This little bugger is creepy as all hell. Cymothoa exigua steals blood from the tongue of a fish until the tongue falls off and then squats there and replaces the tongue. The fish can then use it as a tongue and apparently no other harm is done - but it has to be disconcerting, even to a fish.
If some alien invader replaced my tongue and rode around stealing my life force I’d want something in return, special tongue powers, such as it elongates to a foot long or is resistant to chili or tells me cauliflower tastes nice so I can eat my vegetables.
Mostly elongating, that seems the most useful…