It's like weird Parasites
1. Bit of a mouthful
This tongue-eating louse enters the gills of a fish, latches on to its tongue and sucks its blood. The fish’s tongue eventually shrivels up and drops off, allowing the 40mm long louse to replace it with its own body.
The louse fiendishly attaches itself to the remains of the tongue muscles, and shares every meal with its victim, but seems to work just as well as the original tongue.