A Tongue Imposter

a fish's mouth is opened to reveal a crustacean in the place of the tongue
(Jimmy Bernot)

Cymothoid isopods, also known as tongue-biters, sever the host fish’s tongue, latch themselves onto the remaining nub with hooked appendages, and then nestle down to live in the space vacated by the missing tongue. The isopods then allow the fish to carry on its normal activity, all the while it feeds on the host’s blood.