The contemporary distribution of genetic diversity in widespread freshwater species reflects a complex interplay between historical processes and recent demographic events. We investigated the postglacial recolonization history of the Eurasian perch (Perca fluviatilis L.) across its native range spanning Europe and Western Siberia, aiming to understand how historical and recent demographic processes have shaped contemporary genetic diversity in a widespread freshwater species. Using an integrative genomic approach, we combined whole mitochondrial genome resequencing with nuclear SNP-array genotyping (3660 SNPs) for 382 individuals from 188 locations to reconstruct patterns of lineage divergence, population structure, and admixture. We identified five highly divergent mitochondrial lineages, consistent with the existence of multiple glacial refugia across Southwestern, Southeastern, and Central Europe, as well as Siberia. Nuclear data (3660 SNPs) revealed three major genetic clusters corresponding to Western, Northern, and Southeastern Europe, along with strong regional admixture. The Baltic Sea region emerged as a contemporary hotspot of genetic diversity, likely resulting from the admixture and convergence of distinct maternal lineages during the postglacial recolonization of Northern Europe. Signals of isolation by distance were evident both within and across lineages, highlighting the role of limited dispersal in shaping current genetic patterns. The integration of mitochondrial and nuclear genomic data provided a comprehensive view of the evolutionary history of P. fluviatilis, revealing both deep historical divergence and recent admixture events. The existence of multiple glacial refugia and subsequent secondary contact underscores the complexity of postglacial recolonization processes in freshwater fauna. These findings advance our understanding of how historical and contemporary factors interact to shape biodiversity across Europe.

Understanding how genetic differentiation is maintained between closely related species with overlapping habitats is a central question in evolutionary biology. The sticklebacks Gasterosteus aculeatus and Gasterosteus nipponicus occur in sympatry with reproductive isolation on the Pacific coast of Hokkaido Island. However, their distributions and hybridization patterns in more northern regions remain largely elusive. Here, we analyzed species distributions and hybrid occurrences in Sakhalin, the Northern Territories, the Kuril Islands, the Pacific and Okhotsk coasts of Kamchatka, Chukotka, and the open Bering Sea using genome-wide nucleotide variant data. We found that G. nipponicus is largely restricted to Sakhalin, while G. aculeatus predominates in other regions, with only rare hybrids detected. These findings indicate largely allopatric distributions around the Sea of Okhotsk and the Bering Sea. These allopatric populations provide valuable opportunities for comparing patterns of genetic differentiation and phenotypic divergence between allopatric and sympatric species pairs.

Throughout Russian, Soviet and post-Soviet eras, fish oil has repeatedly changed its role as scientific and technological advances rendered earlier uses obsolete. Originally prized by coastal communities as a dense caloric supplement, it evolved into a cornerstone source of vitamins A and D and later achieved prominence through its rich omega-3 profile, after these fatty acids were recognised for health. In Soviet times, fish oil transcended its nutritional and medicinal roles to become a tool of state health policy and a miraculous lifeline, in particular during the Siege of Leningrad. Today, as a global wellness commodity that still provides a tangible link to marine biodiversity, fish oil epitomises the resilience of natural resources under shifting political, economic and cultural regimes. By tracing these successive 'metamorphoses', this paper illuminates how a single commodity can reveal the dynamic interplay between human agency, ideological imperatives and the latent potential of the natural world. This article was published open access under a CC BY 4.0 licence: https://creativecommons.org/licenses/by/4.0/.

Pacific herring Clupea pallasii and Atlantic herring C. harengus are sister species of Pliocene origin that now both occur in northeastern Europe. Atlantic herring is represented by a huge oceanic population and several small coastal populations in Norway, whereas Pacific herring prevails in the Eastern Barents Sea and White Sea. Two populations in Northern Norway, one in the marine Balsfjord and the other, a putatively unique landlocked Clupea population in the neighboring meromictic Lake Rossfjordvatn, are phenotypically similar to C. pallasii. The Balsfjord herring is predominantly of C. pallasii genetic ancestry, but with 25

