Have you ever heard about the Cymothoa exigua? Well if not, you must! It is a parasitic crustacean of the family Cymothoidae.
Ok so this parasite attaches itself at the base of the tongue of the Spotted rose snapper, and feeds off of it. As the parasite grows, less and less blood is able to reach the tongue, and eventually the tongue just withers up and dies. The parasite then replaces the fish’s tongue with its own body, by attaching to the muscles of the tongue stub! [shutter]
Ok so this parasite attaches itself at the base of the tongue of the Spotted rose snapper, and feeds off of it. As the parasite grows, less and less blood is able to reach the tongue, and eventually the tongue just withers up and dies. The parasite then replaces the fish’s tongue with its own body, by attaching to the muscles of the tongue stub! [shutter]
source:http://www.ripleys.com/weird/daily-dose-of-weird-wtf-blog/unbelievable-but-true/real-life-aliens/