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’s Being Done to Stop Zebra Mussels
Zebra mussels can spread to inland waters either as veligers transported in water, or as adults attached to boat hulls, engines, bait traps, or on other items. Veligers don’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. In moist environments, they can survive out of the water even longer.
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.
For more information on zebra mussels or other exotics, contact:
MN DNR Invasive Species Program 651-259-5131 or MN Sea Grant Exotic Species Information Center 218.726.8106.
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.
- Species Profile: Zebra Mussels
- Zebra Mussels Threaten Inland Waters: An Overview
- WATCH Card: Zebra Mussel
- Infestation in Harbor Impacts Native Mussels, Boaters (Seiche, January 2001)
- Highlights of the 9th International Zebra Mussel Conf. (Seiche, Sept. 1999)
- Zebra Mussels “Pulse” in Duluth-Superior Harbor (Seiche, Dec. 1998)