Freshwater mussels (Bivalvia: Unionoida) play an important role in freshwater habitats as ecosystem engineers of the water environment. Duck mussel Anodonta anatina is widely distributed throughout Europe, Siberia, and Western and Central Asia, which makes it a convenient object for biogeographic studies. In this study, we analyzed the divergence of A. anatina populations and discovered a separate genetic lineage distributed in rivers of the Azov Sea basin. This was confirmed by the high genetic distances between this group and previously defined populations, and by the position of this clade in the Bayesian phylogeny calibrated by an external substitution rate. Based on our approximate Bayesian computation (ABC) analysis, biogeographic scenarios of A. anatina dispersal in Europe and Northern, Western, and Central Asia over the Neogene–Quaternary were simulated. The haplogroup’s isolation in the rivers of the Azov Sea basin most likely occurred in the Late Pliocene that was probably facilitated by rearrangement of freshwater basins boundaries in the Ponto-Caspian Region. Population genetic indices show the stability of this group, which allowed it to exist in the river basins of the region for a long time. The discovery of a long-term refugium in the rivers of the Azov Sea led to a better understanding of freshwater fauna evolution in the Neogene–Quaternary and highlighted the importance of conservation of these freshwater animals in the region as a source of unique genetic diversity.

Freshwater mussels belonging to the tribe Pseudodontini (Bivalvia: Unionidae: Gonideinae) play a keystone role in riverine and lacustrine environments throughout Southeast Asia. Many of them are narrowly endemic and habitat specialists, which need special conservation efforts. Unfortunately, the systematics of this group is rather poorly understood. Here, we show that Pseudodon inoscularis, the type species of the genus Pseudodon, belongs to the eastern clade of the tribe (Southeast Asia, east of the Salween Basin), and, more certainly, clusters with members of the genus Monodontina. Hence, the latter genus becomes a synonym of Pseudodon, while the subtribe Pseudodontina (=Pilsbryoconchina syn. nov.) should be used as a name for the eastern clade. In turn, the genus Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. must be applied as valid names for the western clade of the Pseudodontini (Ayeyarwady to Salween). We present a revision of the genus Indopseudodon with an updated synonymy and a description of a new species, I. indawgyiensis sp. nov. We also link four species-level genetic lineages from the Mekong to available taxonomic names in the genera Pseudodon and Sundadontina (Pseudodon cambodjensis, P. vagulus, Sundadontina harmandi, and S. sulcata). Based on morphological features, we revise the taxonomic position of several recent and fossil species that were traditionally placed in the Pseudodontini and transfer them to the genera Parvasolenaia, Pseudodontopsis, Ptychorhynchus, Simpsonasus nom. nov. (a new name for Nasus Simpson, 1900) (Gonideinae: Gonideini), Pseudobaphia (Unioninae: Unionini), Balwantia and Lamellidens (Parreysiinae: Lamellidentini).

Freshwater mussels are an endangered group of animals, especially in the water bodies of Central and Western Europe. Conservation priorities were established for many endangered freshwater mussel species in the last decade. Here, we present new data on a cryptic refugium of freshwater mussels in the Western Balkans. Two species, Anodonta exulcerata Porro, 1838 and Unio elongatulus Pfeiffer, 1825, were discovered in Montenegro for the first time. The phylogenetic position and taxonomic status of freshwater mussel species from Montenegro were studied by means of an integrative approach, combining morphological data and COI gene sequences. Unio carneus Küster, 1854 stat. rev. represents a species with a restricted range, which is endemic to the Western Balkans. A viable population of Microcondylaea bonellii (Férussac, 1827) was discovered in the Zeta River. This species was not mentioned in Montenegro since the 1900s. A significant level of genetic diversity was revealed for the studied species. In the Skadar Lake basin, freshwater mussels from diverse ecological groups were discovered. Rheophilic species of freshwater mussels may represent ancient lineages, which need special conservation planning. Conservation priorities should be aimed at the protection of freshwater mussel habitats on the Balkan Peninsula.

The accumulation of trace metals in the shells of bivalves allows quantitative assessments of environmental pollution and helps to reconstruct paleo aquatic environments. However, the understanding on how marine and freshwater mollusks control the level of trace elements in their shells remains very limited. Here, we compared the trace element composition of marine and freshwater bivalves from boreal and subarctic habitats, using examples of widely distributed species of marine (Mytilus edulis, M. trossulus) and freshwater (Anodonta anatina, Unio sp., Beringiana beringiana) mussels. Sizable differences in several trace element concentrations were detected between different species, depending on their environmental niches. A multiparametric statistical treatment of the shell’s elemental composition allowed to distinguish the impact of external factors (water and sediment chemical composition) from active metabolic (biological) control. In particular, the obtained results demonstrated that Ba:Ca and Pb:Ca ratios in mussels’ shells are closely related to the primary productivity of aquatic ecosystems. The Mn:Ca ratio allowed to constrain the environmental conditions of mussels’ species depending on the trophic state of inhabited waterbody. Overall, the marine mussels exhibited stronger biological control of trace element accumulation, whereas trace element pattern in shells of freshwater mussels was chiefly controlled by environmental factors. The obtained results might help to use the trace element composition of bivalves in distinguishing marine and freshwater habitats of mollusks in paleo environments.

The duck mussel Anodonta anatina is widely distributed throughout the freshwater basins of Northern, Central, and Western Eurasia, and it has a comprehensive genetic structure. This study was devoted to the analysis of lineages, which are defined based on COI gene sequences. Our new dataset was expanded by samples from freshwater basins of Northern and Central Eurasia. It allowed us to reveal a high level of genetic diversity for the widely distributed trans-Eurasian lineage of A. anatina for the first time. As for results, representative samples from the Russian Plain, Southern Siberia, and the Ural region showed the presence of multiple interactions between duck mussel populations, indicating the existence of connections between freshwater basins in this region during the Late Quaternary. The genetic group from the freshwater basins of Northern Eurasia may be divided into two sub-lineages, which have differences in genetic structure and distribution patterns. It was revealed that there was a post-glacial expansion of duck mussels in the freshwater basins of Northern Eurasia after deglaciations of these territories and that the wide distribution of this species in this region was shaped via ancient connections between periglacial waterbodies. The lineage of A. anatina from the Ponto-Caspian region is a genetically rich and diverged group, which is present in the riverine basins of West-Central Asia related to the Caspian Sea.

Trace elements in freshwater bivalve shells are widely used for reconstructing long-term changes in the riverine environments. However, Northern Eurasian regions, notably the European Russian North, susceptible to strong environmental impact via both local pollution and climate warming, are poorly studied. This work reports new data on trace elements accumulation by widespread species of freshwater mussels Unio spp. and Anodonta anatina in the Severnaya Dvina and the Onega River Basin, the two largest subarctic river basins in the Northeastern Europe. We revealed that iron and phosphorous accumulation in Unio spp. and Anodonta anatina shells have a strong relationship with a distance from the mouth of the studied river (the Severnaya Dvina). Based on multiparametric statistics comprising chemical composition of shells, water, and sediments, we demonstrated that the accumulation of elements in the shell depends on the environment of the biotope. Differences in the elemental composition of shells between different taxa are associated with ecological preferences of certain species to the substrate. The results set new constraints for the use of freshwater mussels’ shells for monitoring riverine environments and performing paleo-reconstructions.