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.

Additional material collected in September 2019 made it possible to clarify the range of conchological variability of Harmozica zangezurica recently described from the Syunik region of Armenia, which largely overlaps with that of Harmozica pisiformis thus rendering impossible the reliable identification of empty shells. At the same time the stability has been proven of a diagnostic feature of H. zangezurica – very long vaginal appendages whose length is approximately equal to the total length of the penis and epiphallus,. This character allows reliable distinguishing of H. zangezurica not only from the conchologically similar species H. pisiformis, but also from other representatives of the genus Harmozica. Some data on the ecology and life cycle of H. zangezurica were obtained.

Mature glochidia of freshwater bivalves can give additional features for taxonomic revisions. To study morphology of glochidia with the scanning electron microscope (SEM), special collecting techniques, storage and preparation are required. Based on extensive personal experience, an overview of various techniques is presented, both positive and negative. The interactions of chemicals used for the storage of glochidia and dissolution of glochidial tissue are described. Techniques for cleaning and mounting the glochidial shells of Unionidae for investigations by SEM are described and compared.

Studying of the demographic characteristics of populations of alien species at the stage of expansion makes it possible to assess their invasive potential. Field studies of alien terrestrial gastropods Xeropicta derbentina (Gastropoda, Stylommatophora, Hygromiidae), Brephulopsis cylindrica (Gastropoda, Stylommatophora, Enidae) and Harmozica ravergiensis (Gastropoda, Stylommatophora, Hygromiidae) have been carried out in Belgorod city, the southern part of the Central Russian Upland (Russia). All studied species are native to the Black Sea region and the Caucasus; in the southern part of the Central Russian Upland, they have been registered over the past ten years. The population density, demographic structure, and spatial structure have been analyzed for three years of observations (2017, 2019, 2020). During this period, the changes in the population density have been noted for all studied species. The most pronounced changes have been observed in X. derbentina, whose population density has significantly decreased, and the spatial structure has changed from clustered type to random distribution. In addition, the age structure of X. derbentina population had a pronounced change at the beginning of the snail activity season of 2020.

The expansion of the other two invaders, B. cylindrica and H. ravergiensis, is proved as more successful. At the same time, H. ravergiensis is distributed throughout the city. This species has stable population density and random type of spatial population structure on most sites. B. cylindrica has the highest population density: in 2020, it has reached 406 ind./m2 on average, with the maximum of 1215 ind./m2. Meantime, the species keeps the clustered spatial structure, which is explained by both high population density and adaptation to the arid conditions of its natural range.

A new species of the family Coryphellidae, Coryphella alexanderi sp. nov. is described based on specimens collected in the Kuril Islands, North-West Pacific, from the upper sublittoral to 200 m depth. An integrative analysis was conducted, including a molecular phylogenetic analysis based on four markers (COI, 16S, H3, 28S), an automatic species delimitation method ABGD, and an analysis of the external and internal morphology using light and scanning electron microcopy. The distinctiveness of Coryphella alexanderi sp. nov. is well established both morphologically and genetically, and it differs from externally similar species in radular characters. Phylogenetically Coryphella alexanderi sp. nov. is closely related to Coryphella trophina, which occurs sympatrically in the same geographic and bathymetric ranges. Coryphella alexanderi sp. nov. appears to be restricted to the middle and northern Kuril Islands, which is consistent with the high numbers of endemic taxa in this area.

On the territory of Republic of Croatia a new subspecies of land snails of the clausiliid genus Ruthenica has been found. Illustrated description is presented. New subspecies differs from other in shell shape, semi-apostrophic aperture and sculpture pattern.

Two subspecies of the schoolmaster gonate squid Berryteuthis magister (Berry, 1913) have been described to date: B. magister nipponensis Okutani et Kubodera, 1987 from the Pacific Ocean off the coast of Japan and Sea of Japan, and B. magister shevtsovi Katugin, 2000 from the Sea of Japan. The gonatid species Gonatus septemdentatus Sasaki, 1915, which was later recognized as a junior synonym of B. magister, was also described from the Sea of Japan. Comparisons among these three taxa and with the nominative subspecies using morphological and ecological features suggest that individuals of the genus Berryteuthis inhabiting the Sea of Japan belong to the species B. septemdentatus (Sasaki, 1915), which should be considered the valid name for this taxon.

The new replacement name Takashius nom. nov. is proposed to solve the homonymy between the fasciolariid genus Okutanius Kantor et al., 2018 and the hymenopteran genus Okutanius D.R. Smith, 1981.

