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Zebra mussels ( D re i sse n a polymorpha) are fingernail-size bivalves indigenous to parts of eastern Europe and the Caspian Sea region of Eurasia. Similar to other mussels, they are planktonic for the first few weeks of life, floating in currents and able to colonize new areas. They then attach to a hard substrate and become sedentary. Females are prolific, producing from 30,000 to over a million eggs during one spawning event.
Zebra mussels spread to western Europe approximately 200 years ago, invading all major rivers via canals. They have since spread to North America, presumably in the ballast water of ocean-going vessels. The mussels were first noticed in 1988 in Lake St. Clair in the Great Lakes region and have since spread to many lakes and river systems in both Canada and the United States. Although they have predators, including several species of freshwater fish and diving ducks, these predators have been unable to stabilize population growth.
In Europe as in North America, the mussels have caused significant ecological and economic harm. Zebra mussels grow together in dense mats attached to a variety of living and nonliving substrates. These dense colonies clog water intake pipes of waterworks, power plants, and other industrial users of water, causing millions of dollars of damage and necessitating the application of chemical treatments or the reconfiguration of the piping of these plants.
Zebra mussels have proven to be a very successful species in their newly colonized territories. They have a formidable ability to filter water, consuming nearly all the available phytoplankton and small zooplankton. They out-compete other species that also feed on microscopic plankton, and in this way, change food web dynamics, impacting larval and juvenile fish as well as other filter feeders. Zebra mussels preferentially remove a variety of nutrients and chemicals from the water column. Phosphorus is removed and sequestered in their shells, changing the phosphorus cycle in aquatic ecosystems where the mussel is found.
North America was home to the greatest biodiversity of freshwater mussels. Many of these populations have declined in numbers or even been extirpated due to combinations of overharvest, pollution, and habitat destruction. Managers are concerned about the threat that zebra mussels pose to many of the remaining native mussel species, such as the endangered winged mapleleaf clam (Quadrula fragosa), found in the St. Croix River in the upper Mississippi watershed.
When the zebra mussel was first encountered in North America, there was no regulatory or legal framework in place to stop ballast water introductions of exotic species. In Canada, ballast water introductions are generally addressed through voluntary guidelines under the Canada Shipping Act. In the United States, however, prompted by the negative impacts of the zebra mussel and by concern over the potential for more new invading species, new legislation was passed. In 1990, the U.S. Congress passed the Nonindigenous Aquatic Nuisance Prevention and Control Act, which was reauthorized, amended, and renamed the National Invasive Species Act of 1996.
Despite these measures, new exotic species have continued to appear. Some scientists believe that the Quagga mussel (D. bugensis), a close relative of the zebra mussel, may pose a more serious threat to native species in the Great Lakes region of North America than the zebra mussel. The Quagga mussel is larger, does not go dormant in the winter, and has a wider habitat range in which it can live. It appears to compete directly with the zebra mussel, and since 2000, the Quagga has replaced the zebra mussel in many areas of Lake Michigan.
- Renata Claudi and Joseph Leach, eds., Nonindigenous Freshwater Organisms: Vectors, Biology, and Impacts (Lewis Publishers, 1999);
- Frank M. D’Itri, ed., Zebra Mussels and Aquatic Nuisance Species (Ann Arbor Press, 1997);
- Thomas F. Nalepa and Donald W. Schloesser, , Zebra Mussels: Biology, Impacts, and Control (Lewis Publishers, 1993).