Oligochaetes play a significant role in aquatic ecosystems, and studying their development can contribute to a deeper understanding of the evolutionary features of embryonic development in the phylum Annelida. In this paper, we initially explored a littoral oligochaete species from the White Sea, which is abundant in the vicinity of the White Sea Biological Station MSU and reproduces in the early summer. A molecular phylogenetic analysis utilizing COI and 18S markers confirmed that the species under investigation belongs to the genus Lumbricillus, family Enchytridae. The early stages of embryonic development were described in detail, and the timing of the onset of key stages was established. The specifics of the cellular patterns observed in the early embryo were elucidated using the fluorescence staining and confocal microscopy. The embryonic development of Lumbricillus sp. exhibits characteristics that are consistent with the typical development of oligochaetes. Additionally, it displays specific characteristics that are typical of representatives of the Enchytriidae family, including equal size of the 2d and 2D blastomeres and a small size of the 4D blastomere in comparison to the large 4d blastomere. The combination of synchrony and low developmental variability, coupled with the accessibility of the object, ease of maintenance and manipulation, makes Lumbricillus sp. a highly promising object for the study of embryonic development of annelids.

The dependence of prolactin gene (Prl1) expression on sex in the brain and its receptors (PrlRa and PrlRb) in the brain, kidneys, gills, and intestine of the three-spined stickleback (Gasterosteus aculeatus L.) was studied in the context of freshwater adaptation. Males and females of the marine morph were adapted to freshwater for 72 hours, and the expression levels of Prl1 in the brain and PrlRa and PrlRb in the examined organs were assessed using RT-PCR. In seawater, no sex differences in Prl1 gene expression in the brain were observed; however, after freshwater adaptation, significant sex differences were detected due to increased Prl1 expression in females. In the brains of females, PrlRa gene expression was significantly lower in seawater compared to males but increased significantly after freshwater adaptation, eliminating sex differences. PrlRb gene expression in the brain increased significantly in females following freshwater adaptation. In the kidneys, no sex differences in PrlRa gene expression were found in either seawater or freshwater, although expression significantly decreased after freshwater adaptation in both sexes. PrlRb gene expression in the kidneys showed no sex differences in seawater but did so under freshwater conditions due to a significant increase in females and a decrease in males. In the gills, the expression of PrlRa and PrlRb genes was independent of both sex and salinity. In the intestine, neither sex nor salinity influenced the expression of the PrlRa gene. PrlRb gene expression in the intestine showed no sex differences and decreased in both sexes after transitioning to freshwater. It is concluded that the osmoregulatory function of prolactin's dependence on sex is manifested in the sex differences in the expression of the prolactin gene itself and in the sensitization of the brain and osmoregulatory organs to it during freshwater adaptation.

Originally described from the northern Norway, Laonice cirrata (M. Sars, 1851) has been considered cosmopolitan and widely distributed in the North Pacific. To clarify the taxonomic status of the Pacific worms, we obtained the genetic characteristics of L. cirrata from Grøtsund Fjord, near Tromsø, one of the sites where Michael Sars collected worms to describe this species. The phylogenetic analysis of sequences of five gene fragments (mitochondrial COI and 16S rDNA, nuclear 18S rDNA and 28S rDNA, and Histone 3) showed significant difference between the Norwegian worms and worms from the north-western part of the Sea of Japan (Russia) earlier identified by morphology as L. cirrata. Common inhabitants of shallow waters in the Sea of Japan, these worms are assigned to the new species Laonice kasyanovi sp. nov. Both Northeast Atlantic and the Northwest Pacific populations exhibit high and overlapping variability of the diagnostic morphological characters of adults, and thus the two species can be considered as siblings. The distribution of these two species in the North Pacific remains uncertain and can only be elucidated by molecular data. Adults and one larva from the White Sea were also sequenced and found to be genetically identical to L. cirrata from Norway. The trochophores of L. cirrata are described and illustrated. They are characterized by two circles of large vesicles in the thick egg membrane and have been incorrectly referred to Aonides by previous authors.

Sex-related differences in the expression of genes encoding prolactin (Prl1) in the brain and its receptors (PrlRa and PrlRb) in the brain, kidneys, gills, and intestine of the three-spined stickleback (Gasterosteus aculeatus L.) were studied in the context of freshwater adaptation. Males and females of the marine morph were adapted to freshwater for 72 h, and Prl1, PrlRa and PrlRb expression levels were assessed using RT-qPCR. In seawater, no sex differences in Prl1 expression in the brain were detected, but they appeared after freshwater adaptation due to significantly increased Prl1 expression in females. In seawater, PrlRa expression in the brain of females was significantly lower compared to males, but increased significantly after freshwater adaptation, becoming comparable in both sexes. PrlRb expression in the brain increased significantly in females following freshwater adaptation. In the kidneys, PrlRa expression was indistinguishable between females and males in either seawater or freshwater, but significantly decreased after freshwater adaptation in both sexes. PrlRb expression in the kidneys was sex-independent in seawater, but freshwater adaptation led to its significant increase in females and a decrease in males. In the gills, PrlRa and PrlRb expression was independent of both sex and salinity. In the intestine, PrlRa expression did not depend on either sex or salinity, whereas PrlRb gene expression also showed no sex differences, while decreasing in both sexes after transitioning to freshwater. It is concluded that the dependence of the prolactin osmoregulatory function on the sex of sticklebacks is manifested in sex differences in prolactin gene expression in the brain, as well as in the sensitization of the brain and osmoregulatory organs to this hormone during freshwater adaptation.

