Luckily for us, the parasite, Cymothoa exigua, which is a type of crustacean called an isopod, is not interested in human flesh. It lives in the Pacific Ocean off the coasts of Mexico and Central America, and only infests fish.

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Beginning life as a swimming form called a manca, a young isopod attaches itself to an appropriate host, passing through its gills and latching onto its tongue. Then it’s dinnertime. The uninvited guest sinks its syringe-like mouthparts into the meaty organ and begins draining it of blood.

The tick-sized isopod may grow as big as a cockroach within a few months. By then, it has extracted so much blood that the tongue withers away.

That’s when things get really weird. The parasite attaches itself to the remaining stub and the fish uses the pest as it used its old tongue, to assist in swallowing prey (smaller fish). This makes the tongue-eating louse the only known animal to replace a body part of another in both form and function. It works so well that the fish eats normally and remains healthy.

How long the isopod lives and works in its host is unknown, but eventually a partner shows up and they mate. The young are raised in the fish’s mouth until they can survive on their own, leaving ‘home’ as the next generation of tongue-eaters.

David Brian Butvill

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Main image © Christian Gloor from Wakatobi Dive Resort, Indonesia, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons

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