Meet Cymothoa exigua aka the tongue eating louse. This parasite enters fish through the gills, and then attaches itself to the fish’s tongue.
Using it’s claws blood is extracted from the fish’s tongue causing it to fall off. It then replaces the fish’s tongue by attaching its own body to the muscles of the tongue stub. This becomes the fish’s tongue and feeds off it’s blood and mucus.
While it sounds horrifying and I don’t know what that fish feels when you’ve got a parasite that’s replacing a body part, these things are harmless to humans and are safe to consume.
If you are interested in a what-if scenario where these things infect humans, the 2012 found footage horror film The Bay depicts a plague of these things that spirals out of control.