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 there, it feeds on the blood supply. As the blood is taken in by the parasite instead of the tongue, the muscle eventually atrophies and dies. The loss of a tongue could be a huge detriment to the fish, making it much less fit, unable to handle its food properly, and possibly even die. 
Of course, a dead host is no good to a fully-grown parasite, as it means a the food stops coming. So Cymothoa has evolved a truly amazing behavior: it acts like the fish's tongue. The fish can control the parasite just like it would its own organ, and thus the crustacean can live and feed off of its host's blood and mucus as long as it lives.
It seems like a nice gesture by the parasite, replacing a lost organ and all. That is, until you think about the fact that the old tongue was totally fine to begin with...
We might use the phrase "cat got your tongue?" when talking to other people, but if we're talking to a fish, " Cymothoa got your tongue?" might be more appropriate.
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