Zebra and Quagga Mussels (Dreissena polymorpha)
French common name: Les moules zébrée et quaggua

Invasive fish and invertebrates - Zebra Mussel

Zebra Mussels

quagga mussel

Quagga Mussels

When placed on a flat surface, zebra mussels are stable on their flattened underside while quagga mussels, lacking a flat underside, will fall over.

Order: Myida
Family: Dreissenidae

Zebra and quagga mussels are invasive freshwater mussels that can be found throughout Ontario, Quebec, and Manitoba. Zebra mussels sit flat, quagga mussels don’t. Both zebra and quagga mussels are light with dark brown markings; zebra mussels have a zigzag pattern and quagga mussels have dark concentric rings. These invasive mussels filter plankton out of the water, which depletes it as a food source for native species. Large colonies can take over fish spawning areas and beaches, cutting the feet of potential swimmers. They also clog water intake lines because of their dense colonies. Zebra and quagga mussels also increase the presence of toxic algal blooms, which can have health impacts on native wildlife. They latch on to boats and are easily passed from waterbody to waterbody. Ensure that all plants, animals, and mud are removed from boats and trailers before leaving an area to help prevent the spread of aquatic invaders.

Zebra Mussels

The zebra mussel is a small shellfish named for the striped pattern of its shell. However, color patterns can vary to the point of having only dark or light colored shells with no stripes. This mussel is typically found attached to objects, surfaces, or other mussels by threads extending from underneath the shells. When placed on a surface, zebra mussels are stable on their flattened underside.


  • Average 2-2.5 cm, reaching up to 4 cm long
  • Sits flat on its underside
  • Triangular in shape
  • Black or brown with white to yellow zigzagged patterns
  • Colour patterns can vary


Quagga Mussels

Although similar in appearance to the zebra mussel, the two species can be distinguished by their shell morphology. When placed on a surface, zebra mussels are stable on their flattened underside while quagga mussels, lacking a flat underside, will fall over. When both zebra and quagga mussels occur in the same area, differentiation can be difficult due to the phenotypic plasticity seen in quagga mussels, and thus genetic identification is necessary at times (Kerambrun et al. 2018, Beggel et al. 2015).


  • Its color pattern varies widely with black, cream, or white stripes
  • When placed on a surface, quagga mussels will fall over
  • Reach up to 4 cm (1.5 in) in size

The introduction of quagga mussels into the Great Lakes appears to be the result of water discharge from transoceanic ships that were carrying veliger (a larval stage), juvenile, or adult mussels. Although adult quagga mussels move very little, they produce larvae that are free-floating in the water. This mobile larval stage contributes to their rapid dispersal. Adult mussels can attach to boats and barges that navigate the waterways. In this way, people moving their boats from infested waters can transfer quagga mussels over land to nearby waterways (Illinois-Indiana Sea Grant). 

Quagga mussels have displaced zebra mussels in all offshore areas of Lakes Michigan (Nalepa et al. 2014; Rowe et al. 2015a), Huron (Nalepa et al. 2018), and Ontario (Wilson et al. 2006; Birkett et al. 2015). There is a gradient of dreissenid domination in Lake Erie, with quagga mussels dominating eastern basins and zebra and quagga mussels coexisting in the western basin (Patterson et al. 2005; Karatayev et al. 2014). A similar gradient was initially observed in southern Lake Ontario, with quagga mussels dominating the west and zebra mussels dominating the east (Mills et al. 1999), but the quagga mussel has since displaced zebra mussels in all offshore regions of Lake Ontario (Birkett et al. 2015). Coexistence is generally only found in shallow, productive systems such as Green Bay in Lake Michigan, Saginaw Bay in Lake Huron, and Western Lake Erie. There are multiple mechanisms by which quagga mussels displace zebra mussels, including differences in growth, reproduction, respiration, and development (Ram et al. 2012; Karateyev et al. 2015). Though zebra mussels have garnered the majority of public and research attention, quagga mussels have a more extensive distribution in the Great Lakes and their abundance far exceeds that of the zebra mussel peak (e.g., southern Lake Michigan; Nalepa et al. 2010).

Distribution Map Provided by EDDMapS

Impacts of zebra mussels on infrastructure

Zebra mussels are notorious for their biofouling capabilities by colonizing water supply pipes of hydroelectric and nuclear power plants, public water supply plants, and industrial facilities. They colonize pipes, constricting flow, and therefore reducing the intake in heat exchangers, condensers, fire fighting equipment, and air conditioning and cooling systems. Zebra mussel densities were as high as 700,000/m2 at one power plant in Michigan and the diameters of pipes have been reduced by two-thirds at water treatment facilities. Navigational and recreational boating can be affected by increased drag due to attached mussels. Small mussels can get into engine cooling systems causing overheating and damage. Navigational buoys have been sunk under the weight of attached zebra mussels. Fishing gear can be fouled if left in the water for long periods. 

