Louse Tongue

Cymothoa exigua has the dubious distinction of being one of the only parasites on the planet that actually replaces one of the host animal's organs.

Upon entering the spotted rosesnapper's mouth, the nubile sea louse attaches itself to the fish's tongue and starts feeding off the tongue's blood. As C. exiqua grows, the tongue atrophies and falls off. But the sea louse remains, firmly grasping the stump of the tongue. The fish is actually able to move and use the sea louse as if it were its own tongue, and sustains no additional harm.

From what I've read, C. exigua only latches on to the spotted rosesnapper's tongue. How does such a specific parasitism arise? Are they too good for other fish? Is there something extra fancy about spotted rosesnapper tongue?

I imagine that when spotted rosesnappers go out on dates, one of the first things they ask each other--certainly before the first kiss--is if they have a sea louse tongue. That might be a deal breaker for some.

Thanks for the photo, Jelo.