The impact of Dreissena polymorpha (PALLAS) invasion on unionid bivalves

Dreissena polymorpha, the zebra mussel, is one of the most aggressive and important invading aquatic species world wide. Its spread has followed the path of human activity, initially following human constructed canals connecting the Black Sea and Baltic Sea basins. One consequence of this invasion is the impact of zebra mussels on native bivalves. Overgrowth by Dreissena can cause a dramatic decrease in unionid density. The extent of this effect is determined by several factors including Dreissena density, time since invasion by Dreissena, biomass of attached Dreissena, and type of bottom sediments (sand versus silt). We found a correlation between overall Dreissena density and the number of zebra mussels per overgrown unionid, and between Dreissena density and the ratio of the mass of attached zebra mussels to the mass of the host unionid. The extensive overgrowth of unionids by Dreissena, resulting in mass mortality, is characteristic of periods of rapid population growth, when Dreissena invade a new waterbody.