Things that’ll keep you up at night, part 1:
This is Cymothoa exigua. That is, the thing thats inside the fish mouth is Cymothoa exigua. Otherwise known as the Toungue Eating parasite, it does exactly what you’d expect it to do: It enters through the gills of the fish, removes its tongue by cutting off blood flow to it, and attaches it self where the tongue would have normally sat.
While this is all rather horrible and nasty, C. exigua doesn’t actually do any major harm to host fish and fish are able to use the parasite as they would use a tongue.
Don’t worry though, it doesn’t affect humans.
(Source: seascienceweekly, via laboratoryequipment)