Zebra mussels (Dreissena polymorpha) are one of the most widespread invasive freshwater animals in the world. They cause economic harm in North America of over one billion dollars per year. Their huge populations attach to hard surfaces, clog intake pipes for water treatment and power generating plants, encrust boat motors and hulls, may greatly reduce lakefront property values, and their sharp shells cut swimmer’s feet. Ecologically, they filter enormous quantities of microscopic algae and alter energy flow through aquatic ecosystems — with potentially large impacts on fish populations — and they smother and cause extinctions of native bivalve mollusks.
What they look like
Zebra mussels are ¼-1 ½ inch-long bivalve (2-shelled) molluscs. They evolved from ancestors similar to surf clams (used to make clam chowder) that invaded fresh waters in southern Russia. They have a D- or wedge-shaped shell, which is often marked by alternating brown and yellow bands in a zigzag pattern. They live on lake and river bottoms, rocks, aquatic plants, docks, lifts, and boats to which they attach using small dark fibers called "byssal threads." Viewed up-close underwater, two tiny siphons can be seen projecting into a narrow gap between the shell valves of each animal — these siphons are used to pump water for respiration and feeding.
Each mussel is either male or female, and they release eggs (500,000 or more per female per year) or sperm into the water. Fertilization yields a tiny (< 1/10th millimeter) larva called a veliger — found in very few other freshwater molluscs. Veligers feed on algae and grow for about 3 weeks, drifting in the plankton, during which time wind and currents can transport them over large distances. After this, they settle down and attach to the lake or river bottom, and after about 12-18 months, they grow to reproductive size.
Where they're found
Zebra mussels are native to large rivers and lakes draining into the Black, Caspian, and Azov Seas of southwestern Russia and the Ukraine. Beginning in about 1800, they began spreading across western and northern Europe and most recently have reached inland waters in the British Isles, Spain, Portugal, and France. They appeared in North America in 1988, and in five years they spread rapidly throughout the Great Lakes and large rivers. In several Great Lakes (particularly Michigan and Erie) zebra mussels have been largely replaced by a related species — the quagga mussel (D. bugensis) — also from the Black Sea. Zebra mussels arrived in the Duluth Harbor in 1989 and the Mississippi River in 1993. As of May 2018, the Minnesota Department of Natural Resources listed 335 waterbodies in the state as infested due to either confirmed zebra mussel presence or connection to a waterbody with a confirmed presence
How they spread
In the 19th century, zebra mussels spread throughout Europe in man-made canals, and in the late 20th century, on recreational watercraft and the nets of commercial fisherman. They were also spread to lakes in Poland and Belarus on the nets of commercial fisherman. In North America, barge traffic and (to unknown extent) larval dispersal were responsible for rapid initial spread throughout the Great Lakes, Mississippi, Ohio and Susquehanna Rivers. Spread to inland lakes has occurred by larvae transported down connected streams and waterways, and overland via mussels attached to vegetation and to surfaces of recreational boats, trailers, docks and lifts. Veliger larvae may also be transported in the "residual water" remaining inside boat compartments when trailered boats are moved between waterways.
How you can help
- Learn how to identify a zebra mussel in our Aquatic Invasive Species ID Guide.
- If you're interested in monitoring your lake for zebra mussels, read these new statewide protocols for early detection monitoring for zebra mussels. You can use the monitoring protocols in this document to assist in early detection monitoring for zebra mussels in a lake where zebra mussels have not yet been found and to apply for treatment if zebra mussels are detected.