parasitic louse (crustacean family) that attacks fish. Enters fish through gills, then attaches itself to base of tongue. Louse restricts blood flow to tongue, feeding off it, causing the tongue to wither and die where it then takes its place. Then it lives in the fishes mouth, working as a fully functioning tongue and living off its blood and saliva. Apparently this does not harm the fish and is the only known case of a parasite replacing a whole organ.