An isopod, and not a louse, it has an odd way of making its living. The way of the tongue-eating louse goes like this- in its early stages of life as a "manca" (a tiny version of an adult) it swims along until it comes upon a suitable fish, in this instance a weaver fish. Entering the fish's gills it works its way to the tongue, where it uses syringe-like mouthparts to begin feeding off of the artery that supplies the tongue. As it does so it robs the tongue of nutrients causing it to atrophy into no more than a nub. Are you ready for this- it latches onto this nub and literally becomes the tongue of the fish. The fish uses it as it would its normal tongue to manipulate food entering its mouth, which is convenient for the little isopod as it shares in the feast.
Photo source: http://www.discoveryon.info/2009/09/rare-tongue-eating-parasite-found.html
Other photos: 1, 2, 3, 4