This fish has an unwelcome guest—a creature is sucking blood from its tongue! The animal is a tiny crustacean called an isopod belonging to the genus Cymothoa. It and all the other species in this group are bloodsucking parasites. These organisms live in or on other organisms, stealing nutrients from their hosts.
Cymothoa males draw blood by attaching to a fish’s gills. A female, though, crawls into the gills, up a fish’s throat, and into its mouth. There, the parasite uses its 14 legs, each with a hooked claw, to latch onto the fish’s tongue. The isopod “has a soda straw structure in its mouth to drill a hole in flesh and draw blood, like a mosquito,” says Richard Brusca. He’s an evolutionary biologist at the University of Arizona who has studied Cymothoa for more than 20 years.
The process is even more gruesome in one particular species, Cymothoa exigua, says Brusca. As it slurps blood from a fish’s tongue, the organ slowly shrivels down to a stub. C. exigua remains attached and acts as the fish’s new tongue. This keeps the fish alive, allowing the parasite to continue to feast.
A fish’s mouth also provides the female Cymothoa with protection from predators while her young grow inside a pouch on her body. When the tiny juveniles are old enough, they’ll swim away in search of fish to feed on. “It’s fascinating to think oceans, lakes, and rivers are filled with these creatures that live by sucking blood from fish,” says Brusca.