Montana Field Guide

New Zealand Mudsnail (NZMS) is a small snail (4-6 mm) with a dextral (right-handed coiling), elongated shell with 5-6 whorls separated by deep grooves. It is generally dark brown to grey in color. This is an introduced species to MT with a stable or expanding distribution in the Missouri, Madison, Yellowstone, and Bighorn Rivers. It is not known in Montana west of the continental divide. In Montana this species was first discovered in the Madison River above Hebgen Reservoir in 1995 (Gustafson pers. comm.). However, the very large population present at that time indicataes that the introduction was a few years earlier. It is a native of New Zealand, but long established in Australia and Europe. This species has been known in North America since 1987 in the Snake River basin of Idaho

Taxonomically, New Zealand mud snails are in the snail family Hydrobiidae. Hydrobiids can be distinguished from other aquatic snail families by having dextral (opening to the right with the spire pointing away from you) shells with an operculum (a hard calcareous flat that can seal the opening of the shell). New Zealand mud snails are small (~ 2-6 mm) and generally dark colored

Potamopyrgus antipodarum can survive passage through the guts of fish and may be transported by these animals (Bruce 2006). Throughout Montana, it has been transported to different waterbodies on the felt soles of fishermen’s wading boots (ANS 2007). It can also float by itself or on mats of Cladophora spp., and move 60 m upstream in 3 months through positive rheotactic behavior (Zaranko et al. 1997). It can respond to chemical stimuli in the water, including the odor of predatory fish, which causes it to migrate to the undersides of rocks to avoid predation (Levri 1998).

Means of Introduction or Expansion: 1) Fish hatcheries and associated stocking operations, 2) Recreational watercraft and trailers, 3) Recreational water users: Particularly when embedded in mud or attached to plant debris, NZ mudsnails may be transported on fishing gear, on waders and boots, swimsuits and swimming toys and even by hunting dogs and horses, 4) Sand/gravel mining, extraction, and dredging: Any waterway operations that remove and transport mud, sand, and other bottom materials from areas with NZ mudsnails can serve as a vector for new introductions, 5) Aquatic plant trade and collections (ANS 2007). Potamopyrgus antipodarum was most likely introduced to the Great Lakes in ships from Europe, where there are nonindigenous populations (Levri et al. 2007, Zaranko et al. 1997) or in the water of live gamefish shipped from infested waters to western rivers in the United States.

New Zealand mud snails appear to prefer flowing water habitats with stable flows. Springs, spring creeks, and river sections downstream from dams are all places that they thrive in. They can survive in cool lakes with suitable habitat. They are most typically found on larger cobble substrates or on pieces of wood.

The NZMS is a nocturnal grazer, feeding on plant and animal detritus, epiphytic and periphytic algae, sediments and diatoms largely on cobbles or submerged woody debris.

The NZMS is a nocturnal grazer, feeding on plant and animal detritus, epiphytic and periphytic algae, sediments and diatoms (Zaranko et al. 1997, USGS 2017).
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 (Collier et al. 1998, Death et al. 2003, Holomuzki and Biggs 2000, Richards et al. 2001, Zaranko et al. 1997).

Potamopyrgus antipodarum is ovoviviparous and parthenogenic. Native populations in New Zealand consist of diploid sexual and triploid parthenogenically cloned females, as well as sexually functional males (less than 5% of the total population). One snail produces approximately 230 young per year. Reproduction occurs in spring and summer, and the life cycle is annual (Gerard et al. 2003, Hall et al. 2003). All introduced populations in North America are clonal, consisting of genetically identical females.

NZMS have great potential for wide-spread colonization because they have a broad environmental tolerance. Although the species occurs in a wide range of aquatic habitat types, including diverse ranges of temperature, osmotic concentrations, flows, substrates and disturbance regimes, clonal lineages may have either narrow or broad ecological tolerances. Likely to find all shallower waters (<50 m depth) as suitable habitat. High spread potential (USEPA 2008). Abundant populations of introduced P. antipodarum may outcompete other grazers and inhibit colonization by other macroinvertebrates (Kerans et al. 2005).

Many times a newly discovered population of NZ mudsnails may be in a river or lake where chemical eradication will not be feasible and physical eradication difficult. This would be the case with large rivers or lakes where it is impossible to isolate the invader and treatment would be difficult to contain. In other situations the invader may occupy too large an area or other ecological or political restraints may rule. Areas where eradication may be possible include small lakes and ponds, waterbodies that can be temporarily hydrologically separated (e.g., curtain, wall), irrigation canals, and fish hatcheries.

Contact information for Aquatic Invasive Species personnel:
Montana Fish, Wildlife, and Parks Aquatic Invasive Species staff
Montana Department of Natural Resources and Conservation's Aquatic Invasive Species Grant Program
Montana Invasive Species Council (MISC)
Upper Columbia Conservation Commission (UC3)

Threats to the NZMS are minimal, other than reservoir drawdowns and river dewatering which will leave the snails prone to desiccation. Although a discrepancy exists when comparing temperature tolerance limits of North American and New Zealand populations. North American populations are generally adapted to warmer temperature regimes than their New Zealand counterparts.