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Subarctic populations of “cryptic” blue mussel species Mytilus edulis (ME) and M. trossulus (MT) are less studied than Arctic and boreal populations. Ecological features of ME and MT in sympatry are poorly known everywhere. We studied the habitat segregation of ME and MT and the interannual dynamics of their mixed settlements at the Murman coast of the Barents Sea, the northeastern boundary of the Atlantic littoral mussel communities. Previous data on mussels from this area are 50-100 years old. The 3-km-long Tyuva Inlet (Kola Bay) was used as the study site. Mussels were found in the littoral and the sublittoral down to a depth of 4 m. Their characteristic habitats were sandbanks, littoral rocks, sublittoral kelp forests and “the habitat of the mussel bed” in the freshened top of the inlet. The main spatial gradients explaining the variability of demographics of the settlements (abundance, age structure, size) were associated with the depth and the distance from the inlet top. ME and MT were partially segregated by depth: ME dominated in the sublittoral and MT, in the littoral. In addition, ME dominated throughout the mussel bed. The ratio of species in the mixed settlements varied over time: between 2004 and 2010 the proportions of MT decreased everywhere, by 22% on average. The habitat distribution of mussels apparently changed with time: we found that mussels were abundant in kelp forests, where they had rarely been observed in the 20th century. We suggest that the spatial and temporal dynamics of subarctic mussels can be partly explained by the competition between ME and MT and their differing sensitivity to environmental factors.

Plankton communities often consist of cosmopolitan species with an extensive gene flow between populations. Nevertheless, populations of some plankton species are genetically structured, owing to various barriers such as ocean currents, hydrological fronts, and continents. Drivers that could explain the genetic structures of most mesopelagic species remain unknown on an ocean-basin scale, and our study aims to analyze the genetic and morphological differences between populations of a cosmopolitan mesopelagic shrimp, Systellaspis debilis, from the Southern and Northern Atlantic Ocean, and the Southwest Indian Ocean. We analyzed the ITS-1 and COI markers of 75 specimens and assessed the genetic integrity and within-species variability of these genes. We also coded 32 morphological characteristics in 73 specimens, analyzed their variability, and assessed the correlation between morphological and genetic characteristics using a Redundancy analysis and Mantel test. Systellaspis debilis was genetically cohesive across the whole Atlantic and Southwest Indian Oceans, which is possibly a result of an intensive gene flow through ecological barriers, the resistance of species to hydrological gradients, a purifying selection of mitochondrial genes, etc. In contrast, we found significant morphological differences between populations from different regions, which mirrors morphological diversification and calls for further genomic approaches in order to understand the basis of these variations and uncover potential local adaptations.

Cryptic and hybridizing species may lack diagnostic taxonomic characters leaving researchers with semi-diagnostic ones. Identification based on such characters is probabilistic, the probability of correct identification depending on the species composition in a mixed population. Here we test the possibilities of applying a semi-diagnostic conchological character for distinguishing two cryptic species of blue mussels, Mytilus edulis and M. trossulus. These ecologically, stratigraphically and economically important molluscs co-occur and hybridize in many areas of the North Atlantic and the neighboring Arctic. Any cues for distinguishing them in sympatry without genotyping would save much research effort. Recently these species have been shown to statistically differ in the White Sea, where a simple character of the shell was used to distinguish two mussel morphotypes. In this paper, we analyzed the associations between morphotypes and species-specific genotypes based on an abundant material from the waters of the Kola Peninsula (White Sea, Barents Sea) and a more limited material from Norway, the Baltic Sea, Scotland and the Gulf of Maine. The performance of the "morphotype test" for species identification was formally evaluated using approaches from evidence-based medicine. Interspecific differences in the morphotype frequencies were ubiquitous and unidirectional, but their scale varied geographically (from 75% in the White Sea to 15% in the Baltic Sea). In addition, salinity-related variation of this character within M. edulis was revealed in the Arctic Barents Sea. For every studied region, we established relationships between the proportions of the morphotypes in the populations as well as between the proportions of the morphotypes in samples and the probabilities of mussels of different morphotypes being M. trossulus and M. edulis. We provide recommendations for the application of the morphotype test to mussels from unstudied contact zones and note that they may apply equally well to other taxa identified by semi-diagnostic traits.

