What zombie snails and teenage mutant frogs tell us about ecosystems

The first mutant frog the kids found probably seemed like a sad fluke. The poor Northern Leopard Frog had one normal hind leg and one frail, fleshless one. But, then the class, which was out on a nature walk in 1995, found another misshapen frog—this one with only one leg—then limped another, and another. Half of the frogs in the southern Minnesota pond were mutants.

The frogs got national news coverage after that and scientists added the pond to the list of mutant hotspots. It was a growing list amid dwindling amphibian populations. Reports of creepy croakers came in from all across the West and Midwest during the mid-90s, and speculation of the cause ranged from pesticides to UV radiation.

Spidey, the Pacific chorus frog (Pseudacris regilla) that has extra hind limbs due to infection with a trematode parasite (Ribeiroia ondatrae, left).
Photo courtesy of D. Herasimtschuk & P. Johnson

Despite the high profile, ecological mystery, it wasn’t until 2010 that I learned about the disfigured frogs. It was during my first visit to Mendocino county where I met Dan Preston, a disease ecology graduate student from the University of Colorado, Boulder, who was there studying the freaky frogs.

Back in Boulder—where he has an eight-legged Pacific Chorus frog named “Spidey”—he works with ecologist Pieter Johnson, who was one of the scientific detectives that solved the mystery.

A year after the Minnesota pond hit news stands, Johnson hopped to another hotspot for misshapen frogs, Santa Clara County, to study 30 ponds. He and his colleagues discovered that afflicted amphibians were always in ponds that also had snails, which happen to be the first host of a vicious parasite – a tiny, flatworm trematode in the genus Ribeiroia.

Flatworm eggs hatch in water and make their way inside certain freshwater snails, such as the Ramshorn snail. Then, they take over. They feed on the snail’s reproductive tract – castrating their victims – and turning them into parasite factories. A month or so later, the more mature worms abandon their zombie snail-homes and set their sites on fish or amphibians. When they invade frogs, the Ribeiroia trematodes settle in the hind limbs—a telltale location.

“I’ve seen some frogs with 20 or so “limb-like appendages” coming off the back end,” Preston says.

Back in the lab, Johnson and his colleagues found that tadpoles swimming in trematode-infested waters developed the same deformities seen in the ponds. But even now, nobody is sure exactly how the trematodes botch leg development.

“There are two lines of thought: that it causes a mechanical disturbance of cells around the limb bud leading to development problems, or that it secretes some type of chemical compound that triggers haywire cell development,” Preston says. “Personally, I think it’s more of a mechanical thing than a biochemical process.”

Regardless of the means, the resulting gimpy frogs probably help the trematodes complete their nefarious plot; the limping croakers are easy targets for predators like birds and mammals – the trematode’s final host. Inside, say a bird, the trematode happily spews eggs into the bird’s feces, which could land back near a pond for the cycle to start again.

A California red-legged frog with an extra hind foot. This species is listed on the
endangered species act. Photo courtesy of D. Preston

Although the mystery might be at rest, the plight of mutant amphibians has only raised more questions, such as “Is this new?” and, “Is it getting worse?” Though Johnson and his colleagues have pieced together historical records of unusual amphibians—some dating back 200 years— it’s tricky to say if amphibian malformations caused by tretmatodes are new. But from what they can tell, they do seem to be increasing.

The boom in buggered pond-life has led Johnson and Preston to start dissecting how the trematode is triumphant and what it might mean for the rest the pond and beyond. At Hopland, Preston was trying to understand how changes in snails matter. After all, they are the first host and their populations might increase with nutrient pollution from urban and agricultural runoff. When I met him in 2009, he had set up rows and rows of black, plastic mini-pools, the size of small bathtubs, in a field at University of California’s Hopland Research station. In each mock pond, he had different types of snail communities, some with just the snails that could become zombie, parasite-factories, and others that had a mix of snails, some resistant to the wicked worms.

He and Johnson just published the results in the journal Ecology, which show that if snail populations were whittled down to just the susceptible species—low biodiversity—the trematode was in welcoming waters. When they compared their results to 320 ponds around California with snail and trematode populations, they found that the snail communities in natural ponds mirrored those in the experiment, suggesting that snail diversity might be able to reduce amphibian deformities.

But snail populations aren’t the only link to leggy hoppers. Johnson and Preston are also looking into how sitting-duck frogs change the pond’s food web and colleagues are examining the impacts of global warming on trematode populations.

In the meantime, amphibian populations are still sinking and malformed frogs have bleak outlooks. Spidey, however, is doing just fine.