Dwindling frog populations have worried biologists for decades, but a rash of frogs sporting extra limbs unnerved them even more. Suspected causes of the deformities, first observed in the early 1990s, included chemical pollutants and increased ultraviolet radiation from ozone depletion, but new research fingers an unexpected culprit: tiny flatworm parasites.
Starting in 1996, Stanford undergraduate Pieter Johnson began tracking the high numbers of deformed Pacific tree frogs in four areas in northern California. He noticed that the frogs shared these habitats with a type of aquatic snail that plays host to parasites called trematodes. In the lab, Johnson exposed Pacific tree frog tadpoles to two species of these parasites and found that one--the Ribeiroia trematode--caused the same deformities he had seen in the field. Eighty-five percent of the frogs were either missing limbs or had extra or deformed ones. Apparently, the trematodes burrowed into the tadpoles' hind limb regions and physically or chemically altered the legs' growth.
Although in this case the trematodes were directly to blame, Johnson doesn't think parasites are the reason frog populations are diminishing worldwide. "This is not the smoking gun of amphibian decline," says Johnson, who has graduated and become a researcher at Roberts Environmental Center of Claremont McKenna College. He is still looking for the reason trematodes suddenly became so numerous, and suspects that man-made pollutants may be the cause. "It could be that something is causing parasitic populations to rise," says Johnson. "And that factor could be of concern to humans."