The threespine stickleback, Gasterosteus aculeatus, has undergone dramatic increases in abundance in parts of its historical native range, and it is also undergoing a major range expansion. We review available information and discuss the vectors and sources of the species' range expansions, the genetic characteristics of recently founded populations and the ecological consequences of both stickleback introductions and increases in abundance. Dramatic range expansions occurred in the Caspian Sea drainage, large rivers in the Black Sea drainage, reservoirs of the Rhine basin, isolated lakes in North America and Japan and remote islands in the Arctic. Likely reasons for these range expansions include canal construction, accidental inclusion with stocking of commercially valuable fish, intentional release by aquarists and fishermen and climate change. In some cases, range expansions of stickleback were likely facilitated by genetic admixture of previously separated lineages, as well as by high-standing genetic variation that promotes rapid adaptation to new habitats. Accordingly, range expansions are often accompanied by striking increases in abundance, although these are two distinct processes. Notably, population growth within the species' native range, particularly in the White and Baltic Seas, has been observed alongside expansions into new areas. Where stickleback colonise new habitats or increase in abundance, extensive ecological impacts on ecosystems typically occur. Given these massive and widespread changes, the species has the potential to provide considerable insight into the evolutionary and ecological effects of human impacts on aquatic ecosystems.

A comparative analysis of juvenile anadromous and freshwater (stream) forms of the Black Sea brown trout Salmo trutta labrax Pallas, 1814 was conducted using 10 meristic and 62 morphometric characters of cranial bones. This analysis included the counts of teeth and pores in the bones, as well as measuring of their shape characters. As a basis for comparison, data on juvenile anadromous and freshwater Atlantic salmon Salmo salar Linnaeus, 1758 were used. The osteological differences between ecological forms were more pronounced in the brown trout than in the Atlantic salmon. In the brown trout, a greater number of meristic characters differentiated the forms compared to the salmon. However, for morphometric characters, the reverse pattern was observed. The freshwater forms exhibited characteristics intermediate to the anadromous forms of the two species, being more similar to the anadromous form of their species than to each other. Juvenile anadromous Black Sea trout typically had the fewest teeth, while juvenile Atlantic salmon had the most, with freshwater forms showing intermediate values. The most significant differences in the shape of cranial bones were also observed between the juvenile anadromous forms of two species. The characteristics pertaining to the number of pores in cranial bones did not reveal clear patterns when comparing the samples. Among the studied bones and characteristics, those showing the most significant differences between Black Sea trout and Atlantic salmon were identified, and species-specific features were noted. The results obtained show that juveniles of different ecological forms within the same species of the genus Salmo exhibit statistically significant osteological differences, suggesting a high genetic determination of these traits and the influence of selection within river systems on the divergence between freshwater and anadromous forms. Despite this, the variability in all studied characteristics indicates plasticity, resulting in the differences between the species and ecological forms within the genus Salmo typically not reaching the threshold for subspecies differentiation. These results align with previous studies on adult individuals and indicate that the freshwater form of the Atlantic salmon displays osteological characteristic of a more ancestral species, namely the brown trout.

This study addresses the problem of concordance in fluctuating asymmetry (FA) across traits by analyzing the relationship between FAs and the mean values of character measurements in a set of morphological traits. Regression slopes vary in natural populations, thus, revealing discordance in FA across traits among these populations. Hence, commonly accepted techniques for measuring developmental instability with FA result in uncertainties. Here, I relate FA to mean as a two-dimensional complex to demonstrate the uniformly negative slopes of standardized FA vs. mean value regressions for sets of morphological traits from eighteen distinct natural marine and aquatic populations. Comprehensive analysis of the FA–mean complex cannot be recommended for wide use in assessing stress and fitness, but it offers promise to improve FA measuring methodologies and to better understand the nature of developmental instability.

The spatial and temporal variability in the dietary preferences of juvenile three-spined stickleback Gasterosteus aculeatus were studied across the typical coastal habitats of Keret Archipelago, Kandalaksha Bay(the White Sea). The sampling of fish using a beach seine was conducted in the late July–early August period and in late August. Additionally, zooplankton samples were collected to conduct a quantitative assessment of the abundance of potential prey items. The similarity percentage (SIMPER) analysis was applied to the data obtained from the stomach content and revealed the five planktonic taxa most prominently contributing to the diet of juvenile sticklebacks. Among these, the copepod Temora longicornis was the most important prey item at marine sites, while Acartia longiremis dominated the diet of fish in the marine lagoon. The calculation of the selectivity index revealed that some taxa (such as A. longiremis) were always avoided by juvenile fish, whereas the selectivity of some taxa increased in late August. In general, juvenile sticklebacks demonstrated a pronounced individual variation in their selectivity, even when a particular prey item was selected positively during the whole period of study. Our data highlight that despite the predominant consumption of easily available prey by juvenile sticklebacks, they demonstrate taxonomic- and size-specific prey selectivity.

