New Zealand Mudsnails

The New Zealand mudsnail, Potamopyrgus antipodarum, is an aquatic invasive species that was first found in the United States in Idaho in the 1987, and has since spread throughout the West. They appeared in California in the late 1990’s in the Owens River, and were found in southern California in Malibu Creek in 2005 and in Piru Creek in the Santa Clara River watershed in 2006. Since their discovery in Malibu Creek, they have spread to several streams in the Santa Monica Mountains. From 2017 on, they have spread into the Bay Area and points north.

Photo by Ken Davis

New Zealand mudsnails are tiny, with adults only reaching 3-5 mm and juveniles even smaller, about the size of a grain of sand. They are usually light to dark brown, and may appear black when wet. They have conical shells that have five, or sometimes six whorls.

New Zealand mudsnails on a leaf. Photo by S. Drill

New Zealand mudsnails reproduce clonally and bear live young. Broods of up to 120 can be produced three times per year in warm climates, and a single female and her offspring are capable of yielding 40 million individuals in a year.

We believe they compete with native invertebrates for food and habitat though research is currently underway. What is known is that they reach huge densities. Over 750,000 individuals per square meter have been recorded in the Yellowstone River. As they provide little in the way of food value, they may have detrimental effects on fish and wildlife. They have a separate piece of shell, called an operculum, that can seal them tightly inside their shells, giving them wide ranging temperature and salinity tolerances. They can survive for several days out of water under moist conditions, and even pass through a fish's digestive tract alive.

Taken together, their small size, dark coloration, and ability to stick to things makes them excellent at invading new systems. They can hitch a ride on fishing gear, sampling equipment, shoes (hiding in the treads and under the laces), and clothes, as well as on the fur of dogs and horses. We know of no way to get rid of them once they invade a river system, other than drastic dewatering or poisoning. Researchers are investigating options for biological control.

NZMS on a boot. Photo by Ken Davis

The best way to manage New Zealand mudsnails and other invasive species is to try and prevent them from spreading. Stay out of infested streams and do NOT to go from one stream to another in wet gear. If you need to go into an infested stream, consider having dedicated clothes and gear that you don’t wear anywhere else. Scrub all gear with a stiff brush before you leave an infested site; mudsnails are experts at hiding, so you can’t trust a visual inspection. Let all gear dry completely between visits, or freeze for a minimum of six hours between uses.

More Information

USGS Map of current New Zealand Mudsnail distribution in the US

UCCE New Zealand Mudnsnail Fact Sheet

Ver un video en español

CDFW Invasive Species Program

Protect Your Waters: Stop Aquatic Hitchhikers

Education about the problems caused by releasing unwanted pets and plants. http://www.habitattitude.net/