Potamopyrgus antipodarum. (New Zealand mudsnail)

Animal Molluscs
Family: Gastropoda
Classification: Prohibited

New Zealand Mudsnail
Mudsnail ID card - USFWS
New Zealand Mudsnail fact sheet
Fact Sheet

The New Zealand mud snail was first found in the Snake River in Idaho in 1987. By 1997 there was a population in the Madison River in Yellowstone Park. In some areas they were in densities of up to 750,000 per square yard. Since then they have been found in the Colorado, Upper Missouri, Owens, Columbia and other Rivers. In Washington there are established populations in the Columbia River at Young’s Bay and Kalama, in the Snake River near the border in Idaho, and in a number of private waterways on the Long Beach Peninsula. They are now present in Capitol Lake in Olympia.

New Zealand mudsnails are small (about 1/8 – 1/4 of an inch long when full grown) that have brown or blackish cone-shaped shells with five whorls. They tolerate a broad range of temperature, salinity, and water quality, and have no natural parasites or predators in the United States. They are able to close their shells, and live out of the water for quite some time. Unfortunately, it only takes one to begin a new population. Females “clone” themselves, producing approximately 230 new female snails each year. Based on a single snail and each offspring in turn reproduces itself, by year two there may be over 52,000  snails, by year three over 12 million snails, and by year four over 2.7 billion snails carpeting the bottom of a river or lake. Preliminary studies indicate that in areas where they have become densely populated they are becoming the dominant invertebrate via displacement and competitive interactions. Large populations may consume up to half of the available food in a stream, starving out insects essential to trout and salmon. A recent study by the U.S. Bureau of Land Management and the Utah State University (Vinson and Baker, 2008) linked these invasive snails to poor condition of trout due to lack of a regular food source and that the snails did not provide sufficient nutritional value when consumed. The snails are very small - between an eighth and a quarter inch in size – and may not be noticed attached to tackle, waders, boats, etc.


Capitol Lake Response

The department will be using this site to update the public regarding the multi-agency response to the New Zealand mudsnail infestation recently found in Capitol Lake. A press release was issued by the department on November 24, and the fact sheet developed by the department and the Washington Invasive Species Council is also available. The photos shown here were taken on November 24, 2009 in the Heritage Park area.

Latest News

  • Agencies seek public’s help in curtailing spread of invasive snails in Capitol Lake
  • New Zealand Mudsnails in Capitol Lake, Olympia - mudsnails on rock New Zealand Mudsnails in Capitol Lake, Olympia - infestation signage
    New Zealand Mudsnails in Capitol Lake, Olympia - log underwater New Zealand Mudsnails in Capitol Lake, Olympia - large grouping
    New Zealand Mudsnails in Capitol Lake, Olympia - rock detail

    Cleaning & Prevention
    To prevent the survival of mudsnails on clothing and equipment recreational water users must first need to clean your gear and then to treat it using either a physical or chemical method.

    • Before leaving a waterbody, remove all plants, algae, or mud from shoes, waders, life vests, boat hulls, trailers, and other gear using a stiff-bristled brush. If you can’t decontaminate on site, place articles in a plastic bag for transport to a location where decontamination can be accomplished with no threat of release into the wild.
    • Select a decontamination treatment.
      • Physical treatments are recommended because they are less expensive, environmentally sound and less destructive to gear.
      • Freeze gear to 14°F (-10°C) or colder for a minimum of 8 hours or 15°F to 32°F (-9°C to 0°C) for 24 hours.  If gear has been used in marine or estuary environments, rinse thoroughly in freshwater before freezing.
      • Soak gear in 120°F (49°C) for a minimum of 30 minutes or 140°F (60°C) water for 5 minutes.  Note: 140°F (60°C) water temperature cannot be achieved using most hot water heaters that are installed for domestic uses, which should be kept at 120°F (49°C) to avoid burns.
      • Dry gear at least 48 hours under low humidity (or2 hours in a clothes dryer).  Gear must be completely dry at least 24 hours.  Felt soled waters require additional drying time.
      • Chemical Treatments SHOULD NOT BE USED NEAR A WATERBODY.  Soak gear in undiluted antibacterial Formula 409 All-Purpose Cleaner for 10 minutes then thoroughly rinse gear in freshwater before using in a waterbody. SOAKING IN FORMULA 409 ALL-PURPOSE CLEANER MAY RESULT IN SURFACE CRAKING OF RUBBER OR LOSS OF WATER REPELLENCY.
    • If using a decontamination solution is not practical, once the item is completely dry to the touch, wait an additional 48 hours to ensure the item is thoroughly dry before contact or use in another waterbody.

    *Laboratory tests show that the snails, as well as other invasive species and pathogens such as Didymosphenia gemnata (Didymo) or Myxobolus cerebralis (whirling disease) live longer in felt sole waders because they remain damp for so long. We recommend always using a decontamination treatment.

    For more information go to: