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"
This is honestly shit for both of them, sure getting your tongue eaten probably isn't great but could you imagine if your whole life was just pretending to be a tongue living inside a fish's mouth??