The Science

Quagga and Zebra Mussels are the Most Costly Ecological Pests ever Introduced to North American Freshwaters

Zebra and Quagga Mussels are freshwater, bivalve mollusks that typically have a dark and white (zebra-like) pattern on their shells. They are alien to North America but have invaded many of our waters.

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Impacts to tourism, hydropower, and infrastructure (irrigation systems, power facilities, and municipal water supply and treatment) could cost more than $95 million a year.

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Once established, invasive mussels will be here forever. They are virtually impossible to remove. The ONLY solution is PREVENTION.


Background Information

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Zebra and Quagga Mussels are freshwater, bivalve mollusks that typically have a dark and white (zebra-like) pattern on their shells. They are alien to North America but have invaded many of our waters.

There are two species of Dreissena, both non-native, in North America: Dreissena polymorpha, commonly called "Zebra Mussels" and Dreissena rostiformis bugensis, commonly called "Quagga Mussels" but may also be referred to as "Zebra Mussels," which is sometimes used as a general term for all Dreissenid mussels. Despite some minor appearance and ecological differences, the species are very similar and pose a significant threat to our waters.

Both species, Zebra Mussels and Quagga Mussels, in general, are usually about an inch or less long, but may be larger. When healthy, they attach to hard substrates. much like marine (saltwater) mussels but unlike any native freshwater bivalve. They are often found in clusters.

Zebra and Quagga mussels are native to Eurasia. Until the mid 1980s there were no zebra mussels in North America. Quickly that changed when they were inadvertently introduced into waters near the Great Lakes region. It is suspected that zebra mussels hitched a ride in ballast water tanks of commercial ships. Zebra Mussels were first discovered in the United States in Lake St. Clair near Detroit, Michigan in 1988. Since the '80s, zebra mussels have spread, unchecked by natural predators, throughout much of the eastern United States. They currently infest much of the Great Lakes basin, the St. Lawrence Seaway, and much of the Mississippi River drainage system. The have spread up the Arkansas River into eastern Oklahoma. Quagga mussels invaded North America later than zebra mussels and have been confirmed in fewer waters, including the Great Lakes, the St. Louis area and now in the Lower Colorado River. In November 2016, state officials announced the first documented presence of zebra and quagga mussels in Montana, after positive tests at sites in the Missouri River system (in Tiber Reservoir, and “suspect” detections in Canyon Ferry Reservoir).

When they are present in North American waters, they are usually millions of them. Zebra/Quagga Mussels are what scientists and engineers call "biofoulers" that block pipes in municipal and industrial water systems, requiring millions of dollars annually to treat. Zebra Mussel densities have been reported to be over 700,000 individuals per square meter in some facilities in the Great Lakes area. They produce microscopic larvae that float freely in the water column, and thus can pass by screens installed to exclude them. Monitoring and control of Zebra and Quagga Mussels costs millions of dollars annually.

Zebra/Quagga Mussels also negatively impact aquatic ecosystems, harming native organisms (including already imperiled indigenous mussels). In huge numbers, they out-compete other filter feeders, starving them. They adhere to all hard surfaces, including the shells of native mussels, turtles, and crustaceans. Zebra/Quagga mussels actively feed on green-algae and may increase the proportion of foul-smelling blue-green algae in water systems.


General Biology

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    Species Names
    • Zebra mussels, Dreissena polymorpha
    • Quagga mussels, Dreissena rostriformis bugensis
    • Microscopic to about two inches long
    • Typically up to 5 years
    Reproductive Potential
    • May spawn all year if conditions are favorable
    • Peak spawning typically occurs in Spring and Fall
    • A few individuals can produce millions of eggs and sperm
    Life Cycle
    • Embryos are microscopic (< 100 microns)
    • Larval stage is planktonic (free floating), carried with currents
    • Adult stage attaches to hard surfaces with threads (like marine mussels), but can detach and move to new habitat
    • Ecological
      • As filter feeders, these species remove food and nutrients from the water column very efficiently, leaving less or nothing for native aquatic species
      • They have the potential of collapsing entire food webs
    • Economic
      • These species clog pipes, ruin boat motors, and damage aquatic recreational equipment
      • Once established in a lake, routine maintenance is necessary and perpetual
      • Management costs are enormous, particularly for industrial raw water users like power stations and water supply agencies


Frequently Asked Questions

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What’s the difference between Quagga Mussels and Zebra Mussels?

