New Zealand Mud Snail

Potamopyrgus antipodarum


The New Zealand Mud Snail (NZMS) is found in many water bodies, including estuaries, brackish waters, lakes, large rivers and small streams. It occurs amongst stream beds and on submerged macrophytes; prefers littoral zones in lakes or slow streams with silt and organic matter substrates. NZMS can tolerate slightly higher flow environments in places where it can burrow into the sediment (Zaranko et al. 1997; Collier et al. 1998; Holomuzki and Biggs 1999; Holomuzki and Biggs 2000; Negovetic and Jokela 2000; Richards et al. 2001; Weatherhead and James 2001; Death et al. 2003; Schreiber et al. 2003; Suren 2005).

Beyond the broad theory that disturbed environments are more prone to the invasion of foreign species (Lodge,1993a,b; D'Antonio et al., 1999), fisheries, environments that are near cities, or are susceptible to agricultural runoff may be also optimal for the NZMS because of their higher nutrient levels and fine sediment. (Quinn & Hickey, 1990a; Collier,1995; Harju, 2007; Quinn et al., 1997).  Even in its native habitat, the NZMS has been shown to become the most prolific macro-invertebrate in natural conditions that are reminiscent of those created by anthropogenic factors (Marshal and Winterbourn, 1979).

This small aquatic gastropod has a history of becoming a pest species in many parts of the world, and its recent introduction into North American waters is cause for concern. In its native waters the mudsnail population is primarily kept in check by trematode (small worm) parasites that sterilize the snail or change mudsnail behavior making it more likely to become eaten by natural enemies.

General Taxanomical Information:

New Zealand Mud Snail (Potamopyrgus antipodarum(Gray 1843), for short often called NZMS, previously was known as Potamopyrgus jenkinsi

This species was originally described as Amnicola antipodanum in 1843 by John Edward Gray    


Kingdom: Animalia 
Subkingdom: Eumetazoa
Phylum: Mollusca
Class: Gastropoda
Subclass: Orthogastropoda
Superorder: Caenogastropoda
Order: Neotaenioglossa           Superfamily: Rissooidea
Family: Hydrobiidae                 Subfamily: Tateinae
Genus: Potamopyrgus              Species: antipodarum 


Clonal Variability:

As shown in many studies, within invasiveranges this parthenogenetic mollusc, P. antipodarum does not belong to one distinct clone, but to a polymorphic set with both visual and genetic
distinctions (Städler et al. 2005).

In North America and Europe, widespread Potamopyrgus populations are composed of couple clones as determined by allozyme genetic markers. All western US populations share an identical six-locus allozyme genotype, and are noncontiguous. This same six-locus genotype is found in Victoria, Australia, which along with DNA sequence similarity suggests that Australia is the source of this invasion (M. Dybdahl and A. Emblidge, unpublished data). Since the Australian populations are about 100 years old (at least 200 generations), this clone likely accumulated genetic diversity through mutation accumulation, some of which might have been introduced to the U.S. populations. (M. Dybdahl and A. Emblidge, unpublished data after Dybdahl ). 


NZMS can reproduce relatively rapidly it often reaches densities greater than 100,000/m² in suitable habitat. The reports of the densities can however vary depending on stream habitat, sampling season, region or time from initial infestation. Densities of NZMS are often lowest in areas with the highest stream velocities (Richards, et al. 2001).

In its native environment, like Lake Alexandrina, New Zealand, the mud snail will still be the most abundant macro invertebrate, but reach its maximum density during the winter at 50,000 snails per square meter (Talbot and Ward, 1987). In Europe one of the highest recorded densities have been reported are 800,000/m2 in Lake Zurich, Switzerland, where this species had colonized the entire lake in less than seven years (Bowler 1991, Dorgelo 1987).

In some reaches, the tiny creatures amounted to more than 90 percent of the invertebrate biomass. That degree of dominance by a single species is typically found only in the most disturbed environments, such as highly polluted streams and sewage ponds.

In the Great Lakes, the snail reaches densities as high as 5,600 m2 and is found at depths of 4-45 m on a silt and sand substrate (Zaranko et al. 1997; Levri et al. 2007)

Examples of various reports of NZMS density: 

  • Montana researchers report densities approaching extremes of 750,000/m² in parts of Yellowstone National Park. 
  • A survey in the upper Madison River, Montana found that NZMS abundance was highest in summer and decreased significantly in the spring (Kerans 2001). 
  • In Firehole River and Nez Perce Creek densities were high in the fall, dropped off in the spring and were high again in the summer (Kerans 2001). 
  •  Densities in Darlington Ditch, Montana peaked at 25,000 snails/m2 (Cada 2003).
  • Middle Snake River, Idaho - densities up to 500,000 snails/m2 have been found in the (Richards, David C. 2003).
  • In the Greater Yellowstone Ecosystem NZMS are the dominant benthic macroinvertebrates and in several rivers reaches densities of 550,000 snails/m2 (Riley 2002) but   can approach extremes of 750,000/m² in parts of the Park.  In three streams within Yellowstone National Park (Firehole River, Gibbon River and Polecat Creek) NZMS density has been observed at a maximum in summer and a minimum in the winter (Hall et al. In preparation).
  •  Site above Hebgen Lake, Montana - densities from 2,000 to 6,000 snails/m2 (Pickett 2002).
  • In the Owens River, California- NZMS densities were about 10,000 snails/m2 have been observed (Becker 2001), but  3 years later Upper Owens River densities reached up to 48,000 snails/m2 in the (Noda 2003).
  • Southern California densities greater than 5,000/m2 have been observed in Malibu watershed (M. Abramson, unpub. data). 
  • Piru Creek  CA-  Based on monthly samples taken from Piru Creek In 2009-10, we found that snail densities fluctuated between very low (less than 10 individuals / m2) following winter high flow events, to relatively high (more than 1500 individuals/m2) during spring and summer months.


In a Southern Californian stream (Piru Creek), mature population densities were greatest, roughly 150,000 per m2, in the dry summer. Relatively constant temperatures, presence of macroalgae and low flow rates are attributed to these densities (Bennet et al., 2015).   

An increase in temperatures, episodic high flow events and drought occurrence may decrease the prevalence of NZMS and their impact on ecosystems in Southern Californian streams. However, when these variables do not changes, such as in dammed waterways, the impacts may be severe, especially for endangered species, such as the arroyo toad (Anaxyrus californicus) since the presence of the NZMS can negatively impact the feeding habitats of the tadpoles (Bennet et al., 2015).  

Ecological Effects

  • can out-compete native sails  

  • do not provide nutrition to fish  

Economic Impacts  

  • can clog grates and pipes  

  • require public awareness campaign to limit spread  

  • change fish hatchery stocking route  

For more detailed information on impacts, go to the Impacts on Streams’ Ecosystems



Where we are not finding NZMS:

While the New Zealand mud snail seems to be invading nearly all types of fresh water bodies, there are some that seem more resistant to invasion.  Fresh water systems that are supplied mostly from snowmelt runoff, or have a high water velocity do not have a high density population of NZMS.  The fact that the NZMS is not present in waters that are supplied with fresh snowmelt runoff, (Statzner & Holm, 1989), as opposed to those that are feed by groundwater, may be due the low dissolved ions concentrations in these waters (Herbst, 2008).  Low ion concentrations, below an SC of 25µS per cm, may reflect the consequence of deficiencies in Na+, K+, and Mg++ for ionic and osmotic regulation.  The dependence on higher ion concentrations can be further supported by the snail's strong presence in brackish water.