Cymothoa exigua, or the tongue-eating louse, is a parasitic isopod of the family Cymothoidae.
Females are 8–29 millimetres (0.3–1.1 in) long and 4–14 mm (0.16–0.55 in) wide.
Males are approximately 7.5–15 mm (0.3–0.6 in) long and 3–7 mm (0.12–0.28 in) wide.
This parasite enters fish through the gills. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female.
The parasite severs the blood vessels in the fish’s tongue, causing the tongue to fall off. It then attaches itself to the remaining stub of the tongue and becomes the fish’s new tongue. – Tiny Fox Lab
It is the only known parasite to replace an entire organ.