The
zebra mussel,
Dreissena polymorpha,
is a
species of small freshwater
mussel, an
aquatic
bivalve mollusc.
This
species was originally native to the lakes of southeast Russia
.
However, it has been accidentally introduced in many other areas,
and has become an
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 genus 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 (5 cm). The
shape of the shell is also somewhat variable.
Anatomy
Three color varieties of the shell of
the zebra mussel
Zebra mussels are relatively small, with adults ranging from 1/4 to
1 1/2 inches long. They have tiny stripes down their shells. 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.
Ecology
Close-up of a typical shell of a zebra
mussel
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. 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. 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.
Life cycle
The life span of a zebra mussel is four to five years. A female
zebra mussel begins to reproduce at two years of age.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 one million eggs per year.
Spawning usually begins in the months from
late spring to early summer by free-swimming larvae (
veligers), which are microscopic in size, thus
invisible to the naked human eye. About two to five percent of
zebra mussels reach adulthood.
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 every day, or 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.
As an invasive species
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
.
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.
North-American invasion
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 Lake Simcoe
and 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.
In July, 2009, The Massachusetts Department of Conservation and
Recreation confirmed that zebra mussels had been found in Laurel
Lake in the Berkshires, the first documented case in a
Massachusetts body of water.
In
September, 2009, 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.
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.
The cost of fighting the pests at power plants and other
water-consuming facilities is $500 million a year in the U.S.,
according to the Center for Invasive Species Research at the
University of California, Riverside.
Effects of zebra mussels
Zebra mussel infestation on the walls
of Arthur V.
Ormond Lock on the Arkansas River
Zebra mussel-encrusted Vector
Averaging Current Meter from Lake Michigan
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. 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 a quart of
water a day.
Recent research has found that zebra mussels don't attach to
copper-nickel alloys, which can be used to coat intake and
discharge grates, navigational buoys, boats, motors, etc., where
the pests tend to congregate.
See also
References
- Center for Invasive Species Research: Zebra
Mussels
- Zebra Mussels
-
http://www.dnr.state.mn.us/invasives/aquaticanimals/zebramussel/index.html
-
http://fl.biology.usgs.gov/Nonindigenous_Species/Zebra_mussel_FAQs/zebra_mussel_faqs.html
- /nationalatlas.gov
- Lisický M. J. 1991. Mollusca Slovenska [The Slovak
molluscs]. VEDA vydavateľstvo Slovenskej akadémie vied,
Bratislava, 344 pp.
- [1]
- Report at ESPN Sports
- Exotic species at PUAF
Further references
Minchin, D. 2003. The Zebra Mussel
Dreissena polymopha
(Pallas) extends its range westwards in Ireland.
Bull.
Ir. biogeog. Soc. 26: 176 -
18.
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