The nominal species name “Theodoxus dniestroviensis” was commonly accepted as introduced by Put’ in 1972 for neritid snails occurring in the upper part of the Dniester River basin. Currently an earlier publication [Put’, 1957] has been found in which this name is established in accordance with the ICZN guidelines, so the dating should be amended to Put’, 1957.

The type series of Theodoxus dniestroviensis is apparently lost but the samples from the type locality were recently traced in the National Museum of Natural History of the National Academy of Sciences of Ukraine (Kiev). An illustrated comparison of topotypic specimens with other morphotypes of regional river nerites is provided. Given the variation in conchological characteristics observed among the specimens of Th. fluviatilis, we consider Th. dniestroviensis to range within its intraspecific variability. This makes Th. dniestroviensis a junior subjective synonym of Theodoxus fluviatilis (Linnaeus, 1758).

The first record of the eastern Mediterranean species Monacha claustralis in Western Ukraine is described, where it was found in one urban habitat of Lviv together with a conchologically similar Monacha cartusiana. Based on the results of our research and literature data, the reliability of some anatomical criteria that can be used to differentiate M. claustralis and M. cartusiana has been analyzed. A considerable variability in the length and shape of the vagina in M. cartusiana in Western Ukraine has been demonstrated, which may cause the erroneous identification of some specimens of this species as M. claustralis. The pattern of the present distribution of M. cartusiana in Western Ukraine are also characterized.

Boreotrophon oparini sp. nov., found in the Sea of Japan, off Russian Maritime Territory, is described and illustrated. The type material was sampled at a depth of 40 m (holotype) and 110 m (paratype) during the 64th expedition of R/V Akademik Oparin. Boreotrophon oparini sp. nov. is compared with the sympatric B. candelabrum and other congeneric species.

Examination of unsorted microgastropod samples collected during the joint Russia-German research “Sea of Japan Biodiversity Studies (SoJaBio) Expedition” in 2010 and preserved in the National Scientific Center of Marine Biology, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, resulted in the finding of six species of the family Rissoidae. Among them, “Alvania” nihonkaiensis Hasegawa, 2014 is newly recorded both in Russian waters and on the northern slope of the Sea of Japan, and Punctulum reticulatum Golikov, 1986 was rediscovered for the first time since its original description. Geographical and vertical distributions of each species and their significance are updated and discussed.

New data on the records of synanthropic slug species of Limacus maculatus and Arion vulgaris in Belarus are analyzed. It is assumed that the penetration of these species into the territory of Belarus occurred from other parts of their modern range together with the imported goods.

The study of the anatomy of Euomphalia aristata from the Western Transcaucasia showed that these populations differ from other species of the genus and from the typical populations of E. aristata from the North Caucasus in anatomy of the genitalia. E. aristata in Transcaucasia is characterized by the separation of the distal parts of the male and female sections of the reproductive system, a long oviduct, a very short proximal part of the vagina, and the location of the vaginal appendages close to the mucous glands. In E. aristata from the North Caucasus (North Ossetia), the male and female parts of the genitals are not separated in the distal part, the oviduct is short, the proximal part of the vagina has an S-shaped bend, and the vaginal appendages are located at a noticeable distance from the mucous glands. On these grounds we distinguish E. aristata in Transcaucasia as a separate species.

E. aristata was described from the vicinities of Kislovodsk that is on the northern macro-slope of the Main Caucasian Ridge; therefore, the name E. aristata is applicable to the snails inhabiting this region. For molluscs living on the southern slopes of the Caucasus Range and further south, we propose the name Euomphalia schileykoi sp. nov.

This paper describes finding of 3 specimens of Skenea profunda (Friele, 1879) in the Arctic Basin from a depth of 3700-3800 The species had been previously reliably known only from the bathyal part of the North Atlantic and is confined to communities of sunken wood. This is the second finding of a taxon obligate for such habitats in the Arctic Ocean and confirms the assumption that there is a fauna associated with wood falls on the path of the Transpolar Drift Flow in the Central Arctic.

It is proposed to exclude Stenoplax lindholmii from the list of Japanese chiton species, which turned out to be a junior synonym of S. magdalenensis. Lectotype of Chiton lindholmii is designated.

One more tiny chiton of the genus Leptochiton is described from the shallow water of the Persian Gulf. The new species Leptochiton persianus sp. n. differs from other species of the genus living on coral reefs in shape of the tail valve with subcentral mucro, long central and first lateral teeth of radula, and round median denticle of the head of the major lateral tooth.