Additional records of the bivalves Mytilopsis leucophaeata (Conrad, 1831) (Dreissenidae) and Arcuatula senhousia (Benson, 1842) (Mytilidae) in the Ponto-Caspian region.
Conrad's false mussel Mytilopsis leucophaeata (Conrad, 1831) and the Asian date mussel Arcuatula senhousia (Benson, 1842) are highly invasive bivalves actively spreading in the Ponto-Caspian region, which includes the basins of the Black, Azov, and Caspian seas. This study provides new records of both species from this region and gives a synopsis of known information on their ecology, substrate preferences, tolerance and adaptive potential to environmental factors such as salinity. Mytilopsis leucophaeata and A. senhousia are at different stages of colonization in the Ponto-Caspian region. While M. leucophaeata has become established in the region, approaching circumferential distribution in each sea basin, A. senhousia is in the initial phase of its dispersal. Although both species are distributed in estuarine and coastal areas, because of different salinity tolerances they are confined to zones of differing degrees of hydrological fluctuation: M. leucophaeata occurs in more estuarine conditions with fluctuating river discharges. Arcuatula senhousia avoids low salinity and is mainly absent near large rivers. In spite of this, A. senhousia generally prefers substrates of riverine origin (so called mixed sediments with prevalence of the soft fraction) and therefore its distribution reflects a compromise pattern with settlement occurring at the outer periphery of estuaries, whereas M. leucophaeata occupies areas with substantial freshwater influence. Unlike A. senhousia, M. leucophaeata usually occurs on hard substrates, however, our findings indicate that it is able to occupy other substrates in the absence of typical biotopes. Compared to A. senhousia, M. leucophaeata colonizes more diverse localities with a broader range of fluctuating conditions such as salinity and of substrate types. Both M. leucophaeata and A. senhousia in the Black Sea tend to settle on substrates that are not fully occupied by native byssus-attached bivalves.