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Frog Legs

The next time you visit a local pond or river, you might become disturbingly surprised if you start looking for frogs. "Frog Legs" on this Moment of Science.

The next time you visit a local pond or river, you might become disturbingly surprised if you start looking for frogs. You may find that some tadpoles or young frogs have extra or deformed legs. Since the mid nineties, scientists have reported widespread deformities in over sixty species of frogs worldwide.

In recent years, the most likely culprit has emerged as a parasitic epidemic targeting frogs and other amphibians. Microscopic parasites called flatworms develop in snails as larvae, then swim free and seek out tadpoles, lodging near the hind legs. The parasite then forms cysts that disrupt normal leg development. Consequently, the tadpole grows extra legs as it develops.

The deformed frog then becomes easy prey for birds, which then inherit the parasite. The parasites mature inside the frog-eating birds, reproduce, and lay eggs which are deposited by bird feces back into the water. And so the cycle begins again, allowing the parasites to infect successive generations of frogs.

Researchers attribute the parasite epidemic to altered habitats such as reservoirs and artificial ponds. Pollutants often cause algae blooms, providing more food for parasite carrying snails. The more snails, the more parasites, which means more infected frogs with extra limbs.

Although pollutants and other environmental stresses may play a role, human alteration of natural habitats has made life difficult for frogs worldwide. It remains to be seen how such alterations harm other animals, including humans. For now, frog deformities may be taken as a dire warning.

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