May 16, 2002
The zebra mussel (Dreissena polymorpha) has become a bane to the Great Lakes. It is a multi-billion dollar problem affecting industrial and public water supplies. They're nonindigenous invaders originally found in Eastern Europe, around the Black and Caspian Seas. In 1988, they were inadvertently introduced to the Great Lakes in the ballast water of a transatlantic freighter and, in less than 10 years, they've spread to all five Great Lakes and have now found their way into a number of major rivers. A female can release up to one million eggs each season, so accidentally transporting just one zebra mussel can lead to big problems in waterways where they've yet to appear.
These freshwater mollusks look like small clams they're usually a yellowish-brown color with dark and light colored stripes. Most zebra mussels are smaller than an inch (2.54 cm) in length, but they can be up to two inches (5.1 cm) long. They're most often found in dense clusters and prefer shallow (6-30 feet or 2-10 m), algae-rich water. Densities of over one million per square meter have been recorded in parts of Lake Erie!
Adult zebra mussels colonize all types of living and non-living surfaces including boats, power plants, water-intake pipes, docks and slow moving animals such as native clams and turtles. They even attach to each other. When they attach to clams, as shown above, they prevent these bivalves from opening. The survival of several clam species and a variety of North American ecosystems are threatened by the spread of these alien invaders.