I’m going with the Cymothoa exigua.
It lives in the fishes mouth and first infiltrates it through the gills, before growing.
It sucks the fishes blood from its tongue until the tongue rots and dies off. Then, it becomes the fish’s new tongue. It eats some of the fishes food each day, as a sort of parasitic tax. It can change gender and make a family too, all of which will live in the fish:
I feel bad for the fish. But it’s incredible what species can evolve to do.