Legless frogs mystery solved

.Scientists think they have resolved one of the most controversial environmental issues of the past decade: the curious case of the missing frogs' legs.

Around the world, frogs are found with missing or misshaped limbs, a striking deformity that many researchers believe is caused by chemical pollution.

However, tests on frogs and toads have revealed a more natural, benign cause.

The deformed frogs are actually victims of the predatory habits of dragonfly nymphs, which eat the legs of tadpoles.

In the late 1980s and early 1990s, researchers started getting reports of numerous wild frogs or toads being found with extra legs or arms, or with limbs that were partly formed or missing completely.

The cause of these deformities soon became a hotly contested issue.

Some researchers believed they might be caused naturally, by predators or parasites.

Others thought that was highly unlikely, fearing that chemical pollution, or UV-B radiation caused by the thinning of the ozone layer, was triggering the deformations.

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"Deformed frogs became one of the most contentious environmental issues of all time, with the parasite researchers on one side, and the 'chemical company' as I call them, on the other," says Stanley Sessions, an amphibian specialist and professor of biology at Hartwick College, in Oneonta, New York.

"There was a veritable media firestorm, with millions of dollars of grant money at stake."

After a long period of research, Sessions and other researchers established that many amphibians with extra limbs were actually infected by small parasitic flatworms called Riberoria trematodes.

These creatures burrow into the hindquarters of tadpoles where they physically rearrange the limb bud cells and thereby interfere with limb development.

"But that was not end of the story," says Sessions.

"Frogs with extra limbs may have been the most dramatic-looking deformities, but they are by far the least common deformities found," he explains.

"The most commonly found deformities are frogs or toads found with missing or truncated limbs, and although parasites occasionally cause limblessness in a frog, these deformities are almost never associated with the trematode species known to cause extra limbs."