Cymothoa exigua

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.[3] It appears that the parasite does not cause much other damage to the host fish,[2] but it has been reported by Lanzing and O'Connor (1975) that infested fish with two or more of the parasites are usually underweight.[4] 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.[2] 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,[5] 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[6] and Ceratothoa imbricata.[7]

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), 1 species of grunt (Haemulidae), 3 drums (Sciaenidae), and 1 species of order Atheriniformes: 1 grunion (Atherinidae). New hosts from Costa Rica include the Colorado snapper, Lutjanus colorado and Jordan's snapper, L. jordani.[8]

In 2005, a red snapper parasitized 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;[9] 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.[10]

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 10 millimetres (0.4 in) in length.[11] The female then makes its way to the fish's mouth where it uses its front claws to attach to the fish's tongue.

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.[12]

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.[8]

• An image of three clownfish, each with a parasitic isopod visible in its mouth, was shortlisted in the underwater category of the 2017 Wildlife Photographer of the Year competition of the Natural History Museum, London.[13]

• A mutated version of Cymothoa exigua was explored in the eco-terror film The Bay.

• The College Humor internet show WTF 101 makes reference to the tongue-eating louse in the first episode.[14]

• This Book Is Full of Spiders revolves around a parasite that replaces the tongues of humans and sometimes controls their behavior.

• Images and discussion