Dreissena is a genus of small freshwater mussels in the family Dreissenidae. The exact taxonomy of the species within the genus Dreissena is, however, uncertain. As Kantor et al. commented, "...the Russian specialists on freshwater molluscs appeared to be “splitters” comparing with their western colleagues. This sometimes leads to extreme differences in opinion on species number and taxonomy of some groups. For example, Graf [2007] recently analyzed the number of species of Unionoidea of Palearctic recognised by Russian malacologists (156 species in 34 genera) and western specialists (45 species in 16 genera). Most of the groups are still awaiting the critical re-assessment."