New Zealand mud snail

tario since 2001, Washington State since 2002, British Columbia since July 2007), most likely due to inadvertent human intervention.

Distribution within the United States

First detected in the United States in Idaho's Snake River in 1987, the mudsnail has since spread to the Madison River, Firehole River, and other watercourses around Yellowstone National Park; samples have been discovered throughout the western United States. Although the exact means of transmission is unknown, it is likely that it was introduced in water transferred with live game fish and has been spread by ship ballast or contaminated recreational equipment such as wading gear.

The New Zealand mudsnail has no natural predators or parasites in the United States, and consequently has become an invasive species. Densities have reached greater than 300,000 individuals per m² in the Madison River. It can reach concentrations greater than 500,000 per m², endangering the food chain by outcompeting native snails and water insects for food, leading to sharp declines in native populations. Fish populations then suffer because the native snails and insects are their main food source.

Mudsnails are impressively resilient. A snail can live for 24 hours without water. They can however survive for up to 50 days on a damp surface, giving them ample time to be transferred from one body of water to another on fishing gear. The snails may even survive passing through the digestive systems of fish and birds.

Mudsnails have now spread from Idaho to most western states of the U.S., including Wyoming, California, Nevada, Oregon, Montana, and Colorado. Environmental officials for these states have attempted to slow the spread of the snail by advising the public to keep an eye out for the snails, and bleach or heat any gear which may contain mudsnails. Rivers have also been temporarily closed to fishing to avoid anglers spreading the snails.

The snails grow to a smaller size in the U.S. than in their native habitat, reaching 6 mm (¼ in) at most in parts of Idaho, but can be much smaller making them easy to overlook when cleaning fishing gear.

Clonal species like the New Zealand mudsnail can often develop clonal lines with quite diverse appearances, called morphs. Until 2005, all the snails found in the western states of the U.S. were believed to be from a single line. However a second morph has been identified in Idaho's Snake River. It grows to a similar size but has a distinctive appearance. (It has been nicknamed the salt-and-pepper mudsnail due to the final whorl being lighter than the rest of the shell.) This morph has apparently been present in the area for several years before being identified correctly as a distinct morph of Potamopyrgus antipodarum. It dominates the typical morph where they overlap, and has a much higher prevalence of males.

In 1991, the New Zealand mudsnail was discovered in Lake Ontario, and has now been found in four of the five Great Lakes. In 2005 and 2006, it was found to be widespread in Lake Erie. By 2006 it had spread to Duluth-Superior Harbour and the freshwater estuary of the Saint Louis River. It was found to be inhabiting Lake Michigan, after scientists took water samples in early summer of 2008. The snails in the Great Lakes represent a different line from those found in western states, and were probably introduced indirectly through Europe.

In 2002, the New Zealand mudsnail was discovered in the Columbia River Estuary. In 2009, the species was discovered in Capitol Lake in Olympia, Washington. The lake has been closed to all public use, including boating and other recreation, since 2009. A heavy cold snap in 2013, combined with a drawdown in water level in preparation, was roughly estimated to have killed 40–60% of the mudsnail population. Other known locations include the Long Beach peninsula, Kelsey Creek (King County), Thornton Creek (King County), and Lake Washington.

In 2010, the Los Angeles Times reported that the New Zealand mudsnail had infested watersheds in the Santa Monica Mountains, posing serious threats to native species and complicating efforts to improve stream-water quality for the endangered Southern California Distinct Population Segment of steelhead. According to the article, the snails have expanded "from the first confirmed sample in Medea Creek in Agoura Hills to nearly 30 other stream sites in four years." Researchers at the Santa Monica Bay Restoration Commission believe that the snails' expansion may have been expedited after the mollusks traveled from stream to stream on the gear of contractors and volunteers.

As of 21 September 2010 In Colorado, Boulder Creek and Dry Creek have infestations of New Zealand mudsnails. The snails have been present in Boulder Creek since 2004 and were discovered in Dry Creek in September 2010. Access to both creeks has been closed to help avoid spread of the snails. In the summer of 2015 an industrial-scale wetland rehabilitation project was undertaken in northeast Boulder to rid the area of a mud snail infestation.

The snail tolerates siltation, thrives in disturbed watersheds, and benefits from high nutrient flows allowing for filamentous green algae growth. It occurs amongst macrophytes and prefers littoral zones in lakes or slow streams with silt and organic matter substrates, but tolerates high flow environments where it can burrow into the sediment.

In the Great Lakes, the snail reaches densities as high as 5,600 per m² and is found at depths of 4–45 m on a silt and sand substrate.

This species is euryhaline, establishing populations in fresh and brackish water. The optimal salinity is probably near or below 5 ppt, but Potamopyrgus antipodarum is capable of feeding, growing, and reproducing at salinities of 0–15 ppt and can tolerate 30–35 ppt for short periods of time.

It tolerates temperatures of 0–34 °C.