|Live zebra mussels underwater with shells open, animals respiring, siphons visible|
The zebra mussel (Dreissena polymorpha) is a small freshwater mussel. This species was originally native to the lakes of southern Russia being first described in 1769 by a German zoologist Peter Simon Pallas in the Ural, Volga and Dnieper rivers. They are still found nearby, as Pontic (Black Sea) and Caspian (Caspian Sea) species. However, it has been accidentally introduced in many other areas, and has become an invasive species in many different countries worldwide.
Zebra mussels get their name from a striped pattern which is commonly seen on their shells, though it is not universally present. They are usually about the size of a fingernail, but can grow to a maximum length of nearly 2 in (5.1 cm). Shells are D-shaped, and attached to the substrate with strong byssal threads, which come out of their umbo on the dorsal (hinged) side.
Zebra mussels and the closely related and ecologically similar quagga mussels are filter-feeding organisms. They remove particles from the water column. The zebra mussels process up to one gallon of water per day, per mussel. Some particles are consumed as food, and feces are deposited on the lake floor. Non-food particles are combined with mucus and other matter and deposited on lake floors as pseudofeces. Since the zebra mussel has become established in Lake Erie, water clarity has increased from 6 inches to up to three feet in some areas. This improved water clarity allows sunlight to penetrate deeper, enabling growth of macrophyte bacteria. These bacteria, when decaying, wash up on shorelines, fouling beaches and causing water quality problems.
Lake floor food supplies are enriched by zebra mussels as they filter pollution out of the water. This biomass becomes available to bottom-feeding species and to the fish that feed on them. The catch of yellow perch increased 5 fold after the introduction of zebra mussels into Lake St. Clair.
Zebra mussels attach to most substrates including sand, silt, and harder substrates. Other mussel species frequently represent the most stable objects in silty substrates, and zebra mussels attach to, and often kill these mussels. They build colonies on native American Unionid clams, reducing their ability to move, feed and breed, eventually leading to their death. This has led to the near extinction of the Unionid clams in Lake St. Clair and the western basin of Lake Erie. This pattern is being repeated in Ireland, where zebra mussels have eliminated the two freshwater mussels from several waterways, including some lakes along the River Shannon in 1997.
In 2012 the National University of Ireland, Galway, said "The discovery of zebra mussels (Dreissena polymorpha) in Lough Derg and the lower Shannon region in 1997 (McCarthy et al 1997, a & b) has led to considerable concern about the potential ecological and economic damage that this highly invasive aquatic nuisance species can cause."
The life span of a zebra mussel is four to five years. A female zebra mussel begins to reproduce within 6–7 weeks of settling. (see Borcherding, J. (1991): The annual reproductive cycle of the freshwater mussel Dreissena polymorpha Pallas in lakes. Oecologia 87: 208-218).
An adult female zebra mussel can produce 1,000 eggs in each reproductive cycle, and over 1 million each year. Free-swimming microscopic larvae, called veligers, will drift in the water for several weeks and then settle onto any hard surface they can find. Zebra mussels also can tolerate a wide range of environmental conditions and adults can even survive out of water for about 7 days.
Research on natural enemies, both in Europe and North America, has focused on predators, particularly birds (36 species) and fish (15 and 38 species eating veligers and attached mussels.
The vast majority of the organisms that are natural enemies in Europe are not present in North America. Ecologically similar species do exist, but it is unlikely that these species are able to eliminate those mussels already established and will have a limited role in their control. 
Crayfish could have a significant impact on the densities of 1 to 5 mm long zebra mussels. An adult crayfish consumes an average of nearly 105 zebra mussels every day, or about 6000 mussels in a season. Predation rates are significantly reduced at cooler water temperatures. It seems that fish do not limit the densities of zebra mussels in European lakes. Smallmouth bass are a predator in the zebra mussels' adopted North American Great Lakes habitat.
As an invasive species
The native distribution of the species is in the Black Sea and Caspian Sea in Eurasia. Zebra mussels have become an invasive species in North America, Great Britain, Ireland, Italy, Spain, and Sweden. They disrupt the ecosystems by monotypic colonization, and damage harbors and waterways, ships and boats, and water treatment and power plants. Water treatment plants are most affected because the water intakes bring the microscopic free-swimming larvae directly into the facilities. The Zebra Mussels also cling on to pipes under the water and clog them.
Grossinger reported it in Hungary in 1794. Kerney and Morton described the rapid colonization of Britain by the zebra mussel, first in Cambridgeshire in the 1820s, London in 1824, and in the Union Canal near Edinburgh in 1834. In 1827 zebra mussels were seen in the Netherlands at Rotterdam. Canals that artificially link many European waterways facilitated their early dispersal. It is non-indigenous in the Czech Republic in Elbe river in Bohemia since 1893; in southern Moravia it is probably native. Around 1920 the mussels reached Lake Mälaren in Sweden.
