Snails are part of nearly every ecosystem in the world. Whether in trees, the ocean or your garden, snails are seemingly everywhere. In New Zealand, a species of snail, Potamopyrgus antipodarum, lives in freshwater habitats as part of the native ecosystem. Halfway around the world, however, these snails, known as New Zealand mudsnails, spread as an invasive species.

New Zealand mudsnails were discovered in the United States in Idaho’s Snake River in the late 1980s. Since then they have been found in watersheds across the West, including multiple locations in Wyoming.
The presence of mudsnails can impact ecosystems, reduce native invertebrate populations and ultimately harm fisheries. Keeping people informed about the snails, how they’re spread and how to prevent their expansion can help reduce the number of waterways with these damaging snails.


 

MADE TO INVADE

When the snails are alive, their shells range from tan to black. The shells are a thin, cone shape with a sharp point and typically have five to six spirals. These invaders prefer habitat with consistent water flow rates, temperature and nutrient inputs.

Like humans, snails rely on feet to get around. Each snail has one foot, which is the part of the snail’s body that contracts to help it glide along a surface. Attached to the New Zealand mudsnail’s foot is a form of protection — a hard plate called an operculum. This plate doesn’t work like a shoe, instead it’s more like a door — closing off the shell opening and protecting the snail when it is retracted inside. Most of Wyoming’s native aquatic snails do not have an operculum, so they aren’t able to protect themselves in their shells the same way a mudsnail can and are more susceptible to drying out.

Another major difference is the orientation of the snail’s opening. Many of Wyoming’s snails are left handed, meaning their shell opens to the left. New Zealand mudsnails, however, are right handed with the shell opening to the right.

Mudsnails are successful invaders because they have many advantages over native species. They feed on algae and decomposing organisms, so they can find a food source in most aquatic habitats. In their native range, mudsnails have natural predators and parasites that limit their population growth and cause them to employ different reproductive strategies where both sexual and asexual reproduction occur. However, mudsnails found in the U.S. are all female and reproduce asexually. Therefore, only one snail is required to start a new population and the number of snails in a population can quickly grow.

These snails can be elusive, even when there are a lot of them. They’re small, only up to a quarter-inch long as adults and smaller as juveniles. They can hide in aquatic plants or floating debris that can be moved from water to water, which expands their range. People can easily move these snails without even knowing they’re there, making humans the primary vector of spread in the U.S.

Mudsnails are commonly found at wading access points and boat launches. Wading anglers often spread populations from these sites unknowingly. Mud and plants concealing mudsnails can be easily transported within and between waters, especially on gear and equipment such as waders, watercraft and anchors. Wading boots, especially those with felt soles, can be difficult to clean and are effective at concealing and transporting mudsnails. Mud from a river bottom brought into drift boats from waders or anchors can contain hundreds of snails. Their small size and dark coloration help them avoid detection during gear inspection.

Using their opercula, mudsnails seal themselves off from the environment and can live outside water for extended periods if they have moisture around them. Studies have shown mudsnails can survive for more than a month on a damp surface. If equipment is not cleaned and allowed adequate time to dry, there is ample opportunity for snails to be moved long distances and remain alive once they enter a new water.

BEATING THE COMPETITION

Mudsnails can quickly become prolific outside their native range. Mudsnail populations have a pattern of booming and busting in these environments. Densities reach extremely high levels, with mudsnails dominating an ecosystem, then sharply decrease. Researchers have observed this pattern in mudsnail populations in Wyoming. Leslie Riley, professor of biology at Ohio Northern University, conducted studies on the effects of mudsnails near Yellowstone National Park in the early 2000s and saw these population dynamics firsthand.

“In Polecat Creek, that population was 500,000 snails per meter squared at one point, and it crashed,” Riley said. However, a crash or bust for mudsnails does not indicate a collapse of the population. In subsequent sampling, researchers found fewer mudsnails but similar issues within the community.
When snail densities are high, ripple effects can be seen throughout the aquatic ecosystem. Having hundreds of thousands of mudsnails in a water body can cause increased competition for food, and a system may not have the capacity to support this population in addition to the preexisting invertebrate community. The food source for mudsnails overlaps with native snails and invertebrates, often to the detriment of those species.

“In all of the cases we looked at, the New Zealand mudsnail was a better competitor,” Riley said. “They were less affected by lower resource availability. They were able to grow just as fast on low resources, whereas the native snails, when there were lower resources, had a harder time.”



Competition for resources includes physical space. Both groups live on the same surfaces, and native species can find themselves pushed out of an area where streambed real estate is in short supply.

