Unio mongolicus Middendorff, 1851 (Bivalvia: Unionidae) was described from Gorbitza mountain stream (in the east of Trans-Baikal Territory, Russia) and the taxonomy has changed over the past century and a half. A specimen of Middendorffinaia mongolica from the Gorbitza stream collected 160 years later is a match with the type illustration of Unio mongolicus from the type locality. Comparison of the shell morphology of the type specimen of U. mongolicus with the modern specimen collected from this location, combined with our analyses of molecular, anatomical and conchological data of M. cf. mongolica species from rivers of the Upper Amur River Basin and the far eastern Russia, revealed that these taxa all belong to a single polymorphic species. The molecular analysis of the COI gene fragment mtDNA of examined Comparatory Species assigned to Middendorffinaia Moskvicheva and Starobogatov, 1973 confirmed they are only intraspecific forms of M. mongolica. Morphological and genetic distances between M. mongolica and Nodularia douglasiae Griffith et Pidgeon, 1833 confirmed their assignment to two independent genera. Differences of morphologically similar species M. mongolica, N. douglasiae and Unio crassus Philipsson in Retzius, 1788 were separated based on conchological characteristics and anatomical features. History of the taxonomic changes for Unio mongolicus is discussed here.

For the first time, the widely spread Antarctic species Leptochiton antarcticus was found at the Emperor Seamounts in the North Pacific Ocean. In spite of a large distance between the Emperor Seamounts and Antarctica, the found specimen have very similar shell, girdle, radula and gill features to the type material. I propose that L. antarcticus spread to the North Pacific from the Antarctic via a deep-water current near the ocean floor, and perhaps it inhabits the slopes of islands and continents from the South Ocean to the Emperor Seamounts.

Two substitution names are proposed for primary homonyms introduced for land snail of the family Camaenidae in Thach [2018].

The genus-group taxon Elbasania Schileyko et Fehér, 2017 has recently been introduced as a subgenus of Metafruticicola Ihering, 1892 for a species occurring in north-western Greece and Albania. Using mitochondrial and nuclear markers, the phylogenetic relationships of Elbasania within Metafruticicolini (Hygromiidae) are reconstructed. The results of these analyses suggest that Elbasania is more closely related to Hiltrudia Nordsieck, 1993, which has a range adjacent to that of Elbasania from Croatia to northern Albania, than to Metafruticicola. Elbasania shares with Hiltrudia and also Cyrnotheba Germain, 1929 a very characteristic microsculpture of the shell and an overall similar genital system, which however differs among these three taxa with regard to its internal structures, especially those of the penis. Therefore, it is suggested regarding Elbasania as a distinct genus here.

The predatory family Streptaxidae is comprised of five species occurring in Thua Thien Hue Province, central Vietnam. The present paper provides an assessment of the literature data relevant to these species and the description of two new species. Haploptychius bachmaensis sp. nov. has an oblique-heliciform shell with elevated spire. The aperture is subcircular, peristome discontinuous, thick and slightly expanded, apertural dentition with one strong parietal lamella. Perrottetia namdongensis sp. nov. has a rounded and protruded shell periphery. The aperture is subcircular, peristome is discontinuous, thick and expanded, apertural dentition with one large, strong parietal lamella, one small upper palatal lamella, one large palatal lamella, one large basal lamella, and one large strong columellar lamella.

All available data and most of materials on Acicula parcelineata from Ukraine are reviewed. Thirteen areas inhabited by the species are reported for Ukraine, some of which include several known sites. Five of these areas were not reported before. Map of general distribution of A. parcelineata is provided (outside Ukraine based on published data). Habitat preferences of this species are reviewed, it occurs in Ukraine almost exclusively in forests with presence of beech, on altitude 300-1100 m. Conservation status of A. parcelineata in Ukraine is shown to be “Near Threatened” according to IUCN criterions, it is recommended to be included into the next edition of Red Book of Ukraine. Shells’ measurements of A. parcelineata are provided and compared with related Platyla jankowskiana. The later species is discussed; it seems to be possible that it represents large abnormal specimens of A. parcelineata.

In present paper, we report the first findings of two large and conspicuous nudibranch species for Russian waters - Goniodoris nodosa and Flabellina pellucida. Both species show wide distribution in the northeastern Atlantic waters or even in the northwestern Atlantic in the case of F. pellucida. The morphology of both species was studied using anatomical dissections and scanning electron microscopy. Molecular diversity was assessed by sequencing of COI marker and reconstruction of haplotype networks. In the Barents Sea, both species show minor genetic differences from their European relatives and similar morphological characters. Putative explanations of new findings for Russian Arctic are discussed.

Anatomical study of five specimens of an endemic species of the Mauritius Island, Erepta odontina (Morelet 1851) showed that within one population disappearing of additional organs of the reproductive tract (epiphallic caecum, penial caecum, penial appendix, flagellum and retractor of penis) occur in four different combinations. It is significant that the shells of all five specimens were practically identical. It is assumed that the described phenomenon has a mutagenic nature and is regarded as a rare (if not unique) case of reduction (up to complete disappearance) of additional organs within one population.