Zebra mussels are also present in British waterways. Many water companies are reporting having problems with their water treatment plants with the mussels attaching themselves to pipeworks. Anglian Water has estimated that it costs £500,000 to remove the mussels from their treatment plants. It has been argued that Zebra Mussels also have had an effect on fish populations, with dwindling fish populations in areas such as Salford Quays.
North American invasion
They were first detected in Canada in the Great Lakes in 1988, in Lake St. Clair, located north of Detroit, Michigan, and Windsor, Ontario. It is believed they were inadvertently introduced into the lakes in the ballast water of ocean-going ships traversing the St. Lawrence Seaway. Another possible often neglected mode of introduction is on anchors and chains, although this has not been proven. Since adult zebra mussels can survive out of water for several days or weeks if the temperature is low and humidity is high, chain lockers provide temporary refuge for clusters of adult mussels that could easily be released when transoceanic ships drop anchor in freshwater ports. They have become an invasive species in North America, and as such they are the target of Federal policy to control them, for instance in the National Invasive Species Act (1996).
From their first appearance in American waters in 1988, zebra mussels have spread to a large number of waterways, including Lake Simcoe in the Great Lakes region and the Mississippi, Hudson, St. Lawrence, Ohio, Cumberland, Missouri, Tennessee, Michigan, Colorado, and Arkansas Rivers. Today the invasion continues. For instance, in 2009 the Massachusetts Department of Conservation and Recreation confirmed that zebra mussels had been found in Laurel Lake in the Berkshires. That same year the Minnesota Department of Natural Resources announced that live zebra mussels have been found in Pelican Lake, Minnesota. This was the first confirmed sighting in the Red River Basin, which extends across the international border into the province of Manitoba. In July, 2010, the North Dakota Game and Fish Department confirmed the presence of zebra mussel veliger in the Red River between Wahpeton, N.D. and Breckenridge, Minnesota. As recently as 2010 California similarly reported invasions. In 2011 an invasion of the mussels has resulted in reduced water supplies during a drought year, worsening water restrictions across the Dallas area 
A common inference made by scientists predicts that the zebra mussel will continue spreading passively, by ship and by pleasure craft, to more rivers in North America. Trailered boat traffic is the most likely vector for invasion into Western North America. This spread is preventable if boaters thoroughly clean and dry their boats and associated equipment before transporting them to new bodies of water. Since no North American predator or combination of predators has been shown to significantly reduce zebra mussel numbers, such spread would most likely result in permanent establishment of zebra mussels in many North American waterways.
The cost of fighting the pests at power plants and other water-consuming facilities is substantial, but the exact magnitude of the damage is a matter of some controversy. According to the Center for Invasive Species Research at the University of California, Riverside, the cost of management of Zebra mussel in the Great Lakes alone exceeds $500 million a year. A more conservative study estimated total economic costs of $267 million for electric generation and water treatment facilities in the entire United States from 1989 through 2004.
Effects of zebra mussels
Zebra mussels are filter feeders. When in the water, they open their shells to admit detritus. As their shells are very sharp, they are known for cutting people's feet, resulting in the need to wear water shoes wherever they are prevalent.
Since their colonization of the Great Lakes, they have covered the undersides of docks, boats, and anchors. They have also spread into streams and rivers nationwide. In some areas they completely cover the substrate, sometimes covering other freshwater mussels. They can grow so densely that they block pipelines, clogging water intakes of municipal water supplies and hydroelectric companies. Recent research has found that zebra mussels don't attach to cupronickel alloys, which can be used to coat intake and discharge grates, navigational buoys, boats, and motors where the species tend to congregate.
Zebra mussels are believed to be the source of deadly avian botulism poisoning that has killed tens of thousands of birds in the Great Lakes since the late 1990s. Because they are so efficient at filtering water, they tend to accumulate pollutants and toxins. For this reason, although they are edible, most experts recommend against consuming zebra mussels.
They are also responsible for the near extinction of many species in the Great Lake system by out-competing native species for food and by growing on top of and suffocating the native clams and mussels.
However, zebra mussels and other non-native species are credited with the increased population and size of smallmouth bass in Lake Erie and yellow perch in Lake St. Clair. They cleanse the waters of inland lakes, resulting in increased sunlight penetration and growth of native algae at greater depths. This cleansing also increases water visibility and filters out pollutants. Each quagga and zebra mussel filters about 1 US quart (0.95 l) of water a day when confined to small tanks. In lakes, their filtering effects are usually spatially restricted (near the lake bottom) due to non-homogeneous water column mixing.
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