X-ray of a deformed frog
X-ray of a deformed frog
For decades we’ve heard of the plight of our environment. After years of chemicals toxins making their way to rivers, and seeping into groundwater there is no doubt some unusual and bad things are arising from human interference in nature, but there is some dispute on just how much human affairs have impacted the eons-old ecology.

In a quest to show the ill effects of humanity’s ill-advised pollution, some environmentalists began trying to show specific problems that these uninvited chemicals were causing. In the late 1970’s, one such example began to gain notoriety: deformed frogs. Because of their thin, permeable skin and the amount of time they spend in still water, the frog is a creature highly influenced by small traces of toxic items in their ecosystem. Pictures of frogs with excessive legs, cysts, or other unusual growths have been used to show the masses what our pollution is costing other occupants of Earth. But are these bizarre deformities the result of the intervention of people, or old Mother Nature pulling one of her sneaky tricks making the frog into easy prey?

Ribeiroia is a parasitic flatworm that generally infects birds and snakes in their adulthood, but have a rather complex life cycle that took several years to untangle because of their propensity to pass through several hosts. Most of the time parasites in the Trematoda family lay eggs within their host, and those eggs are expelled in the host’s feces. When the eggs come in contact with fresh water they hatch into free swimming larva which infect a snail. The snail develops a cyst where the larva has infected it, and after an incubation period the larva busts out, now matured into a larger form with a tadpole-like tail to propel it. Most of the time the larva will seek out a mollusk or arthopod to infect, where it will wait for this host to be eaten by the heron or garter snake that it wants as a permanent home.

The ingenious and insidious part is when the larva emerges from its snail-host and cannot find a mollusk that will deliver it to the actual host to which it aspires. If the tadpole-like larva has the opportunity, it will infect a real tadpole. The parasite will burrow into the larger, amphibious tadpole—usually in the buds from which a frog’s legs will eventually form.

It is in Ribeiroia’s best interest to make sure that the frog that it infects will be caught and eaten by the much coveted heron or garter snake, thus it encysts the limb buds. The parasite causes the leg not to grow, or grow deformed. However if the parasite misses its target the leg will grow normally, and the frog might be too elusive to be caught, thus the parasite falls on a backup plan. The parasite causes extra legs to grow that will inevitably make the frog easier pickings for the host-species to which the parasite truly aspires.

And thus the parasite finally reaches the climax of its bizarre and convoluted life-cycle. It turns out the extra legs often spotted on a small amphibian is just another quirk of nature at work, and not the result of humanity's polluting influence.

Article suggested by Millgate.

Written by Jason Bellows, posted on 26 June 2006. Jason is a contributing editor for DamnInteresting.com.
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