Cymothoa exigua, or the tongue-eating louse, is a parasitic isopod that severs the blood vessels in a fish's tongue, causing the tongue to fall off. It then attaches itself to the remaining stub of tongue and the parasite itself effectively serves as the fish's new "tongue"
Those things are so bizarre. They latch onto the fish's mouth, severs the vein to its tongue so the tongue falls off, and the parasite becomes the new tongue. Apparently the fish functions just fine with its parasitic tongue.
I mean, if the fish didn't survive, the parasite wouldn't get easy meals for life, haha.