An invertebrate community can easily become dominated by mudsnails, which can negatively affect fish. Since many types of fish, such as trout, utilize invertebrates as a primary food source, mudsnails have the potential to destabilize an ecosystem. When groups of native invertebrates are crowded out of an area, fish are forced to leave the area for food or consume different food. If fish switch food sources, they can increase predation pressure on the native macroinvertebrates that are left or consume mudsnails instead. Mudsnails are detrimental to fish growth as they are a less beneficial food source than native species and the soft tissue fish can break down is minimal. The energy required for a fish to consume and digest the snails can exceed the energy they receive, leading to a malnourished fish population. These effects could change fisheries across the state of Wyoming.

Studies have shown up to 50 percent of the mudsnails a fish consumes can remain viable upon excretion. This is bad news for the spread of mudsnails because fish can move farther and quicker than snails, and fish can release the snails into previously uninfested areas, starting new populations.



MUDSNAILS IN WYOMING

Since the discovery of mudsnails in Wyoming in the 1990s, new populations have appeared in isolated locations throughout the state. Eric Hansen, Wyoming Game and Fish Department aquatic invasive species specialist in the Casper Region, was among the first to discover mudsnails in the North Platte River. Within the last few years, Hansen has observed their spread and cyclical life history pattern.

Flaming Gorge Reservoir in the Green River Region has one of the largest observed populations of New Zealand mudsnails. In this case, a member of the public discovered the snails on a beach in 2021. The snails had not been present in samples from previous years.

“We've seen the boom and bust where you can go some years and they're on every surface at the shore, and you go back a couple years later and you have to search pretty hard to find even one or two,” Hansen said.

Hansen has found mudsnails thriving in some areas of Wyoming, particularly where environmental conditions are consistent, as well as near popular launching points for watercraft. “One continuing factor I noticed with the largest populations we have on the North Platte is they are found in highest numbers at high-use areas,” Hansen said. In some areas of the river, Hansen has found hundreds of individuals by simply flipping over a few rocks near the shoreline.

One area of particular concern on the North Platte River is near Game and Fish’s Speas Hatchery where treated, effluent water from the hatchery is released to the river. The combination of stable environmental conditions and high foot traffic at the fishery near the hatchery creates ideal circumstances for the snails to move in and thrive.

“The initial detection of mudsnails in the river near Speas was what we usually see, just spotty findings here and there. And then within one year, the population increased significantly where fresh, oxygenated, nutrient-rich hatchery water was pouring into the river,” Hansen said.

Safeguards are in place to keep the snails from entering the hatchery. The discovery of snails within the facility could prompt an extensive eradication process, which would impact the hatchery’s operations and fish stocking across the state.

Discoveries occurring in high-use areas and in drainages across the state indicate the snails are being transported by humans as opposed to natural dispersion. As is often the case with aquatic invasive species, once a population is established it is difficult to remove.

“Based on what we’ve seen in other states and here in Wyoming, mudsnails would be pretty hard to eradicate. In flowing waters or even a reservoir, I don't think eradication would be a feasible option,” Hansen said.

PREVENTION IS KEY

To help stop the spread of existing mudsnail populations and prevent new populations from forming, Game and Fish continues to monitor waters around the state and informs the public when new populations are identified. The department strongly urges anglers to thoroughly clean their boots and other equipment before going to another location. Boot brush stations have been installed at a limited number of public access points to help with this effort, but ultimately it is the angler’s responsibility to find a way to clean their gear.



An angler’s first step should be to remove all mud and plants from gear, either through rinsing or brushing the material with a stiff-bristled brush. Allow gear to dry completely in low humidity for at least 48 hours. Gear also can be frozen for a minimum of four hours at 26 degrees Fahrenheit or soaked in 120-degree Fahrenheit water for a minimum of 10 minutes. Using hot water is not advised for Gortex. Formula 409 disinfectant spray or Virkon can be effective in killing mudsnails if gear is exposed to the chemical for a minimum of 10 minutes, but chemicals can deteriorate equipment and may have unintended environmental consequences. Bleach is not recommended to kill mudsnails. All equipment and gear including waders — especially those with felt soles — fishing rods and tackle boxes, drift boats and anchors should be cleaned and dried between uses at different locations.

At the end of the day, prevention is the best option for protecting Wyoming’s natural resources and Clean, Drain, Dry remains the most effective method for keeping aquatic invasive species out of the state’s waters.

— Stephanie Estell is a Game and Fish aquatic invasive species specialist in the Laramie Region.