I saw this on Facebook earlier today, so I'm just going to copy the description I read. It is as follows:

Parasites often alter their host's behaviour or appearance, but Cymothoa exigua goes one further - it is the only parasite known to replace an entire organ. Its unfortunate host is typically the rose spotted snapper, which it enters through the gills and lodges itself at the back of the mouth. It hooks its claws at the base of the tongue and interrupts the blood flow, consuming it instead. Due to the lack of blood the tongue atrophies and falls off, where the isopod parasite attaches itself to the remaining stub. It then "becomes" a functional tongue for the fish, able to be moved as the fish would have moved its original tongue.

Like most cymothoid isopods, C. exigua is a hermaphrodite. In one study females were only found in the mouth, whereas males could be found in the gills and the mouth (and in the mouth they were clinging to females, suggesting copulation). A small proportion of extremely unlucky fish were found with females as the tongue and males simultaneously occupying the gills. Despite the "organ-replacement" part, it doesn't appear C. exigua has other adverse effects on the fish.

In case you were wondering (and we're sure it crossed your mind at some point), C. exigua does not affect humans. That said, don't pick one up - they deliver quite a nip.

Photo credit: Dr. Nico Smit.

http://arthropoda.southernfriedscience.com/?tag=cymothoa

http://rcmarinas.ens.uabc.mx/index.php/cmarinas/article/view/885/807