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Pacific Tree Frogs are Sprouting New Limbs

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No, it isn’t from radioactive waste: research conducted by a CNR ecologist indicates that the cause of these mutations is a trematode parasite called Ribeiroia.

A tiny parasitic flatworm, Ribeiroia ondatrae is first hosted by aquatic snails, and then released at night essentially to melt through the skin of tadpoles. As these tadpoles grow, they typically exhibit extra, missing, or malformed limbs. And if the parasite itself does not kill the frogs, the mutations it can cause often make it much more difficult for them to escape birds and other predators.

Kevin Lunde, a graduate student in the Department of Environmental Science, Policy, and Management, has been conducting research on frog mutations since 1998. He has found that almost 50 percent of Pacific treefrogs at the Hopland Research and Extension Center were affected by Ribeiroia. Although some Ribeiroia in nature is harmless, current research and museum records indicate that it is becoming increasingly common. In a paper Lunde co-authored in a 1999 issue of Science, researchers found that fewer than 2 percent of infected frogs in a given pond reach sexual maturity, creating a sharp decline in frog populations.

The question remains whether this increase in Ribeiroia abundance can be attributed to humans. Lunde says that although wide-scale tests have not been conducted, the evidence he’s seen suggests that humans are playing a significant role. Human addition of nutrients such as nitrogen and phosphorus into the frogs’ habitats causes an increase in algae, which in turn creates booming snail populations and more hosts for Ribeiroia. “Wetland loss is another contributing factor,” he says. “Over 90 percent of California’s wetlands have been lost to development. This forces water birds who have consumed the infected frogs onto eutrophic land,” where nutrient-rich waters offer perfect conditions for the further spread of Ribeiroia.

The problem is not limited to treefrogs. “This parasite causes limb malformations in numerous species of amphibians across the western United States,” says Lunde. His continuing investigations will focus on the roles that light, temperature, and nutrients play in expanding populations of Ribeiroia'ssnail hosts.

-Stephanie Ludwig


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