Zebra and Quagga Mussels

The zebra mussel is a small freshwater mussel 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 byssal threads extending from underneath the shells. When placed on a surface, zebra mussels are stable on their flattened underside. 

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. Quagga mussels are more round in shape and have more concentric rings as opposed to the striped pattern of a zebra mussel.  

The introduction of zebra and quagga mussels into the Great Lakes is the result of water discharge from transoceanic ships that were carrying veligers (larvae), juvenile, or adult mussels. Although adult 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, and in this way, can allow them to be easily transported between water bodies.

Zebra mussels were first discovered in the Great Lakes in 1986 in Lake St. Clair and quagga mussels were first discovered in 1989 near Lake Erie. They have since spread throughout the Great Lakes (except Lake Superior, as nutrient availability doesn’t support mussel survival) and many in-land lakes in Ontario, Quebec, Manitoba and the U.S.  

Distribution Map Provided by EDDMapS

Ecological impacts of invasive mussels

Zebra and quagga 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. This reduces the plankton availability for other organisms, and effects can be seen throughout the food web. This filtering increases the amount of sunlight able to penetrate the water, promoting algal blooms and leading to decreased water quality. Invasive mussels are capable of attaching to the shells of native species, suffocating threatened populations of freshwater mussels.

While zebra mussels are limited to colonizing hard surfaces, quagga mussels can also colonize on soft substrates. They are able to survive in low-food environments, so when both species co-exist, quagga mussels are the dominant species and are able to out-compete zebra mussels.  

Invasive mussels retain toxins as they filter the water, which can lead to botulism in mussel-eating fish and waterfowl. Additionally, the nutrients they filter from open water are expelled back as waste products onto the lake bottom. This decreases energy production in the pelagic zone and increases the availability of littoral and benthic energy/resources altering food web structures and energy cycling.

Invasive mussels have the ability to alter energy cycling in invaded lakes through different pathways. One of these cycles involves mercury, which is converted to the neurotoxic form, methylmercury by microorganisms. When invasive mussels form dense mats, this can create oxygen depleted environments which are favourable for mercury methylation by obligate anerobic microbial communities. With a higher concentration of energy and resources in the littoral/benthic zones due to invasive mussels filtering water and expelling waste, this encourages fishes to alter their feeding habitats to rely on these zones where mercury methylation is occurring. A recent study published in 2024 found that mercury concentrations in adult walleye (Sander vitreus) and yellow perch (Perca flavescens) were 72% and 152% higher, respectively in lakes containing zebra mussels compared to uninvaded lakes (Blinick et al. 2024). The threshold for human consumption advisories for sensitive populations by the Minnesota Department of Health is 0.22ppm, and this study found that walleye mercury concentrations exceeded this guideline.

Furthermore, new research suggests invasive mussels are playing a role in the phosphorus cycle in lakes Huron, Erie, Michigan and Ontario which causes uncertainty and variability in these levels. This could have further impacts from phytoplankton communities to large predatory fish. 

Economic Impacts of invasive mussels 

Their ability to rapidly colonize surfaces causes serious economic problems. Invasive mussels 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, cottage intake pipes, and beaches have all been heavily colonized.  

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.

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.