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Zebra Mussels Threaten Inland Waters: An Overview

Zebra mussels (Dreissena polymorpha) were first discovered in the Great Lakes in 1988. Within one year, zebra mussels colonized nearly every firm object in Lake Erie. Zebra mussels quickly spread to all the Great Lakes.

Expansion to inland waters continues at an alarming rate. For example, in 1992 zebra mussels made their way out of Lake Michigan into the Mississippi River basin via the Chicago Sanitary Shipping Canal – an artificial channel that links the Great Lakes drainage basin with the watershed drained by the Mississippi River. At the end of the 1992 season, zebra mussels were being found in isolated pockets from Minneapolis to St. Louis.

Zebra Mussel History

Zebra mussels have been in western and central European waterways for nearly 200 years. While we can learn a lot from looking at how zebra mussels behave in Europe, zebra mussels face a different ecosystem in North America. Comparisons and predictions based on European experience may not be accurate.

Zebra mussels are native to western Russia, near the Caspian Sea. Canals built during the late 1700s allowed the mussel to spread throughout eastern Europe. During the 1800s, canals were built across the rest of Europe. The canals made shipping easier but also allowed rapid expansion of the zebra mussel’s range. By the 1830s, the mussels had covered much of Europe and Britain.

Successful introduction of zebra mussels into the Great Lakes probably occurred in 1985 or 1986, when one or more transoceanic ships discharged ballast water into Lake St. Clair. The freshwater ballast, picked up in a European port, contained zebra mussel larvae and possibly juveniles. Being a temperate, freshwater species, they found the plankton-rich lakes St. Clair and Erie to their liking.

Zebra Mussel Biology

The mussel’s reproductive cycle is one key to its rapid spread and high abundance. Egg production starts when water temperature warms to about 54 degrees F. A fully mature female mussel may produce several hundred thousand eggs per season.

Eggs are fertilized outside the mussel’s body and within a few days develop into free-swimming larvae called veligers. Veligers remain suspended in the water from three to four weeks, drifting with the currents. If they don’t settle onto firm objects, they die. In fact, most do. Those that find a hard surface quickly attach themselves and transform into the typical, double shelled mussel shape. Within a year, a zebra mussel can grow up to an inch and become sexually mature. European studies report mussels may live four to six years, but in Lake Erie three years seems to be the maximum, and the average is much less.

Zebra mussels generate a tuft of fibers known as a byssus, or byssal threads, from a gland in the foot. The byssus protrudes between the two halves of the shell. These threads attach to hard surfaces with a powerful glue that anchors the mussels in place. Small juveniles can actually break away from their attachments and generate new, buoyant threads that allow them again to drift with the currents and find a new home.

Any hard surface that is not toxic can be colonized by zebra mussels – rock, metal, wood, vinyl, glass, rubber, fiberglass, paper, plants, and other mussels. Beds of zebra mussels in some areas of Lake Erie contain between 30,000 and 70,000 mussels per square meter.

Zebra mussels can become established regardless of depth, light intensity, or even winter temperature. Colonies grow rapidly wherever oxygen and particulate food is available and water currents are not too swift – generally less than six feet per second. Colonies are rare in wave-washed zones, except for sheltered nooks and crevices. In most European lakes, the greatest densities are at depths ranging from 6 to 45 feet.

Zebra mussels also colonize soft, muddy bottoms. Hard objects, such as pieces of native mussel shells, act as a base for settling veligers. As a few mussels begin to grow, they serve as substrate for additional colonization. In this way, extensive mats of zebra mussels can form on soft lake and river bottoms.

Zebra Mussels Alter the Environment

Zebra mussels are filter feeders. They strain water for the food they need. Unused food is bound with mucous into a pellet called pseudofeces, which is ejected. Each adult zebra mussel can filter about one liter of water per day. Nearly all particulate matter, including phytoplankton and some small forms of zooplankton, are removed. These microscopic plants and animals are the base of the food chain. Small fish, such as young sportfish or forage fish, depend on this food for survival and growth. The long-term consequences of removing so much food from the environment is still being studied.

Scientists and boaters saw a great increase in water clarity in Lake Erie between 1989 and 1991. Shallow bays are being recolonized by rooted, aquatic plants since sunlight can now penetrate. Much of this change has been attributed to zebra mussels.

Native North American mussels have suffered greatly as a result of being encrusted with zebra mussels. Sometimes several thousand zebra mussels are found on a single native mussel. In Lakes St. Clair and Erie, zebra mussels have severely reduced populations of native mussels. Some mussel species in the St. Croix River are very rare and are officially listed as endangered. As zebra mussels spread, biologists are concerned that these species face imminent extinction.

Zebra Mussels Affect Industry and Recreation

Because zebra mussels prefer hard surfaces at moderate water depth, water intake structures, such as those used by power plants and city treatment plants, are susceptible to clogging by zebra mussels. In fact, since 1989, some facilities located on Lake Erie have reported big reductions in pumping capacity and occasional shutdowns caused by encrusted zebra mussels.

Recreational users of water infested with zebra mussels can also be affected. Unprotected docks, breakwalls, boat bottoms, and engine outdrives can provide the hard surface zebra mussels need to colonize and grow. In lakes Erie and St. Clair, there were numerous reports of boat engines overheating because cooling water inlets were clogged by zebra mussels. Boaters need to make frequent inspections of these areas in the future. Anti-fouling paints containing copper or tin are effective in preventing zebra mussel build-up, but their use is banned in Michigan, and restricted in other states (contact your state’s Department of Agriculture) because they can harm other aquatic life.

What You Can Do

Follow Minnesota Aquatic Invasive Species Laws:

  • Clean all aquatic plants, animals and mud from watercraft, trailers, docks, lifts, anchors and other recreational equipment before leaving access.
  • Drain water-related equipment (boat, ballast tanks, portable bait containers, motor) and drain bilge, livewell and baitwell by removing drain plugs before leaving water access. Keep drain plugs out while transporting watercraft.
  • Dispose of unwanted bait, worms and fish parts in the trash.

Also Recommended:

  • Spray watercraft and equipment with high-pressure water, or
  • Rinse with very hot water, or
  • Dry for at least 5 days.

Report New Sightings — note exact location; place specimens in a sealed plastic bag; and call a Minnesota DNR Invasive Species Specialist (see www.mndnr.gov/invasives/contacts.html), 1-888-MINNDNR or (651) 259-5100; or the MN Sea Grant Program in Duluth, (218) 726-8712.

Know the rules!

Specimens are needed to confirm sightings, but some jurisdictions prohibit possession of thesemussels and other invasive animals and plants. InMinnesota, possession and transport of any Dreissena is illegal, exceptwhen providing themto SeaGrant or DNR for identification. Unauthorized introduction of invasive mussels, fish or plants into the wild is illegal. Protect your property and our waters.

The information in this article comes from a variety of sources, including Zebra Mussels in the Great Lakes: The Invasion and Its Implications by Fred L. Snyder, David W. Garton, and Maran Brainard, published by the Ohio Sea Grant College Program.

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This page last modified on February 14, 2017     © 1996 – 2017 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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