It is proposed to restore the old name of the genus Terenochiton for Leptochiton norfolcensis (Hedley et Hull, 1912). We also provide a new description for the genus since the old description does not reflect the main features associated with the development of the articulamentum. All valves of the genus have rudimentary unslitted insertion plates. Similar rudimentary insertion plates have evolved independently in different families of the primitive chitons.

An annotated and illustrated check-list of species of the subfamily Sphaeriinae (family Sphaeriidae) of the Kama region (the Middle Urals, Russia), with enumeration of species’ localities, is presented. After examination of own collections and critical assessment of literary data, 8 species from two genera (Musculium and Sphaerium) have been included into the final list. One of these species, Sphaerium asiaticum, is recorded for the first time from the studied area. The ecological characteristics of all species as well as remarks on their distribution within the region are provided. From the zoogeographical point of view, the most sphaeriine species of the studied area belong to the European-Siberian faunistic group.

The recent record of Xeropicta derbentina in the south of the Central Russian Upland (Belgorod Region, Russia) provides the opportunity to study the alien populations structure at the stage of colonization of a new territory. High intra-and interpopulation variability of the X. derbentina shell in different parts of the range is revealed. Based on the allozyme and PCR-ISSR analysis, it was established that native and adventitious populations of X. derbentina had the same level of genetic variability. The potential number of multilocus genotypes calculated for the Belgorod population indicate a further complexity of their genetic structure. The result of the ISSR analysis showed significant genetic differentiation (FST=0.746; p=0.01) of two adventive populations from the Belgorod Region, which indicates their different origin from the natural area.

The taxonomic position of the genus Culmenella within Hygrophila remains uncertain. The spermatozoa of Culmenella rezvoji, a species from the Far East of Russia, were examined using a combination of light, scanning and transmission electron microscopy with the objective to assess the utility of sperm characters for clarifying the phylogenetic relationships of the genus. The spermatozoa of C. rezvoji are divided into four regions: head, midpiece, glycogen piece and endpiece. The head contains a slender, cone-shaped acrosome and a conical nucleus with a sinistrally coiled keel. The acrosome consists of an apical vesicle and a thick-walled pedestal with an electron lucent canal partially filled with a patchy electron-dense material. The midpiece contains the mitochondrial derivative that encloses apically three parallel glycogen-filled tracts (helices) positioned in such a way that in the sperm cross section two helices lie opposite each other and equidistant from the third helix. The surface of the sperm above one of the helices forms a high, narrow ridge; the ridges above the remaining two helices have a much lower profile. The boundary between the midpiece and glycogen piece is demarcated by a constriction (annulus) consisting of an anterior electron-dense ring and a posterior conical cylinder connected to the ring with thin filaments. The structure of spermatozoa in Culmenella is consistent with the general pattern of sperm morphology common to all studied species of Hygrophila, but the spermatozoa of Culmenella also have distinctive characters (three glycogen helices and high-profile surface ridge in the apical portion of the midpiece) that should be potentially useful in resolving the taxonomic position of this genus.

This paper provides reliable data on the occurrence of bivalve mollusk Neaeromya compressa in the Sea of Japan. Nine alive specimens (L = 5.9–19.2 мм) were collected in June 2003 in Peter the Great Bay (southern Primorye) at the depths of 66–74 m on mud and sand. These are illustrated and described in detail. The data on ecology and geographical range of the species are summarized.

The SEM image and description of the Boucardicus leopardus radula is presented for the first time. The radula appeared morphologically similar to those of representatives of Alycaeidae. At the same time representatives of Boucardicus differ significantly from South-East Asian Alycaeidae. It is suggested, that presently Boucardicus is not monophyletic and includes representatives of different genera and possibly families.

Oospira haivanensis sp. nov, a new member of the Clausiliidae subfamily Phaedusinae is described from the Hai Van Mountain in central Vietnam. The relationship of the new species to other taxa of the genus and  the zoogeographical importance of Oospira species in Southeast Asia are discussed.

The population structure of the terrestrial mollusk Fruticicola fruticum (Gastropoda, Pulmonata, Bradybaenidae) in different landscapes of Eastern Europe on the basis of conchological measurements, including morphometric parameters and color variants was studied. For all morphometric parameters used, statistically significant differences were found between the studied populations. At the same time, the shell size are determined not by zonal characteristics, but by microbiotopic conditions. Changes in the frequency of shell phenotypes in Fr. fruticum populations are probably of a multifactorial nature. The ratio of different color variants can be caused by the influence of radiation, cyclic fluctuations of climatic factors, succession changes in biotopes, and gene drift.

Two new species of the genus Callochiton are described: C. sonchaensis sp. nov. from Vietnam and C. tongaensis sp. nov. from Tonga. They differ from other congeneric species in having a different number, shape, size and arrangement of grooves on the pleural areas of the intermediate and tail valves, the number of slits, the size of the dorsal spicules, and sometimes the coloration of the girdle. Absence of Callochiton in the Caribbean Sea and along the coasts of Central and North Americas was, perhaps, due to late advancement of species from this genus into the Tethys from the regions near Gondwana.