California Aquatic Invasive Species (AIS)
University of California
California Aquatic Invasive Species (AIS)

Invasive Clams, Mussels, Snails, etc.

Zebra Mussel

Scientific Name

Dreissena polymorpha - Visit ITIS for full scientific classification. 



  • Small freshwater mussels.
  • Two ear-shaped shells.
  • Variable coloration, usually solid light to dark brown, or banded.
  • Shells are less than 2 inches in size.
  • They attach to surfaces by threads, and are sometimes called Eurasian mussels.
  • For detection in the field:
  • Look on ropes and shaded surfaces under structures.
  • Feel for individual irregularly shaped bumps on smooth surfaces.
  • As the population grows, it will feel more like sandpaper.
  • Mussels don't rub off they pivot around point of attachment.
  • Be careful, as large shells can be sharp!
  • Predominantly on hard surfaces, from surface to more than 180 ft.
  • Narrower depth range than the quagga mussel.
Invasion Pathways and Distribution
  • Native to Ukraine and Russia.
  • Widely distributed throughout the U.S. 
  • Believed to have come to the U.S. via ballast water.
  • Continue to spread by attaching to boats, trailers, fishing and other gear, as well as microscopic larvae potentially spreading in water either found in moving vessels, or when water is transferred between streams or lakes in a canal. 
  • Can survive out of the water for extended periods of time, and small juveniles are difficult to detect.
  • Visit USGS for a current U.S. distribution map. 
Life History
  • Adults can spawn multiple times per year producing millions of microscopic larvae each season. 
  • Multiply and reach densities that clog water delivery systems.
  • Filter water, removing plankton needed by native species, and changing water clarity.
  • Microscopic young stages can be transported in water undetected.
  • Adults can survive out of the water for a week or longer depending on temperature and humidity. 
References and Useful Links

For references by category and links to other useful AIS sites see our LEARN MORE page.

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