I had the great pleasure yesterday of walking Rudy Raff back to his office after he gave a lecture on evolution to an auditorium full of undergraduates. Somehow we got to talking about this monster:

Let me introduce you to Cymothoa exigua, otherwise known as the Tongue-Eater or sometimes as the Tongue-eating louse. This little guy enters the gills of a fish and begins comsuming that poor fish’s tongue (its predator phase). When the tongue has either been wholly consumed or necrotic the tongue-eater attaches itself to the now-empty base where the tongue used to be. Helpfully (!!!), that tongue-eater then allows itself to be used as a new tongue for its host (the parasite phase).
One more photo just to make sure you’re paying attention and to confirm that I’m not making this up:

This species takes control of its host’s organ, consumes it, and then functionally replaces that organ for the duration of its life! Like I said, it is both a predator and a parasite, so once it’s attached and performing tongue-y activities, the tongue-eater and the host live happily ever after.
If I had more time I’d mention environmental specialization and niche exploitation, but I’m sure you’re already thinking along those lines…
*checks own tongue*
HA! I know, right? I’ll try to top this one by finding one who prefers human hosts…
Holy hell! I am fascinated and horrified! I had NO idea this existed. What are your feelings about the “immortal” jellyfish, Turritopsis nutricula? Talk about evolutionary!