The tongue-eating louse is the ultimate uninvited guest. It turns up unannounced, eats everything in sight and then vastly overstays its welcome. In fact, it doesn’t actually leave until the day it dies. Hopefully you’ve never had a guest quite like that.
The picture above seems to depict a fish that is in the process of eating some sort of aquatic invertebrate. The invertebrate in question is a type of isopod (a marine relative of the woodlouse), but it’s in no danger of being eaten. It is a parasite called Cymothoa, more commonly known as the tongue-eating louse, and it has a lazy but devilishly clever survival technique.
During its juvenile free-swimming stage, the tongue-eating louse finds itself a suitable host, often a rose snapper fish, and then passes through its gills and latches onto its tongue. Despite its name, the louse doesn’t actually eat the fish’s tongue, although it does start sucking the blood from it. This is how the strange crustacean obtains all of its nourishment, growing in size from little more than a tick to a monster that fills up most of the mouth. Eventually, as it becomes starved of blood, the tongue atrophies and withers away to nothing. The isopod attaches itself to the muscles of the tongue stub and becomes a fully-fledged tongue substitute in its own right, the only known instance of a parasite replacing a host’s organ.
At this stage, the tongue-eating louse reduces the amount of blood that it takes from its host and switches its diet to include food particles in the mouth of the fish, or sometimes just the mucus that its host produces. The rose snapper does not seem too bothered by the presence of the huge parasite in its mouth – or maybe it just accepts that there is nothing it can do about it. Still, if I happened to be that fish, I’d probably be pretty infuriated that I had to share all of my meals with my own ‘tongue’. That’s not exactly something you anticipate when a guest shows up uninvited for dinner.