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.

The results of the experimental study of food preferences of Arion vulgaris in the conditions of Central Russia are presented. Fifty-five native and alien plant species from 22 families were used in the experiment. A. vulgaris causes the greatest damage to the Apiaceae and to a lesser extent to the Polygonaceae, Onagraceae and Ranunculaceae. Assessment of trophic preferences of the A. vulgaris showed a wide food plasticity, providing high adaptability of this species.

A new species of the bythiniid snails, Opisthorchophorus confusus Andreeva sp. nov., is described based on molecular and morphological data. A COI phylogeny inferred on the material of newly obtained sequences and those accessible through the GenBank has releaved the species distinctness of O. confusus sp. nov. as well as the full genus status for Codiella Monterosato, 1894 and Opisthorchophorus Beriozkina et Starobogatov, 1995. The genus Pseudobithynia Glöer et Pešić, 2006, on the other hand, has been revealed as paraphyletic, but the proper status of this taxon is unclear, since no genetic information is available on its type species distributed in Iran. The new species is widely distributed in Russia; its genetically confirmed localities are situated in Krasnodar Krai (south of European Russia) and Western Siberia. Based on morphologically identified specimens, the range of O. confusus sp. nov. embraces European Russia, southern parts of Western and Eastern Siberia as well as Northern and Central Kazakhstan. The new genus is conchologically and molecularly distinct from the species Opisthorchophorus troschelii (Paasch, 1842) [known also as Bithynia transsilvanica Bielz, 1853]. The nomenclatural issues related to the name Bihynia troschelii are discussed.

The snails of the genus Helicopsis belong to steppe species, many of which are listed in the protected lists of European countries. In this work, based on the sequencing of the COX1 mitochondrial gene, the species belonging to eight populations of snails from the genus Helicopsis living in the south of the Central Russian Upland have been identified. These populations were assigned to H. lunulata, H. filimargo, and H. hungarica, and the average genetic distance between them was 0.11±0.01. Next, we analyzed the variability of seven microsatellite (STR) loci developed by us for H. lunulata in one population of H. filimargo, one population of H. hungarica and five populations of H. lunulata in the south of the Central Russian Upland. Both loci specific to H. lunulata and universal for all studied species have been identified. In H. lunulata, six STR loci turned out to be polymorphic and had from 2 to 11 alleles per locus. A total of 28 alleles were identified at all loci in H. lunulata, 14 of which were private and did not occur in more than one population. Molecular Dispersion Analysis (AMOVA) confirmed the high genetic differentiation of H. lunulata populations (Fst=0,425). At the same time, the genetic diversity of H. lunulata in the study area based on STR loci turned out to be higher than in the case of using allozyme loci. The structure of genetic variability calculated on the basis of STR loci turned out to be less pronounced compared to the use of allozyme loci as genetic markers (Fst=0,700). All this confirms the greater selective neutrality and greater diversity of microsatellite loci compared to allozymes. The data obtained can be used to assess the genetic diversity of H. lunulata. populations.

An impressive, medium-sized new camaenid species, Ganesella tamchuc sp. nov., is described from Hà Nam Province, northern Vietnam. Ganesella tamchuc sp. nov. is similar to Ganesella emma (Pfeiffer, 1863) in its general shell coloration pattern but differs by having a smaller shell size; three distinct color belts on the shell surface, including a white belt at the keel and two dark brown belts situated above and below the keel; strong radial ribs that protrude from the shell surface and also present at the umbilicus and within the aperture; a distinctly keeled last whorl that becomes weak behind the peristome; and a widely open umbilicus through which all whorls are visible.

A new deep-water species Stenosemus formosaensis sp. nov. collected in the northeast of the South China Sea southwest of Taiwan is described. From other species of the genus Stenosemus the new species is distinguished by dark olive stripes radially arranged on head valve and lateral areas and longitudinally on pleural areas, and also by the arrangement of grooves of different tegmentum zones.

This paper provides first reliable data on the finding of the bivalve mollusc Thracia septentrionalis in the Sea of Japan. A live specimen (L = 24.4 mm) was collected in June 2003 in Peter the Great Bay at a depth of 70 m on medium-grained sand. The data on geographical distribution and ecology of the species in the northern Pacific Ocean are summarized. The validity of Thracia seminuda Scarlato, 1981, previously synonymized with T. septentrionalis, is confirmed. Diagnostic differences between these species are specified and illustrated.

In the Kherota River (Russian Black Sea coast), freshwater mussels not previously known in the area were discovered and morphologically identified as Unio gontierii Bourguignat, 1856 (Bivalvia: Unionidae). The mussel species identity was verified by analysis of the mitochondrial gene sequence encoding cytochrome c oxidase subunit I (COI). The diversity of the eight studied microsatellite loci in the studied sample was rather high (the average number of alleles per locus was 4.75, the observed heterozygosity was 0.50±0.08). The aforementioned facts, in conjunction with the substantial distribution of the bitterling Rhodeus colchicus Bogutskaya et Komlev, 2001 along the Russian Black Sea coast, whose larvae develop in the bivalves, indicate that Unio gontierii is an indigenous inhabitant of the Russian part of the Caucasian coast and not a recently human-introduced species.

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.