Calcium Requirements and the Spread of Zebra Mussels

) are pistachio-sized freshwater bivalvesnative to the regions around theBlack and Caspian seas. Since theirdiscovery in Lake St. Clair, Michi-gan, in the late 1980s, they havespread distressingly fast through theGreat Lakes and Mississippi Riverbasin, as well as to other lakes andrivers in the eastern United Statesand Canada.Zebra mussels are considered anuisance species because of theirtremendous abundance (they cangrow on top of each other in greatmounds) and habit of colonizing theinterior of any pipe carrying freshwater. They also encrust docks andboats and can literally carpetbeaches with their small, sharpshells. From an ecological perspec-tive, they can alter the natural foodchain by filtering huge amounts ofalgae.