Volume 30

On the territory of Republic of Armenia a new species of land snails has been found. Illustrated description is presented. Relationship between five close related Transcaucasian species of Armenica is discussed. New species differs from closely related A. unicristata in having more large dark shell with well developed axial striation and character of clausilium.

The paper describes a new species of the genus Thermochiton, T. papuaensis sp. nov., found in deep waters off Papua New Guinea. This species differs from T. undocostatus primarily in the dorsal scales, the marginal spicules, sculpture of the jugal area and the shape of the central teeth of radula. There are apparent similarities between the species of the genus Thermochiton, Connexochiton platynomenus, C. kaasi and Ischnochiton crassus. The last species is proposed to be transferred to Connexochiton. Owing to the friable, rusty brown deposits that densely cover the shell and girdle of both specimens of T. papuai, the latter probably lives in areas of high chemical activity.

Based on a study of samples made by the author, 70 species of freshwater bivalves belonging to 6 genera and 2 families are recorded for the Taz River basin (north of Western Siberia). An annotated list of bivalves of the Taz basin is provided, with data on species’ range, their findings in Western Siberia and within the studied area. Some information about bionomics and abundance of bivalves are given. 45 species are for the first time registered in this basin. Most studied molluscan communities are characterized by low species richness; the distribution of species by their occurrence was extremely uneven. 22 species are characterized as rare. The highest species richness of bivalves was found in river channels, rivers and brooks; the lowest – in temporary habitats. The core of the fauna is constituted by species with broadest range (cosmopolitan, trans Holarctic, trans Palearctic) as well as by taxa with European Siberian type of distribution. From the taxonomic point of view, the bivalve fauna of the Taz basin is relatively separated from the faunas of other river basins of Western Siberia and is more similar to the fauna of the Lower Yenisean zoogeographic province (sensu Starobogatov [1986]).

TEM and SEM electron microscopy have been used to study osphradia in 6 species of marine Caenogastropoda. The ultrastructural features of mechanoreceptor cells that perform the Littorina osmoreception function in osphradium organs are presented. Mechanoreception is based on a possible change in the volume of cisterns of microvilli of supporting cells, which can be transmitted by the cilia of nearby mechanoreceptor cells. These cells obviously, have mechanosensory channels on the apical surface. It has been first discovered in predatory molluscs actively searching for food, that single receptor cells with a mobile sensilla consisting of several cilium were joined together. They are located along the groove zone and follow the direction and force of the movement of water along the osphradium lamellae.

A new species Acrotoma (Acrotomina) mallabica sp. nov. from the Western Caucasus is described. The morpho-anatomical relations of the new taxon with closely related species are discussed.

The invasion of the predatory gastropod mollusk Rapana venosa, known as veined rapa whelk, in the Black Sea and its outspread in many regions of the World Ocean have led to complex structural changes in the communities of bivalve mollusks. When capturing a prey, the adult rapa whelk excretes a biotoxin from the hypobranchial gland that causes severe paralysis of the musculature of bivalve mollusks. Under experimental conditions, it was established that the biotoxin, after the short-term exposure of the fertilized eggs of the mussels Mytilus galloprovincialis to it, has a mutagenic effect, causing chromosomal aberrations in the embryos. The acentric groups of chromosomes, single and multiple chromosome bridges, chromosome lags and polyploidy indicate transformation or destruction of threads of the achromatin spindle in the mitosis anaphase. The chromosome fragmentation and emergence of ring chromosomes in the metaphase and anaphase of mitosis of mussel embryos are the consequence of the partial destruction of chromatin. The underdeveloped shell, anomalous development of D-veligers’ velum, and the changed form of the ‘eye spots’ and hepatopancreas of the larvae at the veliconcha stage result from chromosomal mutations during the embryo development. The mechanism of the rapa whelk biotoxin action on the chromosomal apparatus of mussel embryos is discussed.

The extant polyplacophoran tail valve is traditionally considered as a unity despite its clear separation into two distinct regions, which were in relation to the delimiting point – the mucro – termed the antemucronal area for the front part and the postmucronal area for the hindermost region. However, earlier conceptions do exist, which consider the postmucronal area as semiplate, with the antemucronal area as modified “intermediate” plate.

To test the usefulness of the terms “antemucronal” and “postmucronal” in their traditionally sense, three independent attempts were made to demonstrate existing differences between the mucro-separated areas.

Leptochiton rugatus was histologically examined allowing the confirmation of a cardial complex-antemucronal relation. Valve morphology of the brood of Radsia nigrovirescens not only confirms a tegmental development prior the building of the articulamentum but shows that the postmucronal area develops to its final shape before the antemucronal area appears. For the first time it is demonstrated that the antemucronal area of Schizoplax brandtii shows a delayed splitting of the relevant area, characteristic for the conditions found in the intermediate valves of this species only. That leads to the assumption that the underlying valve build processes are of the same nature as in the intermediate valves. Additionally, literature data on valve characters were compiled that show a stronger relationship (61%) of the antemucronal area to the central area of intermediate valves rather than to the merged postmucronal area. In the light of this evaluation it is suggested that the term “tail valve” should be used to demonstrate the merged condition of two independent zones only, the antemucronal area and the postmucronal area.

The monotypic land snail genus Campolaemus, known only from Saint Helena Island, has been classified in the family Hypselostomatidae, a family occurring in the eastern Palaearctic and in the Oriental region. Due to biogeographical reasons and morphological traits, especially the arrangement and morphology of apertural barriers, Campolaemus is moved to the family Streptaxidae, which is a pantropic family, being highly diverse in tropical Africa.

The phenotypic composition of the introduced colonies of Cepaea nemoralis in five settlements of Ivano-Frankivsk and Lviv regions of Ukraine, studied from May to July 2019, is described. The largest colonies were found in Bohorodchany (Ivano-Frankivsk region) and Lviv. In most cases, there was a pronounced dominance (from 40% or more) of the individuals with unbanded and five-banded shells – pink, less often yellow. Among the snails with five-banded shells, phenotypes with the fusion of two or more bands prevailed, among which in Lviv the phenotype (12)3(45) most often occurred, and in Bohorodchany (123)(45). Molluscs with brown shells were found in only one of the studied colonies. In Bohorodchany a differentiation of the phenotypic structure of the C. nemoralis colonies separated by a highway, was noticed. The available data on the present distribution in Western Ukraine of two introduced species of the genus Cepaea: C. nemoralis and C. hortensis are analyzed.

A new deep-water polyplacophoran, Stenosemus nitens sp. nov., from Papua New Guinean waters is described. It differs from other species of the genus Stenosemus by having a sculptureless, smooth and shiny surface of the tegmentum, smooth dorsal spicules with only a few small transverse wrinkles on top, and unusual bends of the front margin of the intermediate valves. Within the genus the new species belongs to a limited group of coloured chitons.