Originally from the Balkans, Poland and the former Soviet Union, zebra mussels (Dreissena polymorpha) are small freshwater mollusks which are invasive. They spread to the Americas in the 1980s via ballast water of a ship. The mollusks were first found in 1988 in Lake St. Clair, Michigan, and are currently known to have infested 29 states and more than 600 lakes or reservoirs in the United States.

They have now reached Lake Bridgeport Texas.

The first page of the Raven & Berg AP Environmental Science textbook details the spread of the invasive species. Invasive species means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.

In 2010, the AP College Board included a question about zebra mussels in the first free response question:  Why are zebra mussels located primarily in areas in the eastern United States rather than in the western United States?

According to the National Atlas: “Zebra mussels upset ecosystems, threaten native wildlife, damage structures, and cause other serious problems. Millions of dollars are spent each year in attempting to control these small but numerous mollusks.“