NOVA put together a video, embedded below, about one of those animals that you have to keep persuading yourself is real, a parasitic crustacean that lives inside the mouths of fishes, eating–and then taking the place of–its host’s tongue.

I can vouch for these beasts, having written about them off and on since I first encountered them in my research for Parasite Rex—most recently on the Loom last year. But I was not aware that it’s the female that wins the Oscar for best performance as a fish tongue. The males just hang out around the gills of the fish and then–yep–mate with the pseudo-tongue.

View Images Clownfish with parasitic isopod. Photograph by Lea Lee / flickr

This discovery led me to wonder about the latest research about tongue-eating isopods. I came across a 2012 master’s thesis by Colt William Cook of the University of Texas, which confirms what you see in the video–that the parasites are born as males, and then when they enter a fish, one turns female. This switch only occurs if there’s no female already installed in the host–otherwise, the males stay male. As this transformation takes place, Cook adds, the female’s body grows enormously. Its eyes shrink, since it no longer has to hunt for a home. Its legs stretch out, to help it anchor itself in the mouth.

View Images Photograph Courtesy of Matthew Gilligan

After one of the males mates with the female, she gives birth to a brood of live male parasites. For their first few days, Cook found, they search madly for another host (each species of parasite seems to only live in a single species of fish). They sniff for the scent of their host, and if a shadow passes overhead when the odor is strong, they shoot upwards through the water. They burn through a lot of energy in the process; if they fail to find a host in the first few days, they settle down and hope they can ambush a fish that happens to be swimming by. It’s a hard way to start your life, and it may explain why several males will huddle inside a fish with only a single female in the offing. Looking for another fish with a single female parasite might be a less promising strategy than competing with the males you’re with.