Cymothoa exigua, more commonly known as the tongue-eating louse, is a parasitic isopod, It is commonly found from the southern Gulf of California to the northern Gulf of Guayaquil but has also been found in parts of the Atlantic Ocean and can be found from around 2 meters to around 60 meters in the water column. It enters its host through the host's gills and typically a male and female will enter together. Their length can vary between 3 and 5cm and the width in females can be around 2.24cm while the males are usually 0.7cm.
Just like the name suggests, the tongue-eating louse does actually eat its host's tongue. As mentioned previously, the male and female both enter the host. The female attaches to the tongue directly and the male attaches itself onto the gill arches. After attaching itself to the tongue, the female will then sever the blood vessels in the tongue and sucks its host's blood. But how does it sever the blood vessels and drink the blood of its host? According to marine biologist Rick Brusca,
“They have five sets of jaws, and all five of them are modified into these stiletto-like devices, and a couple of them are like long lances, if you will, that slice open the tissue of the host fish. And then the others operate together kind of like a soda straw to draw the blood up out of the wound that they've created.”
This eventually causes the tongue to fall off. Once it falls off, the female then attaches herself to the "tongue stub" and acts as the host's tongue where she feeds on leftover mucus and blood from its host's meals and the host itself.
This does not kill the fish and the fish functions pretty normally besides that it now uses the isopod as a tongue.
Not only does this isopod replace the tongue of its host, it also reproduces in the host's mouth. If both the female and male are still alive and in the host, the male will crawl out of the gills and have sex with the female. This eventually leads to the female giving birth to around 400 to 720 male isopods; 200 of which will reach sexual maturity. These male lice then disperse throughout the ocean and some of them will eventually change their sex to be females once more than one is inside the host. The process starts all over again after that.
These isopods have little to no impact on humans. If you catch a fish with a tongue-eating louse as its tongue and want to keep the fish as food, you can simply hold the fish's mouth open, grab a pair of tweezers, and pull the isopod out. Make sure to check the gills for any potential males to pull out as well. If you don't have tweezers, "tickle" the isopod with your finger and it will start to crawl out too. These isopods do not harm humans in any way if one was to eat them so that should not be an issue.
They do pack a bite if picked up by bare hands, so try to wear protective gear if you want to play around with them
This video by CyanKarateBird shows a tongue-eating louse acting as the tongue of a juvenile giant trevally. It gives you a better idea of what they look like while in the fish.
https://futurism.com/the-most-horrifying-parasite-cymothoa-exigua-2/
http://amazinglife.bio/tongue-eating-louse-cymothoa-exigua/
https://en.wikipedia.org/wiki/Cymothoa_exigua
