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Zebra mussel
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The zebra mussel, Dreissena polymorpha, is a species of small freshwater mussel, an aquatic bivalve mollusk. This species was originally native to the lakes of southeast Russia. However, it has been accidentally introduced in many other areas, and has become a problematic invasive species in many different countries.
Although zebra mussels superficially resemble marine mussels in the family Mytilidae, and like them, are attached to solid substrates with a byssus, nonetheless, zebra mussels are not at all closely related to the mytilids; they are much more closely related to the Veneridae, the venus clams.
Zebra mussels get their name from a striped pattern which is commonly seen on their shells, though not all shells bear this pattern.
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The zebra mussel, Dreissena polymorpha, is a species of small freshwater mussel, an aquatic bivalve mollusk. This species was originally native to the lakes of southeast Russia. However, it has been accidentally introduced in many other areas, and has become a problematic invasive species in many different countries.
Although zebra mussels superficially resemble marine mussels in the family Mytilidae, and like them, are attached to solid substrates with a byssus, nonetheless, zebra mussels are not at all closely related to the mytilids; they are much more closely related to the Veneridae, the venus clams.
Zebra mussels get their name from a striped pattern which is commonly seen on their shells, though not all shells bear this pattern. They are usually about the size of a fingernail, but can grow to a maximum length of nearly two inches. The shape of the shell is also somewhat variable.
The native distribution of the species is the Black and Caspian Sea. Zebra mussels are an invasive species in North America, the British Isles, Spain, and Sweden.
Reproduction
In terms of reproduction, zebra mussels are among the most prolific of all animals. An adult female Zebra mussel may produce between 30,000 and 1 million eggs per year. Spawning usually begins in the months from late spring to early summer by free-swimming larvae (veligers).
Spread
The zebra mussel was found and described first in the Roknighani part of Russia, but then it was recognized in the Caspian Sea. In 1991 Lisický described the distribution of this species as Pontic and Caspian.
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. They were discovered in Bohemia in the Elbe river (now in The Czech Republic) in 1893. Around 1920 the mussels reached Lake Mälaren in Sweden.
The first Italian appearance of the organism was in northern Italy in Lake Garda in 1973; in central Italy they appeared in Tuscany in 2003.
In the U.S. and Canada, they were first detected in the Great Lakes in 1988, in Lake St. Clair, located between 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.
From their first appearance in American waters in 1988, zebra mussels have spread to a large number of waterways, including the Mississippi, Hudson, St. Lawrence, Ohio, Cumberland, Missouri, Tennessee, Colorado, and Arkansas rivers disrupting the ecosystems, killing the local unionid mussels, (primarily by out-competing native species for food) and damaging harbors, boats, and power plants. Water treatment plants were initially hit hardest because the water intakes brought the microscopic free-swimming larvae directly into the facilities. The U.S. Coast Guard estimates that economic losses and control efforts cost the United States about $5 billion each year.
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 the North American west. This spread is preventable if boaters would take time to thoroughly clean and dry their boats and associated equipment before transporting these 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.
Ecology
Zebra mussels and the closely related and ecologically similar quagga mussels are voracious filter-feeding organisms. They remove particles from the water column, increasing water clarity and reducing pollution. 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.
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 zebra mussel reduced eutrophication of Lake Erie and increased water quality. The catch of yellow perch increased 5 fold after the introduction of zebra mussels into Lake St. Claire.
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. This has eliminated many native mussel species from affected lakes in North America. 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.
Anatomy
Zebra mussels are relatively small in size, with adults ranging from 1/4 to 1 1/2 inches long. They have tiny stripes down their shells, hence the name "zebra" mussels. Zebra Mussels have a D-shaped shell. They attach to things with 'strings', byssal threads, which come out of their umbo on the dorsal (hinged) side. Removal of the mussel is therefore difficult.
Zebra mussels can best live in 6-28 C weather.
Predators of zebra mussels
There are a number of natural predators of zebra mussel. Zebra mussels have high nutritional value (Walz, 1979) and are consumed in large quantities by crayfish, waterfowl and in smaller quantities by muskrats. The nutritional value changes seasonally, particularly in terms of protein and carbonate content.
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 everyday, or in all about 6000 mussels in a season. Predation rates are significantly reduced at cooler water temperatures.
Several species of fish consume zebra mussels. Of these, roach seems to have the most significant impact on mussel densities. In some Polish lakes the diet of the roach consists almost exclusively (~95%) of zebra mussels (Stanczykowska, 1957). Despite all this, it seems that fish do not limit the densities of zebra mussels in European lakes.
Effects of zebra mussels
Zebra mussels are filter feeders. When in the water, they open their shells to admit detritus.
Zebra mussels are a great nuisance to people. Since colonizing 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.
Also, as their shells are very sharp, they are known for cutting people's feet, resulting in the need to wear water shoes.
Zebra mussels are also 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.
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. Claire. .
They cleanse the waters of inland lakes, resulting in increased sunlight penetration and growth of native algae at greater depths. This proves beneficial for fish most of the time, helping the fish live in better conditions. This cleansing also increases water visibility and filters out pollutants.
Further references
Minchin, D. 2003. The Zebra Mussel Dreissena polymopha (Pallas) extends its range westwards in Ireland. Bull. Ir. biogeog. Soc. 26: 176 - 18.
See also
External links
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- Moules Mariniere recipe from Wikibooks
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- , The Straight Dope, 22 September 2006.
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