Extra legs sprouting on frogs, first observed in North America in the 1990s, were found to be caused by the parasitic flatworm Ribeiroia, writes Carl Zimmer:
Ribeiroia starts out life in snails. It grows and reproduces inside the snails, which it castrates so that they don’t waste time on making eggs or looking for a mate. In its castrated host, the parasite produces a new generation of flatworms that can escape the snail and swim in search of a vertebrate host. They typically infect fish or tadpoles. When they invade tadpoles, the parasites bury themselves in the tiny buds that will eventually grow into legs.An apparent increase in the rate of these deformities may be due to increased concentrations of pesticides in the ponds in which the frogs live. Pesticides can kill off the parasites but they also lower the defenses of the frogs, which may lead to higher infections.
A paper in Nature suggests another factor in the success of leg-deforming parasites: declining biodiversity. In ponds with 'high' biodiversity – up to six species of amphibians – the parasites do much worse at getting transmitted than in low diversity ponds. This is not a small difference: there is a 78.5% decline in deformed frogs in high-diversity sites.