Here’s a riddle: How many legs does a frog
have? That all depends on which parasites infect
it. I’m Anna and this is Gross Science. Throughout North America there’s a parasitic
flatworm, that makes its home in the digestive systems of water birds. A bird’s esophagus is a great place for
these guys—that’s where they find mates, have sex, and pop out thousands of eggs.The
eggs move through the digestive tract, and when the bird poops in freshwater, the eggs
hatch and the baby parasites look for a new home. But finding a new feathered friend isn’t
so easy. The parasite will need to travel through two
other animals and go through four different life stages before it find another bird and
settle down. The first stop is a freshwater snail. When it finds one, the parasite will invade
the snail’s tissues, turn into the next larval stage, and eat away at the snail’s
reproductive organs, castrating it in the process. There it also multiplies asexually, and enters
yet another life stage, turning the snail into a mobile parasite factory. Eventually, these hordes of parasites swim
out of the snail in search of their next host – a tadpole. Once a tadpole is found, the larvae start
to penetrate its tissue focusing on the hind limb buds—that’s where the developing
frog’s back legs will eventually grow. The larvae grow a hard, protective coating
called a “cyst”, and this is where things get really gross. These parasitic cysts interrupt proper limb
formation, causing the frogs to have an unusual number of legs once they metamorphose— anywhere
from zero to ten! And when these froggy monstrosities get eaten
by birds, the parasites finally become adults and the life cycle starts all over again. Now, the thing I find most interesting about
this is that the frog’s weird limb development isn’t just a side effect of the parasitic
infection. Scientists think that causing frogs to have
multiple, or even missing legs, is actually advantageous for the parasite. Manipulating a host’s morphology – in other
words, how it looks – is just one strategy that parasites use to survive. Frogs that have an unusual number of limbs
move more slowly than their four legged counterparts, and that makes them easier for birds to catch. Which of course makes it more likely that
the parasite will end up exactly where it wants to be, inside a bird’s esophagus,
surrounded by mates. And really, isn’t that what we all want
in life? Ew.