September 22, 2009—You might call it a teenage mutant ninja frog—minus the ninja.
In northern California, foothill yellow-legged tadpoles are developing into juvenile frogs with missing legs and eyes, like the animal seen above in 2008. These deformities are possibly caused by outbreaks of an alien parasite from Eurasia that usually attacks fish used in aquaculture and the aquarium trade, a new study says.
Anchor worms flourished in the South Fork Eel River in 2006 and 2008, when heat waves pushed maximum weekly average water temperatures to 75 degrees Fahrenheit (23.7 degrees Celsius), researchers found. (See map.)
Those years were also drier than normal, corralling tadpoles into smaller pools and making them sitting ducks for parasites.
Once the parasitic crustaceans enter the gills of a tadpole victim, they feed off the tadpole's tissues until male and female parasites mate. The male dies soon after. Meanwhile the fertilized female bores her way partially out of the tadpole.
The female worm's head and part of its body then morph into an anchor-like growth attached to the base of the tadpole's tail or to its legs. The parasite's egg sac is attached to this external growth, but part of the female parasite remains inside the tadpole's body and damages internal tissues.
(Related: "Pesticides, Parasite May Cause Frog Deformities.")
It's unclear whether this damage is responsible for the young Eel River frogs' striking defects, said study co-author Alessandro Catenazzi, a postdoctoral researcher at the University of California, Berkeley. But the team has found that many deformed frogs are infected.
The researchers also noticed that the most common malformations occur in the hind limbs—and in some cases the legs are missing altogether. Missing eyes are rare, he said, with only two cases recorded so far.
Infected frogs also tend to be smaller, making them weaker and less likely to survive the winter, according to the study, led by UC Berkeley's Sarah Kupferberg and published online in August in the journal Copeia.
Throughout California, dam construction and river flow management for irrigation, among other factors, have limited the yellow-legged frog's habitat, leading to a "dramatic decrease" of about 54 percent of the frog's native habitat, Catenazzi said. The frog historically was found from northern Oregon south to the San Gabriel Mountains near Los Angeles, as well as higher elevations in the Sierra Nevada.
The anchor worm is yet another blow to the vulnerable amphibians, the authors say, and climate change is expected to spur more parasite-favoring heat waves in California in the future.