»Grossest Animal in the World: Tongue-Eating Louse!«

A diner preparing a Red snapper for lunch found something rather nasty inside the mouth of the fish.
Concerned at what they'd discovered inside the snapper's gob, the diner contacted Lewisham Council's Environmental Health Department who enlisted the help of experts at the Horniman Museum in Forest Hill, London.
Keeper of Natural History at the Horniman, Dr Jim Brock, identified the freaky creature as a type of parasitic crustacean called Cymothoa exigua.
This "tongue-eating louse" is actually a type of isopod and Brock says it's believed to be the only creature known to eat and replace the organ of its host.
Dr Brock said: "I have not seen this in all my 13 years at the museum so it's a remarkable find.
"The tongue louse enters through the fish's gills, using claws to attach itself to the base of the snapper's tongue and survives by drinking from an artery which supplies its blood.
"Eventually the tongue is reduced to a stub. However, the parasite is now large enough to replace the tongue and as it manipulates the fish's food, it also dines out for free on the freed food particles when the fish eats."

Common name: Tongue-eating louse
Scientific name: Cymothoa exigua
Order: Isopoda
Suborder: Flabellifera
Family: Cymothoidae
Symptoms: C. exigua is the only known parasite which replaces the organ of its host. So much blood is removed from the tongue of the fish by the blood-thirsty parasite that the tongue atrophies and shrinks to a stub. The parasite remains in the place of the tongue and is used by the fish in the same way as its tongue was.