Empirical studies of phenotypic variation show that genetic and environmental heterogeneity account for only part of it. Usually, the magnitude of the residual variation is comparable with that of the genetic component, while notably exceeding the magnitude of the environmental component. This can be interpreted in two ways. A deterministic interpretation associates it with artifacts such as measurement error and genetic and environmental heterogeneity that is unaccounted for. An indeterministic interpretation argues that it is random or stochastic phenotypic variation (SPV) resulting from developmental instability - a developing organism's inability to produce a consistent phenotype in a given environment. Classical example of debates between determinists and indeterminists took place about a century ago in quantum physics. In discussing Heidelberg's Uncertainty Principle, Einstein metaphorically expressed his deterministic position: 'God does not play dice with universe'. The indeterministic Uncertainty Principle, however, was eventually widely accepted. Currently, most biologists implicitly or explicitly support deterministic interpretations of phenotypic variation patterns. Here, a wide range of data on morphological traits (studied with analysis of fluctuating asymmetry) and non-morphological traits are analyzed to provide evidence that SPV is not an artifact, but a valid phenomenon. This evidence supports conclusions that observed associations between SPV and stress can be analyzed in the framework of dynamic energy budget theory, and are inextricably linked through energy homeostasis.

A total of 205 COI sequences and 310 cyt b sequences of the threespine stickleback (Gasterosteus aculeatus) from basins of all seas throughout the vast range of this species were analyzed. Median networks of haplotypes constructed in this study, combined with the results of reconstruction of paleogeographic conditions, led to the conclusion that the threespine stickleback emerged as a species in the Pacific Ocean basin and spread to Europe from the south, populating the system of water bodies that existed in the Oligocene. The main water body was the Paratethys Ocean (Sea), which existed 5–34 Mya. In the area of the modern North Sea, stickleback populations, part of which later migrated to the eastern and western coasts of North America, gave rise to the group of haplotypes that has the widest distribution in northern Europe. The stickleback populations belonging to the lineage that dispersed along the Arctic and western coasts of North America displaced the carriers of the haplotypes of the ancient phylogenetic lineage that inhabited the Pacific coast. The ancestors of G. wheatlandi dispersed from the Pacific to the Atlantic Ocean via the Arctic to meet G. aculeatus, which circled the globe from east to west.

Studies of dietary preferences of migratory species are of great importance as these species connect food webs of habitats across the migration route and thus represent trophic relationships between the spatially disjointed communities. Here we described the dietary preferences of threespine stickleback G. aculeatus in the White Sea during the spawning season using stable isotope and stomach content analyses. The two analyses suggested that during the spawning season, when sticklebacks spend the majority of their time inshore, their diet consists mostly of benthic species, while at the beginning of the spawning season when fish migrating from the offshore were feeding on plankton. Additionally, we demonstrated that stickleback eggs contributed greatly to the diet of both male and female fish. Using Bayesian mixing modeling, we showed that dietary preferences in females were broader than in males, and more variable during the spawning season. While guarding their nests, males fed almost exclusively on eggs. Both stomach contents and isotope signatures demonstrate that by the end of the spawning season sticklebacks again increase the consumption of plankton. Isotope analysis proved to be a more reliable tool to trace this change than stomach content analysis. Our results show that stable isotope and stomach content analyses are complementary in understanding seasonal changes in the dietary composition of stickleback.

Studies of speciation and intraspecific differentiation in salmonid fishes are traditionally based on analysis of osteological traits. In the current study the variation of osteological traits was compared between two Atlantic salmon forms: anadromous and landlocked. Each form was represented by three samples: one from a wild population, and two from hatcheries; i.e., we studied six samples in total. In total, 45 osteological characters were analyzed. Differences between the forms were found to be genetically determined to a substantial extent. The factor "origin" explained 48% of the total morphological difference between the fish samples; the factor "habitat", 32%; and their interaction, 14%. Phenotypic plasticity of the anadromous form was observed to be lower than that of the landlocked form. We consider that the higher phenotypic plasticity may compensate for a decrease in genetic diversity which is observed in landlocked forms of the species.

