Zebra Mussels in the Great Lakes

Zebra mussels are small freshwater shellfish, usually no bigger than a fingernail, named for the striped pattern on their shells. They are native to lakes and rivers in southern Russia and Ukraine. In 1988, however, they were discovered in the Great Lakes of North America. Scientists believe they arrived in the ballast water of cargo ships, which take in water at one port to stay stable and then release it at another.

With no natural predators to control them, zebra mussels spread rapidly. A single female can produce up to a million eggs a year. The mussels attach themselves to almost any hard surface, including boats, docks, and the pipes that bring water to cities and power plants. In some places, they have grown so thick that they block these pipes, and removing them costs millions of dollars each year.

Their effects on the lakes themselves are more complex. Zebra mussels feed by filtering tiny particles, including plankton, out of the water. As a result, the water in some lakes has become much clearer. This may sound like a good thing, but it means less food for native fish and other animals that depend on plankton. Clearer water also allows sunlight to reach deeper, which can promote the growth of plants and algae on the lake bottom.