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Encyclopedia Dubuque


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Affiliated with the Local History Network of the State Historical Society of Iowa, and the Iowa Museum Association.


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Although small in size, zebra mussels have had a major impact on the Mississippi River.

ZEBRA MUSSELS. The zebra mussel (Dreissena polymorpha), a small freshwater mollusk, invaded America's rivers and lakes. Originating in the Balkans, Poland, and the former Soviet Union, they appeared in North America in 1986. (1) Biologists believe zebra mussels were carried in the ballast water of a ship and later discharged into the Canadian side of Lake St. Clair.

Although not all shells carry the pattern, zebra mussels are named for the striped pattern of their shells. Usually about fingernail size, this mollusk may reach a maximum length of nearly 2 inches. Zebra mussels live four to five years in fresh water at depths of six to twenty-four feet. A female zebra mussel begins to reproduce at two years of age and produces between 30,000 and 1 million eggs annually. About two percent of zebra mussels reach adulthood.

Being small and free swimming, young zebra mussels are easily spread by water currents. Older zebra mussels attach themselves to hard surfaces by an external organ called a byssus. The mussels may attach to boats, pilings, water-intake pipes, crayfish, turtles, other zebra mussels, and native mollusks. While zebra mussels can attach themselves securely, they may move and can reattach themselves easily if dislodged by storms.

The filter-feeding activity of zebra mussels causes a dramatic increase in water clarity in infested lakes and rivers. This increase in water clarity, however, comes as a result of the zebra mussels removing the tiny animals and algae that are food for the young of native fish. A large zebra mussel population may consume so much food that it will cause a decline in the number of other animals, including native fish, mollusks, and birds.

Zebra mussels can severely effect native mussels and clams by interfering with their feeding, growth, movement, respiration, and reproduction. Zebra mussels can colonize a clam shell to such a degree that the clam cannot open its shell to eat. Zebra mussels exported accidentally with freshwater mussels to Japan have negatively impacted the cultured pearl industry. (2)

Water and environmental management agencies are working to protect endangered native species from the threat of zebra mussels. The primary emphasis is education so that boaters and fishermen do not accidentally transfer mussel larvae from one water body to another. In some rivers, boaters are prohibited from traveling upstream from infected areas to keep the mussels from spreading. The mussel can live forty-eight hours out of water. (3)

In addition to the impact on wildlife, zebra mussels cause many problems for people. They may severely restrict the water flow to power plants or other municipal or private facilities that rely on fresh water. Zebra mussels may foul beaches and create boating and navigation hazards. Both large and small boats suffer increased drag caused by thousands of mussels attached to the boats' hulls. Small zebra mussels may get into engine cooling systems causing overheating and other damage. Navigational buoys have been sunk under the weight of attached zebra mussels. Wood, steel, and concrete are all damaged by prolonged attachment of the mussels.

Once zebra mussels become established in a body of water, they are impossible to eradicate with the technology currently available. Many chemicals kill zebra mussels, but these exotics are so tolerant and tough that everything in the water would have to be poisoned to destroy the mussel. Most commercial water users rely on chemicals such as chlorine, filters, or mechanical scraping to remove mussels from their intake pipes and facilities.

Zebra mussels are controlled with a wide variety of methods. Many plants install equipment to pre-oxidize water at the point of intake, while others rely on different chemical treatments, mechanical controls, or filtration. Physical barriers and chemical coatings are used to prevent zebra mussels from attaching themselves to structures. Removal is accomplished with mechanical scrapers, hot water, air, chemicals, and sound; new methods are under investigation. There is no single, ideal solution for all affected facilities.

Zebra mussels do have a positive impact on some native species. Many native fish, birds, and other animals eat young and adult zebra mussels. Migratory ducks have changed their flight patterns in response to zebra mussel colonies. Lake sturgeon feed heavily on zebra mussels, as do yellow perch, freshwater drum, catfish, and sunfish. The increase in aquatic plants due to increased water clarity has provided excellent nursery areas for young fish and other animals, leading to increases in smallmouth bass populations in Lake St. Clair and the Huron River.

In 1995 an incident occurred on the lower Illinois River that suggested the mussels might not prove invincible. A large colonyt suddenly collapsed perhaps because after starving everything else they starved themselves. The infestation had reached an estiated 94,500 mussels on ten-square foot portions of the river's bottom. At its peak, the infestation dropped the river's oxygen level to less than half of that needed to sustain most species of game fish. Some oxygen was consumed by live mussels while other oxygen was being depleted by decaying mussels. Ninety-nine percent of the mussels died. (4)




1. Reber, Craig. "Mother Nature Flexes a Pesky Mussel," Telegraph Herald, August 6, 1997, p. 1. Online: https://news.google.com/newspapers?nid=aEyKTaVlRPYC&dat=19970806&printsec=frontpage&hl=en

2. Ibid.

3. Ibid.

4. "Zebra Mussels Not Invincible?" Telegraph Herald, January 9, 1995, p. 6

Maury Anderson, interview, Iowa Department of Natural Resources

Zebra Mussels--http://fl.biology.usgs.gov/Nonindigenous_Species/Zebra_mussel_FAQs/zebra_mussel_faqs.html

Zebra Mussels-National Atlas-http://nationalatlas.gov/articles/biology/a_zm.html

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