Genetic markers (sex-specific loci of Ga1 and Ga2) have been used to analyze the sex ratio of the threespine stickleback at different stages of ontogenesis. The progeny of all individually tested females includes both males and females, and no significant departures from a 1 : 1 sex ratio has been found in embryos from the natural nests. Juveniles did not demonstrate a sex bias up to one-and-a-half or two months of age, but in older juveniles, females prevailed (this fact has been reported for the first time). Females prevail among spawners, which mostly results from the predominant elimination of males during the spawning and post-spawning periods. The possible causes of this predominance are discussed.

Sexual dimorphism (SD) in the threespine stickleback Gasterosteus aculeatus reflects the different roles of the sexes in reproduction and their adaptations to different ecological niches. We quantified SD in one population of marine stickleback from the White Sea, collected during the spawning period from three spawning grounds, each at a distance of 5 km or less from the others. We used a landmark-based approach to quantify variation in 44 morphometric linear traits. In total, 749 females and 693 males were analyzed. In males, anterior body parts are larger — the base of the caudal fin and armor structures such as the first and second dorsal spines and the pelvic spine. Females have larger posterior bodies — the abdomen, pelvic girdle and the third dorsal spine. The SD of caudal body parts exhibits complex patterns. In White Sea threespine stickleback, SD patterns are generally similar to other populations of the species, but more often show male-biased patterns. Female-biased size SD may be associated with the female biased sex ratio of White Sea stickleback.

Life-history traits of Pacific ( Clupea pallasii ) and Atlantic ( Clupea harengus ) herring, comprising both local and oceanic stocks subdivided into summer-autumn and spring spawners, were extensively reviewed. The main parameters investigated were body growth, condition, and reproductive investment. Body size of Pacific herring increased with increasing latitude. This pattern was inconsistent for Atlantic herring. Pacific and local Norwegian herring showed comparable body conditions, whereas oceanic Atlantic herring generally appeared stouter. Among Atlantic herring, summer and autumn spawners produced many small eggs compared to spring spawners, which had fewer but larger eggs—findings agreeing with statements given several decades ago. The 26 herring stocks we analysed, when combined across distant waters, showed clear evidence of a trade-off between fecundity and egg size. The size-specific individual variation, often ignored, was substantial. Additional information on biometrics clarified that oceanic stocks were generally larger and had longer life spans than local herring stocks, probably related to their longer feeding migrations. Body condition was only weakly, positively related to assumingly in situ annual temperatures (0–30 m depth). Contrarily, body growth (cm × y −1 ), taken as an integrator of ambient environmental conditions, closely reflected the extent of investment in reproduction. Overall, Pacific and local Norwegian herring tended to cluster based on morphometric and reproductive features, whereas oceanic Atlantic herring clustered separately. Our work underlines that herring stocks are uniquely adapted to their habitats in terms of trade-offs between fecundity and egg size whereas reproductive investment mimics the productivity of the water in question.

Проведена количественная оценка метацеркарий трематод рода Cryptocotyle на поверхности тела у трехиглой колюшки Gasterosteus aculeatus Кандалакшского залива Белого моря. для работы были выбраны три нерестилища, отличающиеся по сочетанию ключевых характеристик среды, влияющих на плотность производителей. отлов рыб проводили трехкратно в течение нерестового периода с 29 мая по 7 июля 2016 г. в начале нереста характеристики заражения у рыб из разных местообитаний были очень сходны, что, скорее всего, указывает на смешивание колюшки во время зимовки. впоследствии во всех биотопах к концу нереста отмечалось увеличение как индекса обилия паразитов (среднее число паразитов на рыбу, включая незараженных особей), так и экстенсивности заражения (доля зараженных рыб). самые высокие значения индекса обилия и экстенсивности заражения наблюдали в лагуне Колюшковая, отличающейся от других нерестилищ колюшки значительной степенью изолированности от моря и замедленным водообменом, способствующими накоплению паразитов у особей. отсутствие у колюшек половых различий по степени заражения указывает на то, что хотя самцы и самки несколько отличаются в отношении пространственного распределения в разные периоды нереста, эти различия недостаточны для того, чтобы привести к различиям в зараженности. отсутствие достоверной связи между индивидуальной зараженностью и размерами тела, вероятно, объясняется высокой вариабельностью размеров рыб разного возраста.

