The tongue-eating louse is a prime example of nature’s affinity for body horror

The tongue-eating louse is a prime example of nature’s affinity for body horror

Nature might be famously red in tooth and claw, but in my opinion if you want to get to the really gruesome stuff you ought to forgo predation entirely. Sharks and lions and etc. get the headlines, but they’re honest in their appetites: they kill their prey pretty fast and eat it pretty fast, and you probably can’t ask for much more than that. Parasites, on the other hand, know how to be truly, viscerally awful.

The great biologist E.O. Wilson famously described parasitic behavior as predation ‘in units of less than one,’ which is both cute and insightful but I think completely fails to capture the various horrors involved. To get you in the proper frame of mind, here’s a short sampling of parasites I find particularly interesting:

  • Pepsis grossa: The tarantula hawk (technically a parasitoid) possesses perhaps the most vicious sting of any wasp. It uses this to paralyze tarantulas. Once its victim is subdued, the tarantula hawk drags the spider back into its own burrow and lays an egg on its body. When it hatches, the wasp grub slowly eats the spider alive, pupates once its long meal is over and emerges as an adult. The tarantula hawk is the state insect of New Mexico, a fact which feels significant but I can’t quite work out of what.
  • Leucochloridium paradoxum: A parasitic flatworm whose lifecycle spans two host species: birds and snails. In birds, the worms breed inside their body, laying eggs in the gastrointestinal tract which are then evacuated in the birds’ droppings. Some of these are then eaten by snails; whereupon they hatch, grow, and hijack the snail’s brain. Eventually the flatworms move into the snail’s eyestalks, making them look like wiggly, juicy caterpillars, and encourage the snail to display itself somewhere conspicuous. If a passing bird takes the bait, the flatworm life cycle can begin anew.
  • Sacculina carcini: A parasitic barnacle which exhibits a behavior known as ‘parasitic castration’. Many animals spend significant amounts of energy on reproductive behavior, so if a parasite can redirect that energy to their own needs, they have the free-est of free rides. Sacculina latches onto a crab, sterilizes it — the process is too complex to go into in a bullet point — and then diverts the nutrients which would have been going to crab reproduction to itself. Yummy.

It’s in parasites where we see evolution at its most intricate, and where I think the true story of nature unfurls. I hope the above have whetted your collective appetite for more parasite stories. They’re horrible, horrible creatures (and sometimes they’re not even ‘creatures’), but I find them absolutely fascinating. Today I’m going to talk to you about one of my favorites: the tongue-eating louse.

Juvenile Cymothoa exigua (which is not a louse at all but a type of isopod) are free-floating swimmers which look for gills of specific types of fish, latch on, and enter the body. There, they move up to the fish’s mouth and carefully sever the blood vessels supplying its tongue. Starved, the tongue dies and falls off, and the louse, still inside the fish’s mouth, replaces it entirely, stealing nutrients by taking pieces of whatever the fish is eating and taking some of the fish’s blood as well.

And yes, this does look very weird:

Mondays aren’t usually this eventful. I found a tongue-eating isopod (purple) in one of our wrasse scans this morning while digitizing it. These parasites attach themselves to the tongues of fishes and effectively become the new tongue…horrifying #backdatwrasseup pic.twitter.com/axlraUrh8W

— Kory Evans PhD (@Sternarchella) August 10, 2020

It’s sort of fun to imagine the evolutionary steps which must have taken place to cause this specific behavior. Obviously, gastro-intestinal parasitism is common in fish and everything else, but we don’t see tongue-severing elsewhere. What happened here?

Here’s my best guess: the tongue-replacement is actually a benefit to the fish. When Cymothoa removes its host’s tongue, it doesn’t just replace the thing spatially, but it performs the functions one would expect of a fish tongue. It’s well matched to its hosts in shape and size, and can be used in more or less the same limited way as an actual fish tongue.

To me, this suggests some evolutionary pressure on fish oral parasites to become less damaging to the fish. All fish have some defense against parasitism, some internal, some involving elaborate mutualistic relationships with other organisms. A large isopod living in one’s mouth, stealing resources and doing nothing useful might be a prime target for removal. But one that is literally functioning as one’s tongue? Irreplaceable. Over generations and generations, these things must have been getting gradually more tongue-like, more selective of the exact blood vessels to nom their way through, and … yeah, increasingly more horrific.

As I mentioned in a previous article about the bird-eating tree species Pisonia, nature can be beautiful, but it’s also frequently uncaring to degrees both profound and grotesque. If there’s a nature-buck to be made somewhere, evolution will have tried it, and in parasites … well, that can get pretty ugly.

Be glad you’re not a fish, I guess?


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