I’ve caught a few freshwater fish with leeches, and seen a few pictures from friends of Musky with large ulcers, but I’ve never seen anything like this. One things for sure the next time I catch a fish I’ll be looking in their mouth for one of these buggers. You can find more information and pictures on the Australian Museum Fish Site.
Cymothoa exigua, a crustacean, is the only known parasite that effectively replaces a body organ. It makes its home in the mouth of a fish, where it drains blood from the tongue until it withers and dies. It attaches itself to the remaining stub and the fish is actually able to utilize it as a replacement tongue to draw in and manipulate food, which the parasite shares.