I was walking the Ohio Erie Canal towpath in the evening near my home in September 2011, when I spotted this young American Bullfrog at the edge of the path. Since there were a number of cyclists out, I decided to move the frog across the path, so it wouldn’t get hit.
When I picked up the little amphibian, I noticed that it held out its front leg in an awkward way. I thought maybe it had been injured. But then I noticed that it had extra legs. The cause of this is a flatworm known as a trematode. The flatworm starts out by parasitizing a snail. The snail serves as an “incubator” for flatworm larva. Eventually they the larva leave the snail and seek out tadpoles. By penetrating the tadpole’s skin, the resulting inflammation of the skin and tissue causes extra legs to grow, making it easier for the flatworm’s final host, a wading bird, to catch the frog and thereby complete the flatworm’s life cycle. You can learn more about this (and see a cool illustration) at Green Museum.org’s website.
The use of fertilizers leads to runoff getting into the water system. This causes an overabundance of algae growth, resulting in more trematode-infected snails that eat it, and ultimately, more frog deformities. In addition, chemicals in agricultural runoff are widely thought to weaken the immune system of frogs and other amphibians.
I teach a community wildlife drawing class for kids and I decided to keep the frog, which I named “Seis,” because it offered an interesting opportunity to explain a little-known phenomenon. Here are a couple of my students drawing Seis’ relatives on “Amphibian Day.”
Sies now lives in a plastic aquarium and seems to be doing fine for the past five months. He eats well, but does not have much control over his “extra arms.” I’d be interested in knowing if anyone has had long-term success maintaining multi-legged frogs.
