• Mississippi National River and Recreation Area

    Mississippi

    National River & Recreation Area Minnesota

Zebra Mussels

Scientific Name: Dreissena polymorpha

What are Zebra Mussels?

Zebra mussels (Dreissena polymorpha) look like small clams with a yellowish or brownish "D"-shaped shell, usually with dark and light-colored stripes (hence the name "zebra"). They can grow to two inches long, but most are under an inch. Zebra mussels usually grow in clusters containing numerous individuals and are generally found in shallow (6-30 feet), algae-rich water.

 
Zebra mussels on a native species

These small, fingernail-sized mussels are native to the Caspian Sea region of Asia. With the opening of the St. Lawrence waterways, ships from all over the world made their way to the Great Lakes. It is believed that ships containing water from Eastern Europe, where zebra mussels are native, discharged not only water but zebra mussel larvae and juveniles as well into the Great Lakes watershed. These mussels found a welcoming environment and quickly began to populate North American waters. The first sighting of zebra mussels was in 1988 in Lake St. Clair, near Detroit, Michigan.

Why Worry?

Zebra mussels are a cause for concern because of their incredible rate of reproduction, because they often form an impenetrable crust of shell on any hard surface. The animals clog water intake pipes used by power plants and water treatment facilities and attach themselves to docks, boat bottoms, dam structures and other human built structures. They also attach to the shells of native mussels in such numbers that they can kill the mussel by holding its shell closed. Zebra mussels are strong competitors not only for living space but for food. Each zebra mussel can filter as much as one liter of water per day plankton that would normally be food for native mussels.

Why Are There So Many?

There are many reasons that the Zebra Mussel population is being so successful in North America:

  • We have not identified any natural enemies of zebra mussels in this environment.
  • Unlike native freshwater mussels, zebra mussels do not require a host for their larvae to mature. This significantly reduces the complexity of their life cycle. For example, the glochidia of the ebony shell mussel requires the snail darter fish as its host. When dams up and down the Mississippi restricted the movement of this fish, ebony shell mussels began to disappear in direct relation to the disappearance of the snail darter. Zebra mussels have no such dependency and can reproduce without regard to the presence of other species.
  • Zebra mussels do not seem to be significantly affected by water conditions. They survive regardless of depth, light intensity, or even winter temperature. It seems that wherever there is food and oxygen, colonies of zebra mussels grow rapidly. This is very different from native mussel species which are bellweather indicators. To some degree, biologists can judge the health of the river by the health of its native mussel population. Zebra mussels are not nearly as sensitive to water conditions as native species.
  • Incredibly, zebra mussels build their own platforms to populate. Where native species have preferences about gravel, sand or silty river bottom and how the currents flow, zebra mussels simply find a hard surface to grow upon. Other zebra mussels can then grow upon the first; in a way, they build their own environment!
  • These animals can even live out of water for several days especially in environments that are moist.

Stopping the Spread of Zebra Mussels

Zebra mussels spread to inland waters either as veligers (mussel larvae) transported in water, or as adults attached to boat hulls, engines, bait traps, or other hard surfaces. Veligers can't survive drying, but they can survive in any small pool of water. Adult zebra mussels can close their shells and survive drying for several days. This time is extended in moist environments.

The Minnesota Department of Natural Resources, in cooperation with Minnesota Sea Grant and the Bell Museum of Natural History, has published a handy field guide to exotic species with this checklist to help you prevent the spread of zebra mussels and other exotic species from one body of water to another:

  • Remove plants and animals from your boat, trailer, and accessory equipment (anchors, centerboards, trailer hitch, wheels, rollers, cables, and axles) before leaving the water access area.
  • Drain your livewells, bilge water, and transom wells before leaving the water access area.
  • Empty your bait bucket on land, never into the water. Never dip your bait or minnow bucket into one lake if it has water in it from another. Also, never dump live fish from one water body into other waters.
  • Wash your boat, tackle, downriggers, and trailer with hot water when you get home. Flush water through your motor's cooling system and other boat parts that normally get wet. If possible, let everything dry for three days before transporting your boat to another body of water.
  • Learn what these organisms look like. If you suspect a new infestation of an exotic plant or animal, report it to your Department of Natural Resources.
  • Check with the DNR for recommendations and permits before you try to control or eradicate an exotic "pest." Remember, exotic species thrive on disturbance. Do-it-yourself control treatments often make matters worse and can harm native species.

Did You Know?

A slow and shallow section of Itaska.

At the headwaters of the Mississippi, the average surface speed of the water is 1.2 miles per hour. People typically walk 3 miles per hour.