Invasive Species

Zebra Mussels
Dreissena polymorpha

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. Zebra mussels are actually 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, making shipping easier but also allowing rapid expansion of the zebra mussel's range. By the 1830s, the mussels had covered much of Europe and Britain.

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 mussels were first discovered in the Great Lakes in 1988. Within one year, zebra mussels colonized nearly every firm object in Lake Erie and quickly spread to all 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. Now, zebra mussels have been located in waters of 20 U.S. states and Canada.

Zebra Mussel Biology

The mussel's reproductive cycle is key to its rapid spread and high abundance. A fully mature female mussel may produce several hundred thousand to a million eggs per season. Eggs are fertilized outside the mussel's body and develop into free-swimming larvae called veligers that remain suspended in the water, 3-4 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. In Lake Erie, mussels may live for three years maximum.

Zebra mussels generate a tuft of fibers known as a byssus, or byssal threads. The byssus protrudes between the two halves of the shell and 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.

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, straining water for the food. 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. However, 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 from being encrusted with zebra mussels. Several thousand zebra mussels can be found on a single native mussel. In Lakes St. Clair and Erie, zebra mussels have severely reduced populations of native mussels, while 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 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 reductions in pumping capacity and occasional shutdowns caused by encrusted zebra mussels.

Unprotected docks, breakwalls, boat bottoms, and engine outdrives provide hard surfaces for zebra mussel colonization and growth. Boaters should frequently inspect these areas. Antifouling 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.

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