Just When You Thought It Was Safe To Go Back In The Water: Here Come the Mud Snails

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The New Zealand mud snail is aquatic, but it can live out of water for up to 27 hours! Image by Dan Gustafson, USFWS Pacific Region

A New Zealand mud snail (Potamopyrgus antipodarum) is only 5 or 6mm long when fully grown. Cone shaped, it varies in color from amber to dark brown, looking almost black when wet—like coffee grounds.

But, small and dull, it can easily catch on fishing gear, boats or boots, nestle into dog fur, or hitch a ride on (or even in) waterfowl. It can close its operculum (the film covering the shell opening) and survive out of the water for as many as 27 hours. This is why the tiny snail has traveled so far.

As its name suggests, this mud snail is a native of New Zealand. An aquatic snail (it lives in water—freshwater), it lives happily in all sorts of rivers and streams: big and small, gravelly and silty, estuarine and montane. It feeds on tiny algae and phytoplankton, as well as plant and animal detritus.

Mud Snail: Have Shell Will Travel

This unlikely world traveler made it all the way to England by 1859, possibly carried there in drinking water on ships returning from the antipodes—but until recently was misidentified as native. It is now considered naturalized in much of Europe (and Australia). It was from Europe that the snails made it to the Great Lakes, this time probably in ship ballast water (ships take on and jettison water in ballast tanks for balance as they load and unload cargo—and all too often also take on invasive species in that ballast water).

But the snails came to the western U.S. first: they were found in Idaho’s Snake River in 1987 where they most likely came in with aquaculture products. By 1995, New Zealand mud snails had spread to Montana, and are now well settled throughout the west and in the Great Lakes, and are establishing a foothold in Washington and Oregon. They can live quite happily in fresh or brackish water, tolerate a substantial range of temperatures (0-34C), and can even survive a trip through the gut of a fish.

Cloned Snails: Single Parenthood

All the New Zealand mud snails in the United States are female, and all are clones. Once in a new home, a mud snail can propagate parthenogenetically (i.e., by virgin birth), producing up to 230 embryos a year.

This ability, combined with the snail’s ability to thrive in virtually any freshwater environment, make it an invasive species of grave concern for fish and river managers across the country. Densities of up to 300,000 snails per square meter can hijack river ecology by crowding out native species; outcompeting native snails by eating the best food first. In Wyoming, they don’t stop there, but also eat the young of native snails.

Removing Invasive Species: Unwelcome Guests Hard to Dislodge

New Zealand mud snails are expected to become established in the Columbia River, which they reached in 1996—and where they will probably foul dams and clog water pipes while they’re at it.

Once they are settled in a new river system there’s really no way to get rid of New Zealand mud snails—or at least no way that wouldn’t also kill every other living thing. In New Zealand, mud snail populations are kept in check by parasites—parasites that do not exist here. The snails easily float downstream and are carried upstream by other animals. Their tough shell can make them hard to digest, so that they can pass through the guts of fish and waterfowl unharmed. Not only do the snails not provide any nutrition, they can also be further spread in this way.

In 2011 a snail was found in the Puget Sound region, in Olympia’s Capitol Lake down the hill from the state capitol. The lake was drained in an effort to kill the snails, but they either survived or returned, because they are there still. The lake was then closed to fishing in the hopes of stopping their spread.

But they have since been found in Thornton Creek, in busy Seattle, and in Kelsey Creek across Lake Washington in Bellevue. There are probably more of them around, but they’re hard to find, at least early in the infestation. (That Thornton Creek mud snail was found in a sample that had been collected in 2009.) It takes a fair amount of effort, skill, and knowledge to first find them and then to tell one snail from another.

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