Zebra Mussels (republished from nationalatlas.gov)

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Lake St. Clair map
National Atlas of the United States®
A small freshwater mollusk called the zebra mussel (Dreissena polymorpha), has been steadily invading America's rivers and lakes. Zebra mussels originated in the Balkans, Poland, and the former Soviet Union. They first appeared in North America in 1988 in Lake St. Clair, a small water body connecting Lake Huron and Lake Erie. Biologists believe the zebra mussels were picked up in a freshwater European port in the ballast water of a ship and were later discharged into the Canadian side of Lake St. Clair.

variety of shell color morphs seen in zebra mussels in North America
 Zebra mussels get their name from the striped pattern of their shells, though not all shells bear this pattern. They're usually about fingernail size but can grow to a maximum length of nearly 2 inches. Zebra mussels live 4 to 5 years and inhabit fresh water at depths of 6 to 24 feet. A female zebra mussel begins to reproduce at 2 years of age, and produces between 30,000 and 1 million eggs per year. About two percent of zebra mussels reach adulthood.

closeup of zebra mussels on stick
S. van Mechelen, University of Amsterdam, The Netherlands
Young zebra mussels are small and free swimming, and can be easily spread by water currents. Older zebra mussels attach themselves to hard surfaces by an external organ called a byssus, which consists of many threads. The mussels may attach to boats, pilings, water-intake pipes, and other hard surfaces, as well as to crayfish, turtles, other zebra mussels, and native mollusks. While zebra mussels can attach themselves securely, they may also move, and can reattach themselves easily if dislodged by storms.

zebra mussel showing bysall threads

Great Lakes Sea Grant Network

Zebra mussels upset ecosystems, threaten native wildlife, damage structures, and cause other serious problems. Millions of dollars are spent each year in attempting to control these small but numerous mollusks.

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Zebra Mussel Threats

clump of zebra mussels on native clam
D. Jude, Center for Great Lakes Aquatic Sciences

Threat to Other Species

Zebra mussels are filter feeders. An adult zebra mussel filters up to a quart of water per day, which multiplied by millions of mussels means that the mussels may be filtering all the water in a lake or stream in a day. The animals and algae that are the food of zebra mussels are also the food for larval fish and other native species, so a large zebra mussel population may cause a decline in other animals, including native fish, mollusks, and birds. The filter-feeding activity of zebra mussels causes a related and frequently dramatic increase in water clarity in infested lakes and rivers.

Zebra mussels can severely effect native mussels and clams by interfering with their feeding, growth, movement, respiration, and reproduction. For example, zebra mussels can colonize a clam shell to such an extent that the clam cannot open its shell to eat. Some native mussels have been found with more than 10,000 zebra mussels attached to them. In addition to colonizing native mussels and clams, zebra mussels may attach to slow-moving species such as crayfish and turtles.

crayfish covered with zebra mussels
GLSGN Exotic Species Graphics Library
Water and environmental management agencies are working to protect endangered native species from the threat of zebra mussels. The primary emphasis of this effort is to education so that boaters and fishermen do not inadvertently transfer mussel larvae from one water body to another. In some rivers, boaters are prohibited from traveling upstream from infected areas in an attempt to keep the mussels from spreading.

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 provides excellent nursery areas for young fish and other animals, leading to increases in smallmouth bass populations in Lake St. Clair and the Huron River. However, these native species do not feed heavily enough on zebra mussels to keep the populations under control.

Threat to Navigation, Boating, and Industry

In addition to the impact on wildlife, zebra mussels cause many problems for people. They may colonize water intake pipes, severely restricting the water flow to power plants or other municipal or private facilities that rely on fresh water. Impacts include damage to the facilities as well as the cost of removing or controlling the mussels. Zebra mussels may also foul beaches and create boating and navigation hazards. Increased plant growth provides an additional hazard to navigation.

Zebra mussels will attach to almost any hard surface, either natural or manmade. On boats, they may attach to the hull, motor, or any item immersed in the water. Both large and small boats can be severely impacted by increased drag caused by thousands of mussels. Small zebra mussels may get into engine cooling systems, causing overheating and other damage.

In addition to threatening boats, zebra mussels pose a threat to navigational buoys, piers, docks, and other structures in the water. 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.

Threat to Outdoor Recreation

Shells of zebra mussels foul beaches and near-shore swimming areas. Bare feet are at risk from the sharp shells, and clean up costs are high. Due to changes in fish populations, zebra mussels also adversely impact recreational fishing.

zebra mussels on Luna Pier beach, Michigan
 M. Parsons, Michigan Sea Grant
windrow of zebra mussels following Lake Erie seiche, Ohio
Michigan Sea Grant
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Removal and Control of the Zebra Mussel

Cost of Removal and Control

Once zebra mussels become established in a water body, 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.

worker removing zebra mussels from water intake pipes
R. Peplowski, Detroit Edison
While accurate cost figures are not currently available, it is known that the cost of dealing with zebra mussels varies widely, depending on the type of facility, the length of infestation, and the control methods chosen. Frequently the highest costs are to retrofit a facility for effective control; control costs usually drop dramatically once retrofits are in place. Because nuclear power plants use large quantities of water they tend to have the highest associated costs per plant, followed by industrial plants, fossil fuel power plants, and drinking water facilities. For many plants, costs average hundreds of thousands of dollars per year.

Methods of Removal and Control

Zebra mussels are controlled with a wide variety of methods. Many plants install equipment to preoxidize 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 to structures. Removal is accomplished with mechanical scrapers, hot water, air, chemicals, and sound; new methods are constantly under investigation. There is no single, ideal solution for all affected facilities.

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