Two blue mussel lineages of Pliocene origin, Mytilus edulis (ME) and M. trossulus (MT), co-occur and hybridize in several regions on the shores of the North Atlantic. The two species were distinguished from each other by molecular methods in the 1980s, and a large amount of comparative data on them has been accumulated since that time. However, while ME and MT are now routinely distinguished by various genetic markers, they tend to be overlooked in ecological studies since morphological characters for taxonomic identification have been lacking, and no consistent habitat differences between lineages have been reported. Surveying a recently discovered area of ME and MT co-occurrence in the White Sea and employing a set of allozyme markers for identification, we address the issue whether ME and MT are true biological species with distinct ecological characteristics or just virtual genetic entities with no matching morphological and ecological identities. We find that: (1) in the White Sea, the occurrence of MT is largely concentrated in harbors, in line with observations from other subarctic regions of Europe; (2) mixed populations of ME and MT are always dominated by purebred individuals, animals classified as hybrids constituting only ca. 18%; (3) in terms of shell morphology, 80% of MT bear a distinct uninterrupted dark prismatic strip under the ligament while 97% of ME lack this character; (4) at sites of sympatry MT is more common on algal substrates while ME mostly lives directly on the bottom. This segregation by the substrate may contribute to maintaining reproductive isolation and decreasing competition between taxa. We conclude that while ME and MT are not fully reproductively isolated, they do represent clearly distinguishable biological, ecological and morphological entities in the White Sea. It remains to be documented whether the observed morphological and ecological differences are of a local character, or whether they have simply been overlooked in other contact zones.

The blue mussels Mytilus edulis and Mytilus trossulus are ecologically and economically important species distributed widely across the Northern Hemisphere. Understanding their behavioral and physiological disparities is crucial for assessing their ecological success and aquacultural value. The recent finding of non-native M. trossulus in the White Sea raises concerns regarding its potential competition with native M. edulis and its prospective spread in light of climate change and surface water freshening. We investigated the responses of M. edulis and M. trossulus to salinity variations by examining shell closure thresholds and tissue levels of 35 metabolic intermediates in mussels acclimated to different salinities (25, 16, and 10). The salinity threshold for valve closure was similar in both studied species, but M. trossulus consistently opened at lower salinities (by 0.2-0.7 practical salinity units) compared to M. edulis. Salinity-induced changes in metabolite levels were similar between the two species. Taurine emerged as the dominant osmolyte, comprising over 50% of the total free amino acid pool, with aspartate and glycine contributing 15-30%. Concentrations of taurine, glycine, and total free amino acids declined with decreasing salinity. Taurine to glycine ratios were higher in M. edulis and increased in both species with declining salinity. Acclimation salinity significantly influenced urea cycle intermediates and methionine sulfoxide content, a cellular biomarker of amino acid oxidation. Species-specific differences were observed in purine metabolism, with higher levels of GMP and AMP found in M. edulis. Likewise, aromatic amino acids and histidine levels were higher in M. edulis compared to M. trossulus. However, no evidence suggests superior adaptation of M. trossulus metabolism to hypoosmotic stress compared to M. edulis. Further research is necessary to elucidate the functional implications of subtle metabolic differences between these Mytilus congeners and their ecological consequences in changing marine environments. [ABSTRACT FROM AUTHOR]

Ecology and biogeography of bivalve transmissible neoplasia (BTN) are underexplored due to its recent discovery and a challenging diagnostics. Blue mussels harbour two evolutionary lineages of BTN, MtrBTN1 and MtrBTN2, both derived from Mytilus trossulus. MtrBTN1 has been found only in M. trossulus from North Pacific. MtrBTN2 parasitizes different Mytilus spp. worldwide. BTN in M. trossulus in the Atlantic sector has never been studied. We looked for BTN in mussels from the Barents Sea using flow cytometry of cells, qPCR with primers specific to cancer‐associated alleles and sequencing of mtDNA and nuclear loci. Both MtrBTN1 and MtrBTN2 were present in our material, though their prevalence was low (~0.4%). All cancers parasitized M. trossulus except one, MtrBTN1, which was found in a hybrid between M. trossulus and M. edulis. The mtDNA haplotypes found in both lineages were nearly identical to those known from the Northwest Pacific but not from elsewhere. Our results suggest that these two lineages may have arrived in the Barents Sea in recent decades with the maritime transport along the Northern Sea Route. A young evolutionary age of MtrBTN1 seems to indicate that it is an emerging disease in the process of niche expansion. Comparing the new and the published sequence data on tumour suppressor p53, we proved that the prevalence of BTN in mussels can reach epizootic levels. The finding of diverse recombinants between paternally and maternally inherited mtDNAs in somatic tissues of M. trossulus was an unexpected result of our study. [ABSTRACT FROM AUTHOR]

