The zebra mussel (Dreissena polymorpha) is a small freshwater mussel, an aquatic bivalve mollusc in the family Dreissenidae. The species originates from the lakes of southwestern Russia and southeastern Europe, but has been accidentally introduced to numerous other areas and has become an invasive species in many countries in the Northern Hemisphere. Since the 1980s, the species has invaded the Great Lakes, Hudson River, Lake Travis, Finger Lakes, Lake Bonaparte, and Lake Simcoe. The adverse effects of dreissenid mussels on freshwater systems have led to their classification as among the world's most invasive aquatic species.
The species was first described in 1768 by German zoologist Peter Simon Pallas in the Ural, Volga, and Dnieper Rivers.Mollusca Slovenska [The Slovak molluscs]. VEDA vydavateľstvo Slovenskej akadémie vied, Bratislava, 344 pp. Zebra mussels get their name from a striped pattern 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 around . Their shells are D-shaped, and attached to the substrate with strong byssal fibres, which come out of their umbo on the dorsal (hinged) side.
<div align=center> Right and left valve of the same specimen:
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Ecology
Zebra mussels and the closely related and ecologically similar quagga mussels are filter-feeding organisms; they remove particles from the water column. Zebra mussels process up to of water per day, per mussel. Some particles are consumed as food, and faeces are deposited on lakes floor. Nonfood particles are combined with mucus and other matter and deposited on lake floors as pseudofaeces. Since the zebra mussel has become established in Lake Erie, water clarity has increased from to up to in some areas. This increased water clarity allows sunlight to penetrate deeper, enabling growth of submerged macrophytes. These plants, when decaying, wash up on shorelines, fouling beaches and causing water-quality problems.
Lake-floor food supplies are enriched by zebra mussels, which filter pollutants from the water. This biomass becomes available to bottom-feeding species and to the fish that feed on them. The catch of yellow perch increased five-fold after the invasion of zebra mussels into Lake St. Clair.
Zebra mussels attach to most substrates, including sand, silt, and harder substrates, but usually juveniles prefer harder, rockier substrates on which to attach. 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 unionid clams, reducing their ability to move, feed, and breed, eventually leading to their deaths. 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 has led to considerable concern about the potential ecological and economic damage that this highly invasive aquatic nuisance species can cause."
The zebra mussel is a freshwater species and cannot survive in the ocean.
Life cycle
The lifespan of a zebra mussel is three to five years. A female zebra mussel begins to reproduce within 6–7 weeks of settling. An adult female zebra mussel can produce 30,000 to 40,000 eggs in each reproductive cycle, and over 1 million each year. Free-swimming microscopic larvae, called veligers, 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 seven days.
Indicators of environmental pollution
Zebra mussels can be used to detect risks to humans from environmental hazards and are considered effective indicators of environmental pollution.Michel C, Vincent-Hubert F. DNA oxidation and DNA repair in gills of zebra mussels exposed to cadmium and benzo(a)pyrene. Ecotoxicology. 2015 Nov;24(9):2009-16. doi: 10.1007/s10646-015-1536-3. Epub 2015 Oct 6. PMID 26438356 Exposure of D. polymorpha gill cells to model contaminants representative of water pollution was found to cause DNA damage measured as the formation of 8-oxodG and DNA strand breaks.Michel C, Vincent-Hubert F. Detection of 8-oxodG in Dreissena polymorpha gill cells exposed to model contaminants. Mutat Res. 2012 Jan 24;741(1-2):1-6. doi: 10.1016/j.mrgentox.2011.10.001. Epub 2011 Oct 8. PMID 22009068
Predators
Research on natural enemies, both in Europe and North America, has focused on predators, particularly birds (36 species) and fish (15 species that eat veligers and 38 that eat attached mussels). Annually, the wintering waterbirds at Lake Constance decrease zebra mussel biomass in shallow areas by >90%. Biomass reduction in deeper areas varies considerably based on substratum; Werner et al. observed no reduction at the lowest observed depth of except for a site at Hagnau.
The vast majority of the organism's natural enemies are not present in North America. Ecologically similar species do exist, but they are unlikely to eliminate already established mussels and have a limited role in their control, unlike their counterparts in Europe.
