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

New Zealand mudsnails are tiny non-native snails that have infested an increasing number of Washington's lakes and streams. These snails multiply quickly and disrupt the food chain, threatening native fish. The species' size makes it easy for anglers and boaters unknowingly to transport mudsnails from one waterbody to another. To date, there is no way to eradicate mudsnails once they have infested a waterbody without damaging the aquatic habitat.


New Zealand mudsnails are small (an average of 1/8 inches long) and cone-shaped. Their shells have five to six whorls, fairly uniform in size, and vary in color from light-brown to black.

This species of mudsnail is hearty, surviving in a variety of salinity, water temperature and quality.  A movable cover at the opening of its shell (the "operculum") allows the mudsnail to protect itself from short-term exposure to most chemicals. The New Zealand mudsnail also survives out of water for quite some time and has no known predators or parasites in Washington state that can keep populations in check.

A single female snail can rapidly reproduce through cloning, adding 230 snails to the population annually. That initial snail, along with its offspring, can build a population into the billions of snails within a four-year timeframe.

New Zealand mudsnails mostly feed at night on algae, sediment, plant and animal detritus – all of which would otherwise be consumed by native snails and insects.

Potential Risks

New Zealand mudsnails are classified as prohibited because they meet the definition of "invasive species" under RCW 77.135.010(13) and pose an invasive risk of harming or threatening the state's environmental, economic, or human resources.. Thanks to rapid self-reproduction, the species can quickly achieve densities of more than 500,000 snails per square meter. These mudsnails feed on the primary food web of algae and detritus important to native aquatic insects. Reductions in native aquatic insects in turn threaten the huge environmental and economic investment our state continues to spend on salmon recovery efforts as those insects are critical as feed to juvenile salmonids. New Zealand mudsnails are not an alternative food source as they have very low nutritional value and most pass through a fish's digestive track unharmed. In addition, They are relatively recent invaders to the United States (original detection in Idaho around 1987) and their potential invasive harm continues to evolve with each new location in which they become established, developing relationships with other invasive species, and the effects of climate change. After moving into a lake or stream, these mudsnails are nearly impossible to remove without damaging other aspects of the habitat. 

History, locations, and management in Washington

Not native to the United States, New Zealand mudsnails were initially detected in 1987 on Idaho's Snake River.  The species is now found in many locations throughout the West, including in Yellowstone National Park, American River in California and parts of Lake Powell in Arizona. New Zealand mudsnails also have spread east to the Great Lakes and the St. Lawrence River. Although fairly widespread geographically, infestations are generally limited to relatively small areas and prevention of further spread is a primary management priority.

New Zealand mudsnails invasive history in Washington State goes back to 2002 with their discovery in the Lower Columbia River estuary and most recently in 2014 at a WDFW fish hatchery on the Columbia River in Eastern Washington. The number of infested sites in the state is still relatively low (~15 areas) and increased awareness and management actions have contributed to preventing rapid and extensive spread. Please see the "NZMS Statewide Action Summary June 2015" report below for infested site locations and an overview of Department management actions related to these infested locations.

The primary statewide management efforts for NZMS are incorporated into the Department's overall aquatic invasive species (AIS) prevention program actions. These include signage at water bodies and boat launches, field gear decontamination protocol development and trainings, and education/outreach to the public. In situations where a NZMS infested site is newly discovered, the Department provides consultation, coordination, and support to local jurisdictions and interested parties on management options including prevention, containment, control, and eradication. As resources allow, WDFW has contributed to additional containment, control, eradication planning, and monitoring actions. In general, the burden of infested site management of prohibited level 3 species is that of the water/land owner or manager.


Boaters, kayakers, anglers and anyone who recreates or works in Washington's waters should take measures to help prevent the spread of New Zealand mudsnails.

For more details on the prevention methods, visit WDFW's webpage on preventing the spread of aquatic invasive species.

Report new infestations

If you observe New Zealand mudsnails or any other known or suspected aquatic invasive species in a previously unreported waterbody, please report the sighting to WDFW.

Find the waterbodies where mudsnails have been reported on this website:

Report new sightings here: http://wdfw.wa.gov/ais/reporting/

Reports and Publications


Legal Classification

New Zealand Mudsnails are classified as "Prohibited level 3" species under RCW 77.135.030(1)(c) for species considered to pose a "moderate to high invasive risk" and which may require management by the department or other affected landowner. Prohibited level 3 species, under RCW 77.135.040(1), "may not be possessed, introduced on or into a water body or property, or trafficked, without department authorization, a permit, or as otherwise provided by rule."