Large-scale climate changes influence the geographic distribution of biodiversity. Many taxa have been reported to extend or reduce their geographic range, move poleward or displace other species. However, for closely related species that can hybridize in the natural environment, displacement is not the only effect of changes of environmental variables. Another option is subtler, hidden expansion, which can be found using genetic methods only. The marine blue mussels Mytilus are known to change their geographic distribution despite being sessile animals. In addition to natural dissemination at larval phase—enhanced by intentional or accidental introductions and rafting—they can spread through hybridization and introgression with local congeners, which can create mixed populations sustaining in environmental conditions that are marginal for pure taxa. The Mytilus species have a wide distribution in coastal regions of the Northern and Southern Hemisphere. In this study, we investigated the inter-regional genetic differentiation of the Mytilus species complex at 53 locations in the North Atlantic and adjacent Arctic waters and linked this genetic variability to key local environmental drivers. Of seventy-nine candidate single nucleotide polymorphisms (SNPs), all samples were successfully genotyped with a subset of 54 SNPs. There was a clear interregional separation of Mytilus species. However, all three Mytilus species hybridized in the contact area and created hybrid zones with mixed populations. Boosted regression trees (BRT) models showed that inter-regional variability was important in many allele models but did not prevail over variability in local environmental factors. Local environmental variables described over 40% of variability in about 30% of the allele frequencies of Mytilus spp. For the 30% of alleles, variability in their frequencies was only weakly coupled with local environmental conditions. For most studied alleles the linkages between environmental drivers and the genetic variability of Mytilus spp. were random in respect to “coding” and “non-coding” regions. An analysis of the subset of data involving functional genes only showed that two SNPs at Hsp70 and ATPase genes correlated with environmental variables. Total predictive ability of the highest performing models (r² between 0.550 and 0.801) were for alleles that discriminated most effectively M. trossulus from M. edulis and M. galloprovincialis, whereas the best performing allele model (BM101A) did the best at discriminating M. galloprovincialis from M. edulis and M. trossulus. Among the local environmental variables, salinity, water temperature, ice cover and chlorophyll a concentration were by far the greatest predictors, but their predictive performance varied among different allele models. In most cases changes in the allele frequencies along these environmental gradients were abrupt and occurred at a very narrow range of environmental variables. In general, regions of change in allele frequencies for M. trossulus occurred at 8–11 psu, 0–10 °C, 60%–70% of ice cover and 0–2 mg m⁻³ of chlorophyll a, M. edulis at 8–11 and 30–35 psu, 10–14 °C and 60%–70% of ice cover and for M. galloprovincialis at 30–35 psu, 14–20 °C.

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 distribution of two common intertidal amphipod speciesGammarus oceanicusandGammarus setosuswas studied along the coast of Svalbard Archipelago. Genetic analysis showed geographical homogeneity ofG. oceanicuswith only one molecular operational taxonomic unit (MOTU) and much higher diversification ofG. setosus(5 MOTUs) in the studied area. Only two MOTUs ofG. setosusare widespread along the whole studied Svalbard coastline, whereas the remaining three MOTUs are present mainly along the northern and eastern parts of archipelago’s largest island, Spitsbergen. Distribution analysis indicates that the demographic and spatial expansion ofG. oceanicusin the northern Atlantic has started already during the Last Glacial Maximum (LGM, ca. 18 ka), whileG. setosusseems to be a long-persistent inhabitant of the Arctic, possibly even through the LGM, with slower distribution dynamics. Combining the results of our molecular study with previous field observations and the knowledge upon the direction of ocean currents around the Svalbard Archipelago, it can be assumed thatG. oceanicusis a typical boreal Atlantic species that is still continuing its postglacial expansion northwards. In recent decades it colonized High Arctic due to the climate warming and has partly displacedG. setosus, that used to be the only common gammarid of the Svalbard intertidal zone.

This paper has been written from the standpoint of the phylogenetic tetrad method which includes comparative anatomy, evolutionary developmental biology (“evo–devo”), paleontology, and molecular phylogenomics. The Bilateria consists of two sister groups such as Cnidaria and triploblastic Bilateria. Their common ancestor was a bilaterally symmetrical diploblastic organism with two circles of tentacles, labial and marginal, around a slit-like mouth. Anthozoa retains a primary bilateral symmetry. Medusozoa develops a secondary radial symmetry as a result of adaptation to a sedentary lifestyle. The common ancestor of the triploblastic Bilateria had a through gut formed by amphistomy, a coelom, segmentation, and metameric limbs. The marginal circle of tentacles gave rise to metameric limbs, the labial circle of tentacles gave rise to perioral ciliated tentacles, ventral ciliated sole (neurotroch), and telotroch. The homology of the ciliated tentacle apparatus in Trochozoa, Lophophorata, and Deuterostomia is substantiated. The ciliary armament of the triploblastic Bilateria larvae follows the general structural plan and includes the adoral ciliary field (a homologue of the ciliary tentacles of adult forms), neurotroch, and telotroch. The homology of pre-oral and oral (=tentacular) segments in triploblastic Bilateria is substantiated. The origin of Ecdysozoa as primarily semi-aquatic organisms is discussed. The chitinous cuticle of Ecdysozoa is homologous to the chitinous theca attached to the epidermis. The primary biramous limbs of Ecdysozoa resulted from the integration of tentacles of the marginal and labial circle of the common bilaterian ancestors. Tentacles of the marginal circle gave rise to exopods, and those of the labial circle gave rise to endopods. Examples of a primary biramous limb in Cambrian Ecdysozoa and recent Chelicerata are considered. The secondary biramous limb of Crustacea is a result from the secondary splitting of the endopod of the primary biramous limb into exopodite and endopodite. The problem of Hexapoda wings homology is discussed. Lobopoda is an Ecdysozoa lineage which adapted to life in loose substrate and acquired a worm-like body shape; the primary endopods became lobopods, while the primary exopods gave rise to metameric dorsolateral spines. Habitation in the sediment (Palaeoscolecida and Cephalorhyncha) caused a complete reduction of limbs and segmentation. The role of progenesis in the nematode origin is discussed.