Plankton communities often consist of cosmopolitan species with an extensive gene flow between populations. Nevertheless, populations of some plankton species are genetically structured, owing to various barriers such as ocean currents, hydrological fronts, and continents. Drivers that could explain the genetic structures of most mesopelagic species remain unknown on an ocean-basin scale, and our study aims to analyze the genetic and morphological differences between populations of a cosmopolitan mesopelagic shrimp, Systellaspis debilis, from the Southern and Northern Atlantic Ocean, and the Southwest Indian Ocean. We analyzed the ITS-1 and COI markers of 75 specimens and assessed the genetic integrity and within-species variability of these genes. We also coded 32 morphological characteristics in 73 specimens, analyzed their variability, and assessed the correlation between morphological and genetic characteristics using a Redundancy analysis and Mantel test. Systellaspis debilis was genetically cohesive across the whole Atlantic and Southwest Indian Oceans, which is possibly a result of an intensive gene flow through ecological barriers, the resistance of species to hydrological gradients, a purifying selection of mitochondrial genes, etc. In contrast, we found significant morphological differences between populations from different regions, which mirrors morphological diversification and calls for further genomic approaches in order to understand the basis of these variations and uncover potential local adaptations. [ABSTRACT FROM AUTHOR]

Cryptic and hybridizing species may lack diagnostic taxonomic characters leaving researchers with semi-diagnostic ones. Identification based on such characters is probabilistic, the probability of correct identification depending on the species composition in a mixed population. Here we test the possibilities of applying a semi-diagnostic conchological character for distinguishing two cryptic species of blue mussels, Mytilus edulis and M. trossulus. These ecologically, stratigraphically and economically important molluscs co-occur and hybridize in many areas of the North Atlantic and the neighboring Arctic. Any cues for distinguishing them in sympatry without genotyping would save much research effort. Recently these species have been shown to statistically differ in the White Sea, where a simple character of the shell was used to distinguish two mussel morphotypes. In this paper, we analyzed the associations between morphotypes and species-specific genotypes based on an abundant material from the waters of the Kola Peninsula (White Sea, Barents Sea) and a more limited material from Norway, the Baltic Sea, Scotland and the Gulf of Maine. The performance of the "morphotype test" for species identification was formally evaluated using approaches from evidence-based medicine. Interspecific differences in the morphotype frequencies were ubiquitous and unidirectional, but their scale varied geographically (from 75% in the White Sea to 15% in the Baltic Sea). In addition, salinity-related variation of this character within M. edulis was revealed in the Arctic Barents Sea. For every studied region, we established relationships between the proportions of the morphotypes in the populations as well as between the proportions of the morphotypes in samples and the probabilities of mussels of different morphotypes being M. trossulus and M. edulis. We provide recommendations for the application of the morphotype test to mussels from unstudied contact zones and note that they may apply equally well to other taxa identified by semi-diagnostic traits. [ABSTRACT FROM AUTHOR]

As predators, Naticidae (Gastropoda) can shape marine soft-sediment communities. Thus understanding of the reproductive biology and development of moonsnails is of great importance. Most moonsnails lay large egg masses, known as sand or egg collars, which are freely distributed on sand or muddy sediments. Here we report upon the abundance of egg collars of two naticid species, Amauropsis islandica and Euspira pallida, from the high-latitude White Sea, as well as describe the morphology of egg collars and hatching success, with a brief description of juvenile feeding in A. islandica. While in the subtidal zone, the egg collars of E. pallida were 10 times more abundant than of A. islandica, the egg collars of the latter species were the only ones that occur in the intertidal zone. The morphology of an egg collar of E. pallida differed from the literature descriptions by having a plicated basal margin. The number of egg capsules inside the collars was twice as high in A. islandica compared with E. pallida, but they were smaller. Amauropsis islandica hatchlings were larger and hatching success was more than twice that in E. pallida. We suggest that these characteristics promote the high abundance of A. islandica populations observed on some tidal flats of the White Sea. Surprisingly, A. islandica juveniles could perform non-drilling feeding in the first month after hatching. This study fills the gap in the knowledge of naticid reproductive biology at high latitudes. [ABSTRACT FROM AUTHOR]

