Cymothoa exigua, or the tongue-eating louse.The parasite severs the blood vessels in the fish's tongue, causing the tongue to fall off. It then attaches itself to the stub of what was once its tongue and becomes the fish's new tongue.
Hatchetfish are deep-sea fish found in the Pacific, Atlantic, and Indian oceans. They use a form of bioillumination to mimic light intensity from the surface, effectively cloaking it from poor-sighted predators. Creepy, but cool.