The activity of lysosomal enzymes in juveniles of the three-spined stickleback Gasterosteus aculeatus L. from three biotopes of the Kandalaksha Gulf of the White Sea (Seldyanaya Inlet, Kolyushkovaya Lagoon, Sukhaya Salma Strait), differing in temperature regime, salinity, water exchange rate, density of aquatic vegetation, and other parameters, was studied. Fry for research was caught twice during the summer period: at the end of July and in August. In the samples, the activity of six acid hydrolases (acid phosphatase, RNase, DNase, β-glucuronidase, cathepsin B and cathepsin D) was determined. Some differences were revealed in the activity of the studied enzymes in the tissues of developing stickleback fry from different biotopes and depending on the time of sampling. The acid phosphatase activity in fry from all water areas practically did not depend on the sampling time. At the same time, the level was significantly higher in the samples from Kolyushkovaya Lagoon taken in August. The RNase activity was the highest in fry from all three biotopes captured in August compared to those taken in July, which indirectly indicates the intensive biosynthesis processes in growing juveniles of fish during this period. A noticeable increase in DNase activity in the group of juveniles from Sukhaya Salma caught in August suggests that they include more long-term mechanisms of adaptive metabolic rearrangements that affect the genome. Judging by the size-mass characteristics, this group is represented by fry of a later generation compared to groups from other biotopes, and they have recently completed morphogenetic transformatios associated with the transition to the juvenile period of development, in which lysosomal enzymes are involved. Compared to those from other biotopes, juveniles from Kolyushkovaya Lagoon have a relatively higher activity of β-glucuronidase and cathepsin B. Both of these enzymes are involved in the synthesis of components that regulate metabolism. It can be assumed that the main factor causing the need to adjust the metabolism of juvenile fish by August in the Kolyushkovaya Lagoon is the diversity and abundance of food components in this biotope. Thus, the active participation of lysosomal hydrolases in the adaptive metabolic rearrangements of stickleback fry under the influence of abiotic (temperature, salinity, tidal cycles) and biotic (feeding patterns) environmental factors has been shown.

This study analyses the potential of stochastic phenotypic variation for investigating the population biology of Eurytemora . Stochastic variation is the third component of phenotypic variance, standing on equal footing with genotypic variation and phenotypic plasticity. This is a manifestation of developmental instability and usually increases under stress. In morphological traits, stochastic variation is most often studied using fluctuating asymmetry (FA) of bilateral traits. Here, using data on the FA of nine populations of three Eurytemora species from Europe and North America, we found no correlation between FA and temperature, salinity or tidal amplitude. Invasive American E. carolleeae in the Gulf of Finland (Baltic Sea) had lower FA than the same species in its native Chesapeake Bay, or than E. affinis in its native Gulf of Finland. This pattern may be caused by global warming, which brought Chesapeake Bay temperatures beyond E. carolleeae ’s optimal conditions, but made the Gulf of Finland a more suitable environment. Stochastic variation in life history traits is technically more difficult to study, but it may provide important information on fitness. In particular, it manifests in bet-hedging, a risk-spreading strategy beneficial in unpredictable environments. As resting eggs are common in Eurytemora , bet-hedging can be considered a genus strategy. Understanding how stochastic variation contributes to total phenotypic variance may help to interpret changes under unpredictable environmental conditions. Therefore, studies of stochastic phenotypic variation may supply information about the population biology of Eurytemora and other copepods.

This review summarizes and analyzes data on the threespine stickleback Gasterosteus aculeatus L. of the White Sea, which is currently the most abundant fish in the region and, therefore, plays an important role in inshore and offshore communities. The threespine stickleback was abundant in the 1920s–1940s; its numbers declined significantly between the late 1960s and late 1990s and have increased again since then, showing a positive correlation with water temperature. In order to reveal the mechanisms of changes in the population of this species and to assess its role in the marine ecosystems, various aspects of the population biology of the species (interannual and seasonal population dynamics, spatial heterogeneity, age and sex structure, lipid and fatty acid status, homing, and fluctuating asymmetry), as well as its interactions with other organisms (feeding characteristics of adults and juveniles, role in feeding predatory fish, association with eelgrass, parasite composition and spatial distribution, and relationships with competing species) are analyzed.