Cymothoa exigua, or the tongue-eating louse, is a parasiticisopod of the family Cymothoidae. 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. Females are long and wide. Males are approximately long and wide. 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.
Behavior
Using its front claws, C. exigua severs the blood vessels in the fish's tongue, 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. It appears that the parasite does not cause much other damage to the host fish, but it has been reported by Lanzing and O'Connor that infested fish with two or more of the parasites are usually underweight. 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 assumed to be 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, and only cymothoid isopods are known to consume and replace the host's organs. Other species of isopod known to parasitise fish in this way include Cymothoa borbonica and Ceratothoa imbricata.
Distribution
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 to almost deep. This isopod is known to parasitize eight species in two orders and four families of fishes—7 species of order Perciformes: 3 snappers, 1 species of grunt, 3 drums, and 1 species of order Atheriniformes: 1 grunion. New hosts from Costa Rica include the Colorado snapper, Lutjanus colorado and Jordan's snapper, L. jordani. In 2005, a red snapper parasitized by what could be Cymothoa exigua was discovered in the United Kingdom. As the parasite is normally found south the gulf of California Mexico, 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 was an isolated incident.
Reproduction
Not much is known about the life cycle of C. exigua. It exhibits sexual reproduction. It is likely that juveniles first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills. If there is no female present, within a pair of two males, one male can turn into a female after it grows to in length. The female then makes its way to the fish's mouth where it uses its front claws to attach to the fish's tongue.
Influence on humans
It is currently believed that C. exigua are basically not harmful to humans, except that they will bite if separated from their host and handled. In Puerto Rico, C. exigua was the leading subject of a lawsuit against a large supermarket chain. The isopod C. exigua is found in snappers from the Eastern Pacific which are shipped worldwide for commercial consumption. The customer in the lawsuit claimed to have been poisoned by eating an isopod cooked inside a snapper. The case, however, was dropped on the grounds that isopods are not poisonous to humans and some are even consumed as part of a regular diet.
