DESCRIPTION: New Zealand mudsnails are relatively small (average length of 4-5 mm in western USA), with a maximum of 11 mm in native habitats. They reach maturity at 3 mm in length in rivers in western Montana and Idaho. Their shell usually consists of a right-handed coiling of 5-6 whorls. The shell varies in color (gray, light to dark brown). An operculum (i.e., plate) covers the opening of the shell. New Zealand mudsnails have triploid, parthenogenetic female populations: asexual females are born with developing embryos in their reproductive system. Diploid, sexual male and female populations are extremely rare in western USA. Asexual females generally produce twice the number of daughters as sexual females. The adult New Zealand mudsnail may easily be confused with various native and exotic species which can be similar in appearance, and all newly discovered populations should be verified by experts. The shell of the New Zealand mud snail is narrower, longer, and has more whorls than most hydrobiid snails native to the United States. New Zealand mud snails are live bearers (they release embryos and not eggs), and therefore, the presence of newly released young may indicate a possible population. New Zealand mudsnails can inhabit lakes, ponds, streams, rivers, lagoons, estuaries, canals, ditches, water tanks, and reservoirs and occupy a wide variety of substrates including silt, sand, mud, concrete, vegetation, cobble, and gravel. They are capable of tolerating a wide range of temperatures with upper thermal limits of 28°C and lower thermal limits near freezing. The also have a wide salinity tolerance range from saline and brackish to freshwater. Populations in saline conditions produce fewer offspring, grow more slowly, and undergo longer gestation periods. Individuals of this species are able to tolerate turbidity, clear water, and degraded conditions (including sewage and may pass through the digestive tracts of many fish species.

PATHWAYS/HISTORY: This species is native to fresh and brackish habitats of New Zealand and adjacent islands and has been naturalized in Europe and Australia. The means of introduction in the United States is unknown, but possibly occurred with the transfer of fish eggs and live game fish and in ballast waster.

RISKS/IMPACTS: The New Zealand mudsnail 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. Since the mid-1980’s, North American population densities in some infested streams have reached up to 3/4 million individuals per square meter. New Zealand mudsnails could displace native invertebrates. Five species of mollusks (all native to the Snake River) have recently been listed as "endangered" in part due to the establishment of the New Zealand mud snail and its potential impacts. Establishment is expected to have negative impacts on native fauna (e.g., decrease in densities of herbivorous invertebrates, decrease in attached filter-feeding organisms). There is evidence for a negative correlation between populations of mayflies, stoneflies, caddisflies, and chironomids and New Zealand mud snail densities of <28,000 per square meter in a spring creek in southwestern Montana. This species may have the potential to impact the food chain of native trout and other fish species and have the potential to disrupt the physical characteristics of invaded ecosystems (e.g., reduction in the biomass of periphyton and the resulting interactions can have wide-ranging affects on stream ecosystem processes). They also have the potential to become a pest species of freshwater supplies: in Australia New Zealand Mudsnails actually emerged from domestic water taps.

MANAGEMENT: Heat, desiccation, and subjecting them to a hard freeze will kill the New Zealand mudsnail. A trematode native to New Zealand may be of assistance in the development of a biological control, but further research is needed.

WHAT YOU CAN DO: Transportation is believed to occur mainly via contaminated equipment of recreational boaters and anglers, and therefore, the following will assist in containing the spread: Scrub and thoroughly rinse boat, gear, and equipment before exiting an infested area. Allow everything to dry in low humidity for at least 24 hr before entering another body of water. Scrub and thoroughly rinse off all mud and debris (e.g., aquatic vegetation) which may be adhering to boots, waders, clothing, etc. before leaving an infested area. Allow to dry in low humidity and high temperature > 30 ºC for at least 24 hr before entering another body of water.

PROFILE CREDIT: Dani Crosier and Dan Malloy - IMAGE CREDIT: D.L. Gustafson