ZEBRA MUSSEL

Dreissena polymorpha

History

• Zebra mussels are native to the Black Sea in Europe and were introduced to North America through ballast waters in ships in the late 1980s in their larval stage (veligers). 
• In North America, there are reports of this species appearing in the Great Lakes in 1986-1987 (Lake Erie, Ontario) and in Lake St. Claire (Ontario) in 1988 and rapidly spreading throughout the Great Lakes of North America. 
• Since their original introduction to North America, they have been found throughout the Great Lakes and inland lakes of Ontario, Quebec, and Manitoba. As of 2025, there are no reported sightings of zebra mussels on PEI. However, zebra mussels were recently discovered in the Saint John River in New Brunswick (September 2023) and therefore have a high potential for spread into PEI waters.

Identification, Biology, and Life Cycle

Identification

Zebra mussels are bivalves that inhabit fresh water. They are named for the striped pattern of their shells. They can be identified through the following characteristics:

• Very small; on average 2 to 2.5 cm long, reaching up to 4 cm long.
• Distinct triangular shape with a sharply pointed hinge end.  
• Flat bottom. 
• Prominent light and dark banding on the shell with a smooth or zigzag appearance. 
• Hair-like byssal threads that extend underneath their shells and attach to solid surfaces such as rocks, docks, vegetation, and native mussels.

Biology

• Zebra mussels are filter feeders and consume plankton and algae. They can each filter approximately one liter of water per day.
• They attach to surfaces using byssal threads secreted by a byssal gland.
• They occur in freshwater and colonize hard surfaces in clear or running waters. Typical habitats include estuaries, rivers, and lakes that have suitable surfaces for attachment. They can tolerate a wide range of physical and chemical characteristics in water. The lower temperature limit is 0°C as it cannot survive freezing temperatures and the upper temperature limit is between 28°C and 32°C.
• The abundance of zebra mussels in a waterbody depends on the substrate type, size, water quality, water depth, pH, and nutrient content (in particular calcium levels).
• They typically live between 2 – 4 years but can live up to 9 years depending on environmental conditions.

Life Cycle

• In the spring or summer, female zebra mussels release eggs into the water which are fertilized by sperm released by male zebra mussels. Females usually reproduce in their second year. Spawning begins at 12 – 15°C. A female zebra mussel can produce as many as 1 million eggs per year.
• Microscopic larvae (veligers) emerge within 3 – 5 days and are free-floating for up to a month. The optimal temperature for larval development is between 20 – 22°C. The veliger stage has the highest mortality rate. 
• The juvenile stage (postveliger) begins when larvae sink to the bottom where they search for a suitable attachment site. Survival of this stage depends on the availability of suitable substrate. 
• Once the larvae find an attachment site, they begin metamorphosis and the adult shell is secreted. They can reach extremely high densities on surfaces. 
• They reach maturity within 1 – 2 years. 

Potential Lookalikes

• Zebra mussels can be confused for Quagga mussels, however, Quagga mussels are larger, have a distinct D-shape, rounded bottom, and have more concentric rings instead of a striped pattern. Quagga mussels are invasive and any suspected observations should be reported to the PEIISC.
• Zebra mussels may be confused for native mussels, however, they have strong threads that are used to attach to surfaces. Native mussels lack these threads as they are free-living and do not attach to any surface.

What it Does to the Environment

Environmental impacts

• Female zebra mussels can release up to one million eggs during a single breeding season. This allows the species to colonize in extremely high densities, with reports of over 700,000 individuals/m² on a variety of surfaces such as boats, docks, rocks, plants, and native mussel species. This accumulation on surfaces is referred to as “biofouling”. 
• Zebra mussels have harmful impacts on native species of mussels by outcompeting them for food and suffocating them by attaching to their shells. A native mussel with zebra mussels attached loses the ability to move, feed, breathe, and breed which eventually leads to the death of the native mussel. Zebra mussels have been noted to cause huge declines in native mussel populations.
• They can alter fish spawning habitat by colonizing substrates needed for egg laying which can impact the survival of fish eggs. 
• They can alter food webs by depleting the water column of phytoplankton and algae, removing important food sources for many aquatic invertebrates and fish. They have been found to cause major shifts in the plankton communities of lakes and rivers.
• Zebra mussel filter feeding increases water transparency and allows sunlight to penetrate deeper into the water. This increases the growth of submerged aquatic plants (including invasive species) and creates toxic algal blooms. 
• They produce a large amount of suspended nutrients through filtering a significant amount of water (up to 1 L per day each). These nutrients are released from zebra mussels as pseudofeces. This affects nutrient cycling, particularly nitrogen (N) and phosphorous (P), which influences plankton community structure and favors the growth and development of blue-green algae. 
• They bioaccumulate toxins as they filter water which has been shown to lead to increased instances of botulism in mussel-eating fish and waterfowl.

