It is an idea so horrifying that it sounds made up: A parasite which bites off its host's tongue and then replaces it, living inside its mouth for the rest of its life.
But in fact, the tongue-eating louse is real and lives inside the mouths of fish - one of which was discovered recently by a shocked student in South Africa.
Don Marx, 27, from Cape Town, was fishing near Cape Agulhas when he hooked the six-pound carpenter fish and discovered the stowaway inside its jaws.
Marx, who happens to study marine biology, had heard of the lice but never seen one in the wild - so snapped a picture to document the moment.