It is pointed out that crayfish could have a significant impact on the densities of -long zebra mussels. An adult crayfish consumes around 105 zebra mussels every day, or about 6,000 mussels in a season. However, predation rates are significantly reduced at lower water temperatures. Additionally, certain fish, such as the smallmouth bass, are predators in the zebra mussels' adopted North American Great Lakes habitat, but in European lakes, fish do not seem to limit the densities of zebra mussels.
There have been some high winter mortalities, for example in the winter of 1994–1995 in the invasive population of Lake Simcoe. Evans et al., 2011 attributes this to predation by the crayfish Orconectes propinquus. Nonetheless this has not been sufficient to eradicate the problem.
Other control On 4 June 2014, Canadian conservation authorities announced that a test using liquid fertilizer to kill invasive zebra mussels was successful. This test was conducted in a lakefront harbour in the western province of Manitoba. However, outbreaks continue in Lake Winnipeg.
Similar tests were run in Illinois, Minnesota, and Michigan, using zequanox, a biopesticide. Niclosamide proves effective in killing invasive zebra mussels in cool waters.
As an invasive species
Europe
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 ecosystems through monotypic colonisation and damage harbours 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. Zebra mussels also cling to pipes under the water and clog them.
Grossinger reported it in Hungary in 1794. Kerney and Morton described the rapid colonisation 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 nonindigenous in the Czech Republic in the Elbe River in Bohemia since 1893; in southern Moravia, it is probably native. Horsák M., Juřičková L., Beran L., Čejka T. & Dvořák L. (2010). "Komentovaný seznam měkkýšů zjištěných ve volné přírodě České a Slovenské republiky" [Annotated list of mollusc species recorded outdoors in the Czech and Slovak Republics]. Malacologica Bohemoslovaca, Suppl. 1: 1–37. Around 1920 the mussels reached Lake Mälaren in Sweden.
The first appearance of the organism in northern Italy was in Lake Garda in 1973; in central Italy, they appeared in Tuscany in 2003.
Zebra mussels are present in British waterways. Many water companies are reporting problems with their water-treatment plants, with mussels attaching themselves to pipework. Anglian Water has estimated that it costs £500,000 per year to remove the mussels from their treatment plants. Zebra mussels arguably have also affected fishing, for example at Salford Quays, where their introduction has changed the environment for the fish.
Zebra mussels were first reported in Ireland in 1997, but probably arrived in 1994 or earlier. First identified near Lough Derg, they may have been introduced to Ireland through several vectors but have since spread through much of the River Shannon and its adjacent waters. In the summers of 2023 and 2024, zebra mussels were one contributing factor to a major bloom of toxic cyanobacteria in Lough Neagh, the largest body of freshwater in the United Kingdom. In 2021 the similar quagga mussel was identified in the Shannon, which is capable of tolerating a wider range of environmental conditions. The spread of quagga mussels is expected to reduce zebra mussel populations over time while increasing the overall environmental impact.
The mussels have displaced native species of molluscs in Lake Constance, reaching densities of up to . The mussels present a food source to waterfowl and have caused bird numbers to double over the last 30 years. By the end of winter, birds decimate zebra mussel populations and reduce them by 95–99% up to the maximum depth reachable by birds of c. . The estimated quantity of consumed zebra mussels is . Zebra mussel populations recover annually, indicating that waterfowl may control infested bodies of water but not reverse the infestation status entirely. The presence of zebra mussels has also allowed the colonisation of the Swiss lakes by the normally strictly marine common eider, which typically feeds on marine mussels.
In Italy, zebra mussels were first recorded in the early 1970s in Lake Garda, in the north of the country. They then spread over tributaries and nearby basins, reaching the Apennine Mountains in the 1990s, namely Tuscany, Umbria, Abruzzo, Molise and Sicily. Zebra mussels are more likely to be found at low to medium elevations in the Italian regions where large streams have fluviatile deposits. Italy's north and centre seem to be more vulnerable. The majority of medium- to high-risk lake and river reaches are found in northern and central Italy.
North America
They were first detected in Canada in the Great Lakes in 1988, in Lake St. Clair. They are thought to have been inadvertently introduced into the lakes by the ballast water of ocean-going ships that were traversing the St. Lawrence Seaway. Another possible but unproven mode of introduction is via anchors and chains. 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).