Ecological impacts of zebra mussels

Zebra mussels can have large impacts on the ecosystems they invade. They primarily consume phytoplankton, but other suspended material is filtered from the water as well. Large populations of zebra mussels in the Great Lakes and Hudson River have reduced the biomass of phytoplankton significantly. 

The zooplankton community has also been affected by the invasion of zebra mussels. These effects can be attributed to the reduction of available food (phytoplankton) and direct predation on microzooplankton. Increased competition in the zooplankton community for newly limited food should result from zebra mussel infestation. The size of individual zooplankters might decrease. 

Effects may continue through the food web to fish. Reductions in zooplankton biomass may cause increased competition, decreased survival, and decreased biomass of planktivorous fish. Alternatively, because microzooplankton are more heavily impacted by zebra mussels, the larval fish population may be more greatly affected than later life stages. This may be especially important to inland lakes with populations of pelagic larval fish such as bluegills. Experimental evidence exists that zebra mussels can reduce the growth rate of larval fish through food web interactions (Raikow 2004). Conclusive negative impacts on natural populations of fish, however, have yet to observed (Raikow 2004). Many species of birds known to be predators of zebra mussels occur in the Great Lakes region. 

Ecological impacts of quagga mussels

Quagga mussels can colonize all regions of a lake and form larger populations. They may filter larger volumes of water and may have greater system-wide effects, especially in deep lakes, than zebra mussels, which are restricted to shallower portions of lakes. Shortly after the initial invasion, as populations increase, both dreissenids will have their largest effects on communities, and most of them will be direct effects. After the initial stage of invasion, impacts are less predictable and more likely to be caused by indirect effects through changes in the ecosystem. (Hydrobiologia, 2015)

Quagga mussels are water filterers, removing substantial amounts of phytoplankton and suspended particulate from the water. As such, their impacts are similar to those of the zebra mussel. By removing the phytoplankton, quagga mussels in turn decrease the food source for zooplankton, therefore altering the food web. Impacts associated with the filtration of water include increases in water transparency, decreases in mean chlorophyll a concentrations, and accumulation of pseudofeces (Claxton et al. 1998). Water clarity increases light penetration, causing a proliferation of aquatic plants that can change species dominance and alter the entire ecosystem. As the waste particles decompose, oxygen is used up and the pH becomes very acidic and toxic byproducts are produced. In addition, quagga mussels accumulate organic pollutants within their tissues to levels more than 300,000 times greater than concentrations in the environment, and these pollutants are found in their pseudofeces, which can be passed up the food chain, therefore increasing wildlife exposure to organic pollutants (Snyder et al. 1997). 

Impacts of quagga mussels on infrastructure

This species ability to rapidly colonize hard surfaces causes serious economic problems. These major biofouling organisms can clog water intake structures, such as pipes and screens, therefore reducing pumping capabilities for power and water treatment plants and posing costs to industries, companies, and communities. Recreation-based industries and activities have also been impacted; docks, breakwalls, buoys, boats, and beaches have all been heavily colonized. Quaggas are able to colonize both hard and soft substrata, so their negative impacts on native freshwater mussels, invertebrates, industries and recreation are unclear.

Fact Sheets

Best Management Practices

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Zebra versus quagga mussels: a review of their spread, population dynamics, and ecosystem impacts

AY KaratayevLE BurlakovaDK Padilla – Hydrobiologia, 2015 – Springer
Dreissena polymorpha (zebra mussel) and D. rostriformis bugensis (quagga mussel)
continue to spread in Europe and in North America, and have large ecological and
economic impacts where they invade. Today many more waterbodies are invaded by zebra  …

[BOOK] Quagga and zebra mussels: biology, impacts, and control

TF Nalepa, DW Schloesser – 2013 – content.taylorfrancis.com
When our first edited book on zebra mussels (Dreissena polymorpha) was published
(Nalepa and Schloesser 1993), it was only a few years after this species had been reported
in North America, and our intention was to document initial impacts, summarize control …

Invasion of zebra mussel, Dreissena polymorpha, in Lake Simcoe

DO Evans, AJ Skinner, R Allen, MJ McMurtry – Journal of Great Lakes …, 2011 – Elsevier
This study documents the timing of invasion, and initial settlement rates, density and
biomass of zebra mussels in Lake Simcoe. Pumped water samples, multi-plate tower
samplers, and scuba and benthic airlift surveys were used to sample veligers, post-veligers …

Current Research and Knowledge Gaps

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Further Reading

The Invasive Species Centre aims to connect stakeholders. The following information below link to resources that have been created by external organizations.