Tongue-Eating Louse

Cymothoa exigua, or the tongue-eating louse, is a parasitic crustacean of the family Cymothoidae. This parasite enters fish through the gills, and then attaches itself to the fish's tongue. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female. Females are 8–29 milli metres (0.3–1.1 in) long and 4–14 mm (0.16–0.55 in) in maximum width. Males are approximately 7.5–15 mm (0.3–0.6 in) long and 3–7 mm (0.12–0.28 in) wide.The parasite severs the blood vessels in the fish's tongue, causing the tongue to fall off. It then attaches itself to the stub of what was once its tongue and becomes the fish's new tongue.

C. exigua extracts blood through the claws on its front, causing the tongue to atrophy from lack of blood. 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. It appears that the parasite does not cause any other damage to the host fish. Once C. exigua replaces the tongue, some feed on the host's blood and many others feed on fish mucus. This is the only known case of a parasite functionally replacing a host organ.When a host fish dies, C. exigua will detach itself from the tongue stub after some time, leave the fish's mouth cavity, and can then be seen clinging to its head or body externally. It is not fully known what then happens to the parasite in the wild.

There are many species of Cymothoa,but only C. exigua is known to consume and replace its host's tongue.

C. exigua is quite widespread. It can be found from the Gulf of California south to north of the Gulf of Guayaquil, Ecuador, as well as in parts of the Atlantic. It has been sampled in waters from 2 metres (6 ft 7 in) to almost 60 m (200 ft) deep. This isopod is known to parasitize eight species in two orders and four families of fishes [7 species of order Perciformes: 3 snappers (Lutjanidae), 3 grunts (Haemulidae), 3 drums (Sciaenidae), and 1 species of order Atheriniformes: 1 grunion (Atherinidae)]. Females of this isopod were found in the mouths of three species of snappers. New hosts from Costa Rica include the Colorado snapper, Lutjanus colorado and Jordan's snapper, L. jordani.

In 2005, a red snapper parasitised by what could be Cymothoa exigua was discovered in the United Kingdom. As the parasite is normally found off the coast of California, this led to speculation that the parasite's range may be expanding; however, it is also possible that the isopod traveled from the Gulf of California in the snapper's mouth, and its appearance in the UK is an isolated incident.