    Short Answer: Not Much
    • These are two species within the same genus Dreissena
    • Zebra Mussels invaded North America First (in mid-1980s)
    • Quagga Mussels invaded a few years later (1989)
    • There are morphological differences, but they are subtle
    • There are ecological differences, but more research is needed on North
      American quagga mussels to assess these differences
    • The practical implications of zebra and quagga mussels are essentially

What temperatures can zebra mussels and quagga mussels tolerate?

Short Answer: Between 1-30° C (33-86° F)

    Heat Tolerance
    • Zebra Mussels can survive in waters as warm as 30° C (86° F).
    • Quagga Mussels may be able to survive in waters as warm.
    Cold Tolerance
    • Both zebra mussels and quagga mussels can survive cold waters near freezing, but cannot tolerate freezing.
    • Zebras need waters above 12° C (54° F) in order to reproduce.
    • Quaggas need waters above 9° C (48° F) in order to reproduce.
    Temperature Preference
    • Zebra Mussels survive and reproduce best in waters near approximately 18° C (64° F).
    • Quagga Mussels survive and reproduce best in waters slightly cooler, approximately 16° C (61° F).

What do they eat?

Short Answer: Algae and bacteria in the water column

  • Both species are filter feeders.
  • Quagga Mussels are more efficient filter feeders than Zebra Mussels

What eats zebra and quagga mussels?

Short Answer: They have no natural predators in North America

  • Many species do eat these mussels, including diving ducks, red-eared sunfish and some catfish, but predators cannot keep up with the explosive reproductive potential of these invasive mussels

At what depths can you find these mussels?

Short Answer: At any depth, but quaggas can be found in deeper water

  • Zebra mussels are typically found from just below the surface to about 12 meters (40 feet)
  • Quagga mussels are typically found at any depth as long as oxygen is present
  • Both species prefer to avoid light and are usually found in shaded areas or below the depth that light penetrates water

Why aren't they a problem in Europe?

Short Answer: They are, but most Europeans have been dealing with them for over 200 years. Their industrial facilities were designed with these in mind.

How do they spread?

Short Answer: Larvae flow downstream. Adults attach to
recreational boats and equipment (anchors, bait buckets, etc).

  • Eggs and larvae will naturally flow downstream of established populations.
  • Larvae can also be transported in water carried by recreational boats, trailers, and other aquatic equipment.
  • Adults can also be spread by recreational boats, trailers, and aquatic equipment.
  • Adults can survive out of water for weeks if temperatures remain cool and humidity remains high.
  • Quagga mussels were probably transported overland at least 1000 miles from their source population (most likely the Great Lakes)
  • Resident boats (those boats that are moored or held in a slip) are much more likely to harbor zebra and quagga mussels than day boats (boats that are removed from the water after each use).

How can we prevent additional spread?

Short Answer: Educate boaters and other water recreationists.

  • Preventing downstream invasions is practically impossible.
  • Convincing recreational boaters to clean their boats and equipment before transporting them to new waters is essential.
  • Simple steps are necessary every time a boat is retrieved from a lake or
    other water body:
    • Remove all aquatic plants, animals, and mud from everything that
      came in contact with water.
    • Drain all water, including bilges, live-wells, cooling water from
      the motor.
    • Clean and dry everything that came in contact with water
    • Dispose of any live bait.
  • If mussels are seen attached to a boat or other recreational equipment, it must be decontaminated using more stringent guidelines.
    • A decontamination protocol is attached.