Distribution of Mollusks of the Genus Dreissena in Water Bodies and Watercourses of the Middle and Lower Volga

摘要

The current distribution of the Ponto-Caspian alien mollusk species of the genus Dreissena (Dre-issena bugensis and D. polymorpha) in the water bodies of the Middle and Lower Volga is shown. Since the invasion of quagga mussel (D. bugensis) which tookplace 20 years ago, the occurrence ratio of this species has remained stable. This fact provides no evidence for the ongoing replacement of one species by another. The distribution area of D. polymorpha is expanding presently owing to its penetration into rivers where this species was never registered before.