Economic and Social Impacts

• Dense colonies clog intake structures such as pipes and screens in power stations and water treatment plants, causing millions of dollars of damage annually. 
• Zebra mussels negatively impact recreation-based activities. Docks, buoys, and boats can be heavily colonized. Boat engines can be overheated and damaged by small mussels that get into cooling systems. The weight of attached zebra mussels can sink navigational buoys.
• Where zebra mussels colonize shallow beaches, their razor-sharp shells can cause injuries and subsequent infection. This can also lower the aesthetic value of recreational areas.  
• Zebra mussel growth reduces water-front property value as dense colonies lower aesthetic value and omit a foul odour.

Pathways of Spread

The most significant pathways of spread of zebra mussels are human-mediated. They are capable of attaching themselves to the surfaces of boats or other aquatic equipment. Improper cleaning of watercraft or equipment when leaving an invaded waterbody can spread this species to uninvaded areas. Females have been found to survive for up to 30 days out of water in high humidity conditions, increasing their chances of dispersal. 

Zebra mussels have also been found in moss balls sold in stores in Canada, thus facilitating their spread through aquarium products. 

The larval stage is free-swimming, microscopic, and planktonic and thus has a high probability of spread. Larvae can survive in boat bilge waters, bait buckets, and engine cooling systems. However, they are incapable of swimming horizontally towards an object and rely on water currents for dispersal. They can remain suspended in the water column for up to 2 weeks.

Prevention

Once established, zebra mussels become almost impossible to eradicate, thus prevention is key to managing this species. Learn to recognize the physical characteristics of Zebra mussels to know if a waterbody is infested. Zebra mussels depend on water currents to disperse and are therefore more likely to spread downstream. Be sure to Clean, Drain, and Dry all items that come into contact with water. The larval stage (veligers) are small and can survive for several days outside water in moist conditions.

• Clean – Remove all plants, animals, and mud from all equipment. Pay close attention to crevices and other hidden areas. Zebra mussels are extremely small and can easily be missed. A hot water sprayer (40°C) should be used after your equipment is in infested waters or you find zebra mussels.
• Drain – Ensure that all water has been drained from the watercraft and other equipment. 
• Dry – Ensure that you allow adequate time for your watercraft to dry completely before launching it into another waterbody. 
• Dispose – Remember to dispose of unused bait. The microscopic stage of zebra mussels (veligers) can live in bait buckets.

The use of anti-fouling or foul-release coatings on boats can help minimize the spread by removing attachment surfaces. Anti-fouling coatings release an aquatic toxin to repel zebra mussels whereas foul-release coatings provide a slippery surface that prevents attachment. Zebra mussels may also spread contaminated moss balls in aquariums. DO NOT DUMP aquarium contents into a waterbody. If you find a zebra mussel on a moss ball, you must destroy it and properly dispose of it. Moss balls can be destroyed by freezing them for 24 hours, boiling them in water for at least a minute, and submerging them in bleach/vinegar. Following this, moss balls can be disposed of by sealing them in a clear plastic bag marked “INVASIVE SPECIES” and throwing them in the residential black waste bin (never in the green compost bin). DO NOT DISPOSE in a storm drain or waterbody as this could lead to the spread of invasive species and damage waterways.

Detection and Monitoring

Zebra mussels are almost impossible to eradicate from waterbody once established. Thus, detecting the species early and monitoring the environment are important aspects to managing this species. Several survey methods exist that can be used to detect zebra mussels. These include:

• Visual and Tactile Surveys
  • These methods can be used to detect mussels at least 3 mm in diameter. Choose sites that are easily viewed or reached and have structures that are at high risk of infestation. High-risk sites include the undersides of docks, the undersides of navigational buoys, or natural materials (rocks, native mussels, or aquatic plants). The easiest tactile survey method is to run a bare hand along the length of the surface to feel for mussel shells. Deeper waters will require SCUBA gear to conduct surveys.
• Artificial Substrate Surveys
  • These methods are useful in areas where attachment sites do not exist or are not reachable such as the bottom of a dock. They involve placing an artificial substrate that mussels can attach to. These surveys are typically used to monitor the post-larval and juvenile stages and take advantage of the settlement behavior where mussels seek an attachment site. Artificial substrates can include plastic PVC pipes or flat plates, cement blocks, ceramic flowerpots, and synthetic wood.
• Plankton Surveys
  • This method can detect the presence of the larval stage (veligers). This can indicate the presence of an infestation before the adult stage has been found. This method utilizes special equipment (plankton nets). Collected samples must be viewed under a microscope. Thus, this method requires technical expertise and can be time-consuming and costly.

Learn More

Fisheries and Oceans Canada –  Zebra Mussel

New Brunswick Invasive Species Council – Zebra Mussels

Invasive Species Council of British Columbia – Zebra and Quagga Mussels

Alberta Invasive Species Council – Zebra Mussel and Quagga Mussel factsheet

Invasive Species Centre – Zebra and Quagga Mussels

Nature Conservancy of Canada – Zebra Mussel

U.S. Geological Survey, Nonindigenous Aquatic Species Database – Zebra Mussel  

CABI Compendium – Zebra Mussel

Acknowledgements

This webpage was developed with the financial support of the Department of Fisheries and Oceans Canada through the Aquatic Invasive Species Prevention Fund.

Image Gallery

Featured image: Amy Benson, U.S. Geological Survey, Bugwood.org