Relative importance of biotic factors (i.e., predation) in shaping community structure at high-latitude marine habitats is likely underestimated due to lack of relevant research. Size-selective predation on dominant species can strongly affect the demography of the prey resulting in indirect community-wide effects. Moon snails (Naticidae: Gastropoda) drill shells of other mollusks, and their feeding habits are thus comparatively easy to trace. Here we test the predation of the Iceland moon snail Amauropsis islandica, preying upon the Baltic tellin Limecola balthica, for size-selectivity in the high-latitude White Sea, combining caging experiments and a 17-years' survey. The results from both data sources confirm that drilled valves of Limecola are generally larger than living clams. Sizes of the drillholes on the Limecola shells in the sediments evidence that with increasing predator size its' size-selectivity shifts towards bigger prey, and larger range of sizes becomes available for consumption (0.5–7 mm range for the 2.5 mm Amauropsis and 4–20 mm for the 22.5 mm Amauropsis). In the caging experiment, larger Limecola (>8 mm length) are more likely consumed by larger predators (12.1–14.8 mm in height) than by smaller ones (7.1–8.6 mm in height). Size-selective predation by Amauropsis could shape Limecola population as evidenced by the size variation of potentially edible Limecola clams in samples with different density and size of Amauropsis. Since the large Limecola clams may have different ecological functions than small ones, Amauropsis could indirectly influence the intertidal community structure. [ABSTRACT FROM AUTHOR]

Polar shallow marginal seas are of high importance as they are the most productive regions of the Arctic Ocean and serve as filters for terrestrial runoff. Salinity and turbidity gradients create diverse habitats for planktonic organisms in coastal areas. In the present study we aimed at assessing the degree to which environmental gradients influence the abundance and community structure of the zooplankton in a shallow Arctic sea affected by terrestrial runoff. Zooplankton distribution was studied in a coastal zone in the southeastern part of the Barents Sea (Pechora Sea) in July 2014 and September 2016 along the archipelago that stretches from the continent towards the open sea. The ecosystem was in a spring state in July (2014) and in a summer state in September (2016). A clear positive gradient of salinity and a negative cline of turbidity were revealed, directed from the coast towards the open sea. A horizontal salinity gradient was detected in both seasons. The turbidity gradient was most pronounced during summer. Distribution of several species of marine zooplankton (e.g. Pseudocalanus spp., Temora longicornis, Microsetella norvegica) was associated with the salinity gradient. Parameters of community structure (species richness, diversity, evenness, total zooplankton abundance) correlated with turbidity while only diversity and evenness were influenced by salinity. A gradient was observed from a more diverse and less abundant zooplankton community in areas with high turbidity and low salinity towards a less diverse and more abundant community in the open sea. This heterogeneity influences higher trophic levels including commercial fishes and reflects how marginal filters function. [ABSTRACT FROM AUTHOR]

Many size-selective predators switch their diet during ontogeny. At the White Sea, the adult moonsnails Amauropsis islandica feed mostly on Macoma balthica clams. The diet of juveniles was largely unknown. We conducted a field survey and a caging experiment to find out if juvenile moonsnails can prey on Hydrobia ulvae, and whether they prefer this snail to their usual prey. Live molluscs and their intact or perforated shells were collected from 34 sediment cores. We exposed the single-prey cages with 50 Macoma (MP) or 50 Hydrobia (HP) together with five Amauropsis juveniles, as well as the cages where both prey species were in a 25:25 proportion (HMP). While live Hydrobia was more abundant in the natural assemblages, Amauropsis preferred Macoma, as indicated by proportions of perforated shells. The caging experiment produced similar results. Per capita Macoma consumption rate was significantly higher than Hydrobia consumption rate (6.4 ± 0.5 mg day−1 ind.−1vs. 1.4±0.2 mg day−1 ind.−1 in MP and HP respectively). Prey consumption rates in the single-prey treatments were higher than in mixed-prey cages regardless of prey species. Different mechanisms explain this variation: for Hydrobia it is a consequence of the dietary shift, while for Macoma it reflects the ‘floor’ effect in HMP cages, where virtually all Macoma had been drilled by the end of exposure term. While Macoma is the preferable prey of young Amauropsis, Hydrobia can supplement the diet of juveniles when Macoma is scarce in certain locations. [ABSTRACT FROM PUBLISHER]

