It is an isopod (
Cymothoa sp.), a crustacean related to woodlice and water lice. It eats the tongue of the fish, and then lives in the place where the tongue would be. The fish usually lives a normal life with the isopod instead of its tongue. It is not dangerous to humans (unless someone has a tale about diving with their mouth open and ending up being mistaken for a fish!).
Tongue-eating lice hit catfish farm | Practical Fishkeeping magazine
UnderwaterTimes.com :: View topic - Fish Tongue-Eating Parasite Spreading?
I hadn't heard of these before. Would they have been around Greece a few thousand years ago? Here's why I ask: (From wikipedia)
Re: Anaximander: Teacher of Pythagorus
In De Die Natali (IV, 7), the 3rd
century Roman writer Censorinus reports:
" Anaximander of Miletus considered that from warmed up water and
earth emerged either fish or entirely fishlike animals. Inside these
animals, men took form and embryos were held prisoners until puberty;
only then, after these animals burst open, could men and women come
out, now able to feed themselves.
Anaximander put forward the idea that humans had to spend part of this
transition inside the mouths of big fish to protect themselves from
the Earth's climate until they could come out in open air and lose
their scales.[22] He thought that, considering humans' extended
infancy, we could not have survived in the primeval world in the same
manner we do presently".
Perhaps Anaximander thought that the embryo/larval stage of humans resembled these. Not exactly mainstream science today!