Parasitic Crustacean
What could be more sci-fi than a case of replacing body parts? Sure, we might figure to do it with machines or cloning instead of another species, but fish don't get that luxury. Snappers have a wonderful parasite that works with them, to a point, to replace a part of their bodies.
Of course, the parasite - a crustacean (Cymothoa exigua) - first causes the part it's replacing to die - so it's not exactly a voluntary procedure. And what part does this lovely parasite replace?
The tongue.
Cymothoa exigua enters the fish's mouth through the gills and latches on to the base of the tongue. From…