Large freshwater mussels (Unionoida) are declining throughout the world. The European bitterling Rhodeus amarus (Bloch, 1782) female spawns its eggs inside the unionids’ shells, where fertilisation and further embryonic development take place; thus its reproduction depends fully on the presence of large freshwater mussels. Unio crassus, previously regarded as one of the most numerous unionids in Europe, is now listed in the IUCN Red Data List as being globally endangered. Despite its previous prevalence, it was never reported as a host for the bitterling. A large population of U. crassus was studied in small river at the Świętokrzyskie Mts (Poland), where also electrofishing was conducted. In each bitterling territory located on the study plots, we found individuals of U. crassus, with the bitterling eggs or larvae developing on mussel’s gills. That proves that this species can be also used by the bitterling for reproduction. We suggest that this relationship has not been reported to date due to the mussels’ rarity and ongoing decline. However, it is also possible that the endangered mussel is a novel host for the bitterling, which is expanding its range throughout Europe.

A new large population of the Asian clam Corbicula fluminea (O. F. Müller, 1774) was found within a reach (ca 210 km long) of one of the largest European rivers, the Vistula. The largest population and the largest individuals were found at the outlet of a channel collecting heated water from the cooling process at the Połaniec power station and adjacent parts of the river. In the northern part of the study area, bivalves occurred at the channel margins, in groyne fields, and in the shallows of sand banks or sandbars. The clams were less numerous in places where the river was regulated with straightened banks and stone ripraps. Twenty-five other taxa of mollusks were found altogether at the sampling stations where Corbicula was observed, including other non-indigenous mollusk species.