Correction: Correction published 5/1/99 follows: PHOTOGRAPHS OF A TADPOLE AND A FROG ACCOMPANYING AN ARTICLE ON FROG DEFORMITIES YESTERDAY SHOULD HAVE BEEN CREDITED TO STEVEN HOLT/STOCKPIX.COM.

A common parasite can cause limb deformities in frogs, making it a prime suspect for the malformed amphibians that have been discovered around the country in recent years, researchers reported yesterday.

Field surveys and lab experiments indicate that small parasitic flatworms known as Ribeiroia trematodes can infect tadpoles, causing their legs to grow abnormally, according to a report in today's issue of the journal Science.

Frogs with missing, deformed and extra legs have been reported throughout the United States and in parts of Canada, raising concerns that they may be a harbinger of a serious environmental problem. The cause has been a mystery, but researchers have been investigating a variety of possibilities, including pollutants or exposure to ultraviolet light because of ozone loss. But scientists have always suspected that some natural cause must account for at least some deformities because abnormal limbs have been observed in amphibians for hundreds of years.

While studying at Stanford University, Pieter Johnson and several fellow students identified two trematodes that appeared to be associated with high rates of limb abnormalities. When they exposed Pacific tree frog tadpoles in the lab to the same parasites, one of the trematodes formed cysts at the bases of developing legs that caused the same kinds of deformities.

Johnson's experiment confirms a link between parasites and amphibian abnormalities first proposed almost 10 years ago by Stan Sessions, a biologist who also studied deformed Pacific tree frogs in Northern California.

Johnson said it is unclear whether parasites interfere mechanically with developing limbs or in some way influence the chemical signaling that normally regulates limb development. Sessions, now at Hartwick College in New York, argues for mechanical disruption in a companion article in today's issue of Science, but this view is not widely shared among other scientists.

Sessions also examined extra legs in frogs from several West Coast sites -- most of them animals he collected in the late 1980s -- and determined that their structures were not consistent with experimental evidence suggesting that a class of chemicals called retinoids might cause such defects.

Johnson called the trematodes a "cosmopolitan" parasite commonly found across North America. "It certainly merits further study in other parts of the country," he said.

Mike Lannoo, U.S. coordinator of the Declining Amphibian Populations Task Force, agreed that parasites need careful evaluation wherever deformities are being reported but emphasized that little is known about the distribution of trematode parasites or the vulnerability of amphibians to infection. He said his own comparisons of frogs from Minnesota with frogs from Johnson's sites using high-resolution radiographs showed different bone malformations that suggest different causes for the deformities in the two locations.

Other researchers echoed praise for Johnson's work while saying the cause of deformities in different species at other locations is still an open question.

David Hoppe, a herpetologist investigating malformed frogs in Minnesota, reevaluated frogs from one of his sites where the deformity rate is so high that the whole population appears to be in jeopardy. He failed to find any correlation between trematode cysts and deformities but said nothing could be ruled out at this point.

"I seriously doubt that any one cause will explain all that is going on," said Hoppe. "I haven't seen anything yet that explains even one Minnesota site."

Jim Burkhart, a biochemist with the National Institute of Environmental Health Sciences, which is working with the Minnesota researchers, said his group has narrowed its investigation to a handful of chemical contaminants that have been shown to cause limb defects in the lab. Burkhart said those results had been achieved using doses comparable to what they found in the environment.

Cutline: A tadpole, above, that has been experimentally exposed to the parasitic flatworm Ribeiroia grows two extra limbs.

Leg deformities such as the ones exibited in this Pacific tree frog have led to new concerns about pollution.