Certainly one of the more disgusting species I’ve come across, the Cymothoa exigua or Tongue eating louse (louse being singular form of ‘lice’) is a parasitic crustacean that typically grows to 3 or 4 cm in length (1 in – 1.6 in). The parasite attaches itself at the base of a fish’s tongue entering the fish’s mouth through its gills.

It then proceeds to extract blood through the claws on its front three pairs of legs. As the parasite grows, less and less blood reaches the tongue and eventually the organ atrophies. The parasite then replaces the fish’s tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue and it appears that the parasite does not cause any other damage to the host fish. Once the insect replaces the tongue, it can feed on the host’s blood and/or feed on the fish’s mucus. This is the only known case of a parasite functionally replacing a host organ, making it a scientific marvel and poster-child for repulsion.