Someone sent me something mentioned this same parasite earlier today.
http://www.bogleech.com/bio-para.html
Worth linking for "My other Tongue is a Sea Louse" and their terrifying picture of it I think. Also really liked this bit:
"Roughly 80% of all human beings on earth have one or more internal parasite infestations. For america, the estimate is 95%."
which gets slotted in my mind alongside a tv special I once watched on microscopic life that claimed there were more creatures (not including bacteria) living on the average human body than there were people on earth.
back to the fish though, what do you suppose it made of the tongue? What a nightmare if it understood what was happening during it's upgrade.