Extra Frog Legs

People around the world began documenting unusual frogs in the 1700s. Like the frog pictured, they had extra or missing legs. The phenomenon is due to an infection by a parasite called Ribeiroia ondatrae.

R. ondatrae is a flatworm. During its life cycle, the parasite travels between three different host organisms. Newly hatched flatworms first hijack a freshwater ramshorn snail’s reproductive system, where they produce larvae—the immature stage of their life cycle. One-fifth of the snail’s body mass becomes a “parasite factory,” explains Pieter Johnson, an ecologist at the University of Colorado Boulder.

The snail releases thousands of flatworm larvae from its reproductive tract into the water. These larvae burrow into a tadpole’s limb buds—the areas where legs will eventually grow. If the parasite attacks a tadpole before its buds fully develop, the frog is likely to have missing legs. Older tadpoles with more mature buds can grow up to six extra legs—that’s 10 total.

This makes it hard for a frog to hop. That helps the parasite invade its next host—birds. “What better way to slow a frog down than to disable the back legs?” says Johnson. A bird can easily catch and eat the frog. The parasites end up in the bird’s gut, where they mate and lay eggs. The bird excretes the eggs in its poop, and the flatworms hatch to terrorize snails and frogs once again.