Cymothoa, the family of infamous tongue-eating lice, with Cymothoa exigua (pictured here)being best known.
C. exigua will crawl into their host through their gills, settle down at the base of the fish’s tongue, then use her sturdy claws to pierce and extract blood from the base and eventually the tongue will waste away due to lack of circulation. The parasite then attaches itself to the muscles of the tongue stub and replaces the organ. This way C. exigua can rifle through everything the fish swallows without doing any hunting or scavenging of her own.
The louse acts as a fully functional tongue and the fish doesn’t seem to suffer much because of it.