Zebra Mussel, Dreissena polymorpha

A relatively new invader, the zebra mussel is believed to have come to America in the ballast tanks of a transoceanic ship. Native to Europe's Caspian and Black Seas, the zebra mussel was first found in North America in 1988 in Lake St. Clair between Lakes Huron and Erie. Since then, the zebra mussel has spread throughout the Great Lakes and waterways of at least 19 States, including the Hudson, Illinois, Mississippi, Ohio, and Tennessee Rivers. Currently, the zebra mussel is extending itself to the West via the Arkansas River and other tributaries. It remains unstopped by predators and parasites, leaving all waterways of North America wide open for invasion.

Zebra mussels are very small with brown and black bands. Fully grown, they are only 2 inches (5 cm) long. By themselves, these mussels don't look threatening. But 1 adult female can produce 30,000 young each season, and occasionally there are two spawning seasons in a year. The first season is from June until mid-July and the second, when it occurs, takes place between August and October. The zebra mussels' young can attach themselves to any hard surface—a rock, a pipe, or even the shell of another sea creature.

The threat of the zebra mussel lies in its ability to reproduce in large numbers and in the fact that it can stick to anything hard. Oftentimes, zebra mussels will block intake pipes to factories and water treatment plants. The mussels will blanket marinas and can crowd out native bivalve species like freshwater clams. Sea creatures with shells often fall victim when literally hundreds of zebra mussels cover them, preventing them from feeding, growing, moving, breathing, and reproducing. Two years after the zebra mussel's introduction into Lake St. Clair, all hard-shelled native species in the lake were encrusted by the invasive mussel. Several native species were destroyed completely. This pest is a plague and shows that even pint-size invaders can cause big trouble.