Yahara Portal

Explore the legacy and future of the Yahara Lakes and Watershed

Zebra and Quagga Mussels

Yahara Lakes Most Unwanted Aquatic Hitchhikers


The zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) are tiny—1/8-inch to 2-inch—bottom-dwelling mussels native to Caspian and Black Seas. They were brought to the Great Lakes in the ballast water of ocean-going ships. The zebra mussel takes its name from its striped shell. Zebra mussels were introduced into the Great Lakes system in 1985 or 1986 and first turned up in Lake St. Clair.

The quagga is easily outcompeting its cousin in the Great Lakes because it can adapt more readily to the Great Lakes environment.The zebra must attach to hard surfaces in relatively warm water, which has made it the most common in near-shore areas. In contrast, the quagga mussel can thrive on any surface except mud and prefers silt and sand. Quaggas can also tolerate both colder and warmer water temperatures, can spawn in colder water, feed year around, and are found at depths of 300 feet and greater.

Zebra mussels were first documented in Wisconsin waters in Lake Michigan in 1989. Since then they have spread to over 100 lakes and streams statewide. After their initial discovery in Lake Monona in 2002, extensive benthic sampling was conducted from 2002-07 on lakes Mendota and Monona. Three transects were taken on each lake and over 250 bottom samples were analyzed from Lake Mendota and 119 samples from Lake Monona. No zebra mussels were found in any of the samples from either lake (Karatayev & Burlakova, 2008). Monitoring protocols established by DNR need be followed to determine if Lake Monona meets the criteria for delisting. Zebra mussel shells were found in the water of upper Lake Mendota in 2011 (attached to the hull of a boat that had recently been transported from a highly infested lake nearby), but neither DNR or the UW Center for Limnology have found evidence that zebra mussels are living in the waters of Lake Mendota.

Problems associated with this invader

The exotics clog water-intake systems of power plants and water treatment facilities, along with the cooling systems of boat engines. According to the DNR Fisheries Biologist, Kurt Welke, their presence can also damage the aquatic ecosystem. They can severely reduce and may eliminate native mussel species. Because they filter plankton (microscopic plants and animals) from the surrounding water, water clarity may improve, but the long-term effects aren't well understood at this time. Newly hatched larval fish need zooplankton and phytoplankton to survive and zebra mussels, which are incredibly efficient filterers, are competing with native fish for the same food source. Ironically, even though zebra mussels filter out many planktonic species, the nuisance blue-green algae—often the summer bane of the Yahara lakes—is not one of them. In simple terms, zebra mussels take out the good stuff, but not the bad stuff. 


  • Look like small clams with a "D" shaped shell, usually with dark and light colored stripes. 
  • Most are under an inch but can be up to two inches long
  • Usually grows in clusters and are generally found in 6-30 feet of algae-rich water 
  • Zebra mussels are the ONLY freshwater mollusk that can firmly attach itself to solid objects: submerged rocks, dock pilings, boat hulls, water intake pipes, etc. 
  • Quagga mussels can survive better in silty and sandy environments and don’t need to attach themselves to hard surfaces.