Here's a better photo of the parasite that eats its fish host's tongue, and then pretends to be the tongue.
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.
Reader comment: Mark McGrouther, Fish Collection Manager, Australian Museum says:
"I enjoyed the Boing-Boing Tongue-eating bug entry. I'm a little late, but you might be interested to add a link to the Australian Museum page, which shows some interesting pics of these little blighters." I like the photo with two tongue-eating bugs in one fish's mouth!