Expression of prolactin axis genes (prolactin Prl1, prolactin-like hormone Prl2, prolactin receptor “a” PrlRa, and prolactin receptor “b” PrlRb) have been studied in the brains of females and males of the three-spined stickleback Gasterosteus aculeatus upon adaptation to fresh water. The expression of the Prl1 gene in females was significantly higher after 24-h adaptation to the freshwater relative to the control group (seawater) and to that in males at the same time point. No changes in Prl2 gene expression in females and Prl1 in males were detected. In males, after 12 and 18 h of adaptation to fresh water, higher expression of the Prl2 gene was observed relative to the control group. The expression of the PrlRa gene in males in seawater is higher than that in females; upon adaptation to fresh water, it does not change in females, and it decreases in males gradually after 3 h of being transferred to fresh water. No differences in the expression of the PrlRb gene between females and males in seawater were detected; however, after 24 h of freshwater adaptation, this parameter was higher in females than in males. Changes in the expression of prolactin axis elements during freshwater adaptation are gender-dependent, and the contribution of prolactin and prolactin-like hormone to adaptation of females and males of the three-spined stickleback to fresh water is significantly different.

The species composition and structure of bottom communities with a predominance of bivalve mollusks of thePortlandiagenus, which are widespread on the shelf of the Russian Arctic, were analyzed. Molecular genetic analysis confirmed thatPortlandia aestuariorumandP. arcticaare valid species; it has been shown that allP. arcticafrom different regions belong to the same species regardless their morphological variations. Ten different variants of benthic communities from the White, Kara, Laptev, and East Siberian seas were described, in whichP. aestuariorumandP. arcticaplay the dominant role. No associated species complexes have been identified forPortlandiaspp. Communities practically do not have common species, and their composition and structure are determined by the selection of species with similar ecological preferences from the local fauna. The differences in communities are determined by both geographical and environmental factors.

We investigated the evolution of the panmictic experimental populations of Drosophila melanogaster under the influence of artificial selection for reduced fitness. Slowly developing flies were selected among offspring from parents with low sexual activity and low fecundity. As result of this selection carried out for 16 generations, there was a strong decline in the various components of fitness. Our results confirm the results of the experiments by Kaidanov and his staff. The more rapid decrease in fitness observed by us seems to be due to the use of panmictic populations instead of inbred lines. Thus, the resulting fly selection observed in our studies likely reflects a preexisting genetic variation, rather than new mutations.

Five genetically distant groups of mussels possessing high intragroup homogeneity were identified among 65 specimens of 14 East European Unionidae "comparatory species" by genetic analysis of nuclear and mitochondrial markers. By shell morphology other than the shape of the convex contour of the shell, the identified groups correspond to five "taxonomic species" according to Zhadin's classification. The use of the comparatory method for Unionidae species identification is unjustified.

Hydroids are common components of fouling communities in the sea, but they are often inconspicuous and easily overlooked.In such cases, the appearance of their medusae in plankton is an obvious indicator of the species' presence in a locality.In this study, we present the first record of medusae Bougainvillia principis from the White Sea.We hypothesize that hydroids of the species B. principis inhabit the White Sea, as well, but they do not usually produce medusae and consequently the species does not exhibit sexual reproduction in the White Sea.

We have obtained the first data demonstrating the capability of multicellular organisms for longterm cryobiosis in permafrost deposits of the Arctic. The viable soil nematodes Panagrolaimus aff. detritophagus (Rhabditida) and Plectus aff. parvus (Plectida) were isolated from the samples of Pleistocene permafrost deposits of the Kolyma River Lowland. The duration of natural cryopreservation of the nematodes corresponds to the age of the deposits, 30 000–40 000 years.

Phoronida is a phylum of marine invertebrates; they have a worldwide distribution and form huge benthic aggregations in some areas. The taxonomy of Phoronida remains very poorly developed; only 13 species of phoronids have been described worldwide, while approximately 40 varieties of competent larvae are known. Morphological description of phoronid larvae and the search for correspondence between the larval and adult forms make an important contribution to the taxonomy of this enigmatic group. We present an original description of the larval forms of Phoronis australis Haswell, 1883. The larvae and adults of P. australis were collected in the plankton of Nha Trang Bay in the South China Sea. For the first time, the external morphology of larval forms of P. australis was described at the stages of 4, 8, and 20 tentacles. The basic distinguishing features of Phoronis australis larvae were determined: opaque integuments, the presence of a characteristic dark pigmentation of the oral field epidermis, of the unpaired ventral stomach diverticulum, and more than 20 tentacles in the competent larvae. A molecular-genetic analysis based on comparison of the DNA sequences of the 18S rDNA and 28S rDNA nuclear genes confirmed the identity of the phoronid larvae that were found in the planktonic samples from Nhatrang Bay and adult P. australis that were collected in the same area. The problems of Phoronida taxonomy and correspondence of larval and adult forms can be solved only with a complex approach that involves both morphological and molecular-genetic information.

Южный океан характеризуется сложной системой океанических фронтов, формирующих широтную биотопическую зональность. Фронты разграничивают водные массы с характерными для каждой гидрофизическими параметрами. Цель работы состояла в выявлении генетической неоднородности широко распространенных в Южном океане видов зоопланктона в условиях жесткой биотопической зональности. Проанализирована изменчивость фрагмента митохондриального гена СО1 для трех массовых видов копепод Южного океана Calanus simillimus, Rhincalanus gigas, Metridia lucens, формирующих основу биомассы зоопланктона в Южном океане. Для копепод C. simillimus и R. gigas в пределах изученной акватории не установлено наличие генетически различающихся группировок. Для M. lucens показано существование субантарктической и антарктической генетически дифференцированных группировок, встречающихся совместно только в Полярной фронтальной зоне.