The paper describes changing patterns of commercial fish catch in the downstream part of the Neva River and the eastern Gulf of Finland and analyzes drivers of these changes for the period 1929-1995. We summarize catch data on 20 species and species groups of fishes and lamprey, as well as available abiotic data (salinity, temperature and water transparency). Water transparency gradually decreased during the 20th century being inseparable from a number of non-quantified anthropogenic factors, thus it can be used as an integral index of anthropogenic loading on the ecosystem. Because fisheries statistics were not published regularly, catch data were extracted from archives and various publications. Fishing locations, gear and target species changed over time in relation to each other, reflecting technological developments in fisheries, commercial demands for fishery products and the abundance of fish populations. Until the 18-19th centuries, fisheries took place mostly in rivers where weirs and set nets targeted sturgeon, salmon and whitefish. By the end of the 19th century, herring and smelt were the main targets of fixed nets in coastal areas. A century later, the main commercial species, herring, was harvested with pelagic trawls operating offshore in the Gulf. This evolution in fisheries, along with other anthropogenic activities, caused severe declines in diadromous species. Spawning migrations that make them easy to catch, and their high market value, make diadromous fish more vulnerable than other groups. Canonical correspondence analysis showed that catches of most diadromous species decreased with increasing transparency, which may reflect their response to anthropogenic pressure. Marine and freshwater fish suffered from anthropogenic pressure, but to a lesser extent probably because of a wider distribution and dispersal, and more capital-intensive fishing methods. Catches of marine species, except herring, significantly increased in the 1970-1980s when salinity was comparatively high. We found no correlation of fish catches with temperature. [ABSTRACT FROM AUTHOR]

Mussels are ecosystem engineers, and fluctuations in their abundance and population structure could be important to the associated community. There is, however, little understanding of this connection. In the present study, based on quantitative monitoring (1997-2011) of three mussel beds in a fjord-like White Sea bay, two hypotheses were tested: (1) mussel assemblages are temporally unstable and local population fluctuates cyclically as a result of negative adult-juvenile interactions; and (2) oscillations in mussel size-structure are correlated with changes in the associated community structure. A negative correlation found between the abundance of small (length < 21 mm) and large (length > 20 mm) mussels suggests that adult mussels indeed suppress recruitment. Such interaction implies an auto-oscillatory pattern of population dynamics, with Large- and Small-dominated stages temporally replacing each other. This cyclic pattern was clearly revealed for one mussel bed only, but long-term replacement of the Large-dominated stage by the Small-dominated stage was revealed for the other two assemblages also. In general, temporal variations of mussel populations were significantly correlated with the dynamics of the associated community, although several abundant taxa ( Tubificoides benedii, Littorina saxatilis, Macoma balthica, and Gammaridae) were insensitive to mussel changes. In contrast, filamentous algae and mud snails Hydrobia ulvae tended to be more abundant during the Large-dominated stage, whereas polychaetes Dipolydora quadrilobata were most abundant during the Small-dominated stage. Several other abundant 'sensitive' taxa were obviously dependent on algal bloom. Thus, mussel beds are unstable systems, whose dynamics are shaped not only by the ecosystem engineer but also by the associated community. [ABSTRACT FROM AUTHOR]

Populations of marine bivalves are known to be unstable but it is unclear whether this instability has internal or external causes. In this paper we summarize results from long-term (1978-2007) monitoring of an Astarte borealis (Astartidae) population in the White Sea. We sought significant long-term changes among a population of subtidal clams existing under relatively stable conditions. The studied population was initially (in 1978-1995) formed exclusively by adult clams, with no recruitment occurring until a mass inflow of unidentified astartid's juveniles in the late 1990s. We assessed the suitability of periostracum microsculpture for identification of astartid juveniles and identified the juveniles recruited the population as A. borealis. Thus significant changes in population structure were detected. Our study suggests that A. borealis may have a very long life span (>20 years) and a prolonged recruitment failure could be associated with scarcity of hard substrates needed for egg attachment in a silted environment. As old clams died, their valves provided a hard substrate on the bottom surface, facilitating recruitment. We propose a new conceptual model of subtidal clams' population dynamics predicting long-term cyclic changes driven by internal mechanisms associated with an unusual type of positive influence of adults upon recruits. [ABSTRACT FROM AUTHOR]

Sea stars Asterias rubens are important natural enemies of the blue mussel Mytilus in the North Atlantic. We asked whether these predators distinguish between the cryptic species M. edulis and M. trossulus that occur sympatrically in the White Sea. In mixed experimental stocks, the odds of being eaten by sea stars were about four times greater for M. trossulus. We also showed that A. rubens preferred smaller mussels to larger ones, irrespective of their species affinity. Our findings support earlier indirect observations showing that sea stars recognize M. trossulus as a more preferable prey than M. edulis. Dramatic differences in the vulnerability to sea star predation may explain the segregation of habitats between the two mussel species in contact zones; M. trossulus usually tends to occupy habitats where the sea star predators are scarce., Introduction Blue mussels (Mytilus spp.), well-known ecosystem engineers, are an important component of coastal communities (Seed and Suchanek, 1992; Buschbaum et al., 2009). The structure and dynamics of their populations [...]