Invasive snails on the run leave DNA evidence behind

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DNA sleuthing helped scientists discover early outcrops of an expanding snail invasion in Pennsylvania. The method can be used to track down the unwanted critters before they do serious damage. This way, conservationists can prevent them from conquering even more places.

New Zealand mud snails became a global pest in part because they can reproduce asexually — just one snail in a new area can essentially clone itself until there are 500,000 snails in a single square meter. They are about the length of a pencil eraser so that they are hard to find until there are so many in one location that it is nearly impossible to stop their spread.

“It’s kind of weird because as an invasive ecologist I never really want to find this thing,” said Edward Levri, a professor at Pennsylvania State University-Altoona and senior author of a new study. study published this week in the magazine Biological invasions. “It’s kind of an exciting feeling. It opens the door for us to be able to use this technology to detect the snail on a much larger scale.”

Levri and his colleagues successfully used DNA present in the environment (what researchers call environmental DNA or eDNA) to track down the tiny snails. But for the past decade, Levri has spent a lot of time looking for the little rascals by turning rocks and casting nets. “It’s a tedious process and success is very limited,” he says The edge.

So this time, he teamed up with lead study author James Woodell to collect water samples from eight different Pennsylvania locations and search them for the snails’ DNA. Just as humans shed skin cells in the shower, snails shed DNA into the water. The researchers used a technique called a polymerase chain reaction, or PCR, to amplify small amounts of snail DNA in water samples. They use a dye to make the DNA glow so that it can be spotted with a special machine.

Biologists led by the University of Iowa used a special technique called eDNA to discover an invasive species of tiny snails in streams in central Pennsylvania, where the snails’ presence was unknown. The invasive New Zealand mud snail has spread to the east coast after arriving in the western United States decades ago.
Image: Edward Leviv

They eventually found eDNA from the mud snails at five of the eight locations they sampled. Levri has since been able to go back and find a real New Zealand mud snail at one of those locations. At the other locations, he hasn’t found the snails yet — he hasn’t been able to be out in the field as much as he’d like because of the pandemic. While there’s a possibility that some of the eDNA was floated there from other locations, Levri and co-authors hope the state will consider the eDNA findings as enough evidence to take action.

“From a conservation standpoint, that’s enough to say, ‘Hey, we really need to be more careful with these sites,'” said Maurine Neiman, an associate professor of biology at the University of Iowa and another author of the study.

The snails are small enough to hitch a ride on fishing gear undetected, which is one of the ways they’ve spread across the US and other parts of the world (they can also travel on boats). The state could put up signs warning people fishing in these locations to take extra precautions, such as cleaning their waders with certain disinfectants before visiting a new fishing location (if you put them in the freezer overnight, you’ll achieve the same).

The New Zealand mud snail was first found in Pennsylvania in 2013 near Spring Creek, and now there are “millions,” according to Levri. That’s bad news for the fish in the area and the people who love to catch them. The snails are difficult to digest and have been known to emerge alive from the other side of a fish after being eaten. Because they are a poor source of nutrition, studies have shown that fish that eat them lose weight. The snails also displace native snails and aquatic insects. In some cases, the invasive snails make up 90 percent of a site’s invertebrate biomass. That’s “amazing for just one species to do,” Levri says.

“These snails are small, but they seem to have the potential to have a really big ecological impact on these invaded ecosystems,” Neiman says.

eDNA could help researchers find the snails a year or more earlier than they otherwise would. That gives conservationists crucial time to stop the snails before people unknowingly give them a lift to other, snail-free locations. Conservationists haven’t been very successful in getting rid of the snails once they’ve appeared, Levri says, so that’s all the more reason to stop their spread in the first place.