It’s pretty rare that an animal comes along that makes you say “you’re a sick bastard, nature,” but Cymothoa exigua, or the tongue-eating louse, evokes just such a reaction. This unassuming little isopod takes parasitism to a strange place when it enters a fish’s mouth, eats the fish’s tongue, and then replaces said organ with its own body.

The beginning of this stomach turning process starts when a bunch of juvenile males will enter the gills of a fish and stay there until adulthood. C. exigua is what is known as a “protandric hermaphrodite” which means that it starts out as a male and can turn into a female once it needs to breed (or finds its inner self). When one such male has made this sexual transition, it will travel to the fish’s mouth, latch down onto the fish’s tongue and suck it dry of blood – well, as dry as something can get under water. Regardless, the tongue soon withers away and C. exigua takes its spot.

Understandably displeased. (Two Fish Blog)

But even after it issues its host a new tongue, scientists suspect that C. exigua manages to do something even more disrespectful once it’s in there. Once the newly-gendered individual has taken her station in the fish’s mouth, it is thought that the males will work their way forward from the gills to mate with her. So, C. exigua isn’t content just stealing your tongue, she also wants to get busy in your mouth with her boyfriend. Lovely.

The deal isn’t quite as raw for the fish as it may seem. Though I bet most fish would just as soon keep their original organ, once the isopod is in place, it still acts as a fully functional tongue for the fish. This is the only known case of a parasite replacing a host’s organ yet retaining that organ’s functionality. While this keeps the fish alive for a while, both parties are eventually doomed. Once the female has given birth, she detaches and dies, with the now tongueless fish soon behind it.

Gross? Sadistic? Or in its own way biologically charming? Tell us in the comments section below.

HT: Nova, Wired