision, and a beak- or hook-like basal edge of the outer peristome.

In this study, we report the discovery of two invasive freshwater mollusc species belonging to the genus Galba (Gastropoda: Lymnaeidae) on the territory of Azerbaijan. This is the first record of the species Galba cubensis  and G. schirazensis in this country, based on the analysis of morphological traits and sequencing of the mitochondrial cytochrome c oxidase subunit I gene (COI marker). Also, the phylogeographic analyses of both species were performed based on a median-joining network approach of the COI sequences. The find of G. cubensis is the first record of this invasive snail of New World origin in the ex-USSR territory. The taxonomic position of G. schirazensis sensu Kruglov et Starobogatov is discussed.

The gastropod feeding apparatus originally comprises the radula and jaws. While the radula has been extensively studied, data on jaws remain limited. Nevertheless, investigating jaw morphology is essential for understanding feeding mechanisms and morphofunctional adaptations. This study addresses this gap by presenting novel data on jaw morphology across a range of gastropod species from various clades, including Vetigastropoda, Littorinimorpha, and Neogastropoda. Based on our data and previously published research, we propose the existence of two distinct types of jaws: simple and complex. Simple jaws (found in the majority of Patellogastropoda and some Neogastropoda) consist of solid plates, whereas complex jaws (found in Vetigastropoda, Caenogastropoda, and Heterobranchia) are composed of both a solid homogeneous layer and a layer of solitary structures, such as rods or hooks, referred to as rodlets (as in some heterobranchs). Electron microscopy data enabled us to homologize the complex jaws of Vetigastropoda and Caenogastropoda. The jaws of representatives from both subclasses exhibit a similar synthesis mechanism, in which a single gnathoblast synthesizes an individual rodlet. The jaws of Neogastropoda and Tonnoidea display considerable diversity, both in their localization within the buccal complex and in their morphology, ranging from paired smooth or complex structures to highly modified structures acting as a stylet. Furthermore, the varied localization of jaws in neogastropods suggests a high potential of the buccal epithelium for the formation of diverse jaw structures.

The main goal of the present paper is to revise the diversity of the nudibranch genus Ziminella and update the geographic ranges of its species, based on historical museum material and newly collected specimens of Ziminella japonica (Volodchenko, 1941) from the Sea of Japan. We provide morphological examinations, a molecular phylogenetic analysis based on four markers, and species delimitation analyses of Ziminella. Our results indicate that Z. salmonacea (Couthouy, 1838) and Z. circapolaris Korshunova et al., 2017 represent a single species but display some intraspecific variation in the COI marker (up to 2.54% between individuals). The species delimitation analyses cannot explicitly support the distinct status of Z. japonica and Z. vrijenhoeki Valdés et al., 2018, but in this case the minimal interspecific p-distance in COI comprised 7.95%. Moreover, several differences in the coloration and external morphology are found in specimens from the type localities of these two species. We also show that Z. japonica displays high morphological diversity across its putative geographical range. Since this variation could be either intraspecific or interspecific, we suggest that specimens collected outside the type localities and lack molecular data, cannot be precisely identified. The molecular phylogenetic analysis does not support close relationships of the shelf species Z. japonica and the abyssal species Z. abyssa Korshunova et al., 2017, thus a hypothesis of a shelf origin of the latter species cannot be confirmed by the presented data. Finally, we provide novel evidence on the hidden diversity of flabellinids in the deep-sea waters of the Sea of Japan.

For the first time in the Russian waters of the Sea of Japan, the tropical-subtropical species of bivalve mollusk, Musculus cupreus (Gould, 1861), was found. An illustrated description of shells, synonymy, information on distribution and ecology are provided. An assumption is made about the probable introduction of M. cupreus into Peter the Great Bay.