When I first saw this, I thought it was fake:
There’s apparently a parasite that eats (and eventually replaces) fish tongues! This parasite, called Cymothoa exigua, snacks on the fish’s tongue until it eats it down to just a stub. But by then, the thing is big enough to replace the fish’s tongue, which it does. The fish is pretty much unaffected by the presence of the parasite, since the “tongue louse” functions like a tongue, but that parasite also gets the pickings of food particles that the fish lunches on.
This parasite was thought to exist solely on the Gulf of California, but a red snapper at a fish market in London was recently found with one: http://animal.discovery.com/news/briefs/20050912/tongueeater.html
A very similar parasite exists in Japan. This story, with its accompanying photos, was taken from "Fishing Fury"'s website:
On Sunday fishing near Yokosuka I caught a fine example of Japanese horse mackerel (Trachurus japonicus) and when I was preparing the fish that evening, it turned out to be harboring a parasite known here in Japan as ‘uonoe’ (Rhexanella verrucosa). Apparently these creatures affix themselves to the tongue and help themselves to whatever the host fish eats. As you can see in the photos I have attached, with their multiple legs like hooks, they seem well adapted to their niche; I had to behead the mackerel to remove the parasite from its tongue. According to the literature here, these parasites are common in red sea bream and yellowtail amberjack (kingfish), but the two examples I have come across personally have both been in horse mackerel. In both cases, the fish appeared outwardly healthy and did not seem emaciated or unwell at all. The parasitic infection did not appear to affect their flesh either, as they were quite delicious.
This post, although in Japanese, it worth checking out if you want more gnarly pictures.
Up next? Maybe I'll write about the fish parasite that eats, replaces, and hangs from the fish's eyeball! Anyone posted on that yet?