Cymothoa exigua, or the tongue-eating louse, is a parasitic isopod of the family Cymothoidae. It enters fish through the gills and the female attaches to the tongue, with the male attaching on the gill arches beneath and behind the female. Females are 8–29 mm (0.3–1.1 in) long and 4–14 mm (0.16–0.55 in) wide. Males are about 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 remaining stub of the tongue and becomes the fish's new tongue.
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. The parasite apparently does not cause much other damage to the host fish, but Lanzing and O'Connor (1975) reported 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.[clarification needed] This is the only known case of a parasite assumed to be functionally replacing a host organ. When a host fish dies, C. exigua, after some time, detaches itself from the tongue stub and leaves the fish's mouth cavity. It can then be seen clinging to its head or body externally. What then happens to the parasite in the wild is unknown.
Many species of Cymothoa have been identified, and only cymothoid isopods are known to consume and replace the host's organs. Other species of isopods known to parasitize fish in this way include C. borbonica and Ceratothoa imbricata. Different cymothoid genera are adapted to specific areas of attachment on the host. This includes scale-clingers, mouth- or gill-dwellers, and flesh-burrowers.
C. exigua is quite widespread. It can be found from the Gulf of California southward to north of the Gulf of Guayaquil, Ecuador, as well as in parts of the Atlantic. It has been found in waters from 2 m (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—seven species of order Perciformes: three snappers (Lutjanidae), one species of grunt (Haemulidae), three drums (Sciaenidae), and one species of order Atheriniformes: one grunion (Atherinidae). 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 C. exigua was discovered in the United Kingdom. As the parasite is normally found south of the Gulf of California, Mexico, this led to speculation that the parasite's range may be expanding; however, the isopod possibly traveled from the Gulf of California in the snapper's mouth, and its appearance in the UK was an isolated incident.
Not much is known about the lifecycle of C. exigua. It exhibits sexual reproduction. The species starts as a juvenile in a short, free-living stage in the water column. Juveniles likely first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills. The fertilized eggs are held in a marsupium, similar to a kangaroo. If no female is present within two males, one male can turn into a female after it grows to 10 mm (0.4 in) in length. The female then makes her way to the fish's mouth, where she uses her front claws to attach to the fish's tongue.
Influence on humans
C. exigua is not believed to be harmful to humans, except it may bite if separated from its host and handled.
In Puerto Rico, C. exigua was the leading subject of a lawsuit against a large supermarket chain; itis 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.
In popular media
- 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.
- A mutated version of C. exigua was explored in the ecoterror film The Bay.
- The CollegeHumor internet show WTF 101 makes reference to the tongue-eating louse in the first episode.
- This Book Is Full of Spiders revolves around a parasite that replaces the tongues of humans and sometimes controls their behavior.
- Vampire: The Requiem antagonist sourcebook Wicked Dead features a fictional relative of the tongue-eating louse, C. sanguinaria, which infests mammals (including humans) and gradually takes control over their behavior to seek out blood (its primary food source) and breeding grounds.
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- Brusca, R. C.; Gilligan, M. R. (1983). "Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda)" (PDF). Copeia. 3 (3): 813–816. doi:10.2307/1444352. JSTOR 1444352. Archived from the original (PDF) on 2019-07-19.
- Finley Sr, Reginald (8 March 2016). "The Tongue-eating Louse (cymothoa exigua)". Amazinglife. Archived from the original on 4 December 2018. Retrieved 3 December 2018.
- Ruiz-Luna, Arturo (March 1992). "Studies on the biology of the parasitic isopod Cymothoa exigua Schioedte and Meinert, 1844 and its relationship with the Snapper Lutjanus peru (Pisces: Lutjanidae) Nichols and Murphy, 1922, from commercial catch in Michoacan". Ciencias Marinas. 18 (1): 19–34. doi:10.7773/cm.v18i1.885.
- Thatcher, Vernon E.; de Araujo, Gustavo S.; de Lima, José T. A. X. & Chellappa, Sathyabama (2007). "Cymothoa spinipalpa sp. nov. (Isopoda, Cymothoidae) a buccal cavity parasite of the marine fish, Oligoplites saurus (Bloch & Schneider) (Osteichthyes, Carangidae) of Rio Grande do Norte State, Brazil" (PDF). Revista Brasileira de Zoologia. 24 (1): 238–245. doi:10.1590/S0101-81752007000100032.
- Parker, D.; Booth, A.J. (2013). "The tongue-replacing isopod Cymothoa borbonica reduces the growth of largespot pompano Trachinotus botla". Marine Biology. 160 (11): 2943–2950. doi:10.1007/s00227-013-2284-7. S2CID 85025367.
- Bates, Mary (18 September 2012). "Tongue-eating parasites inspire new horror movie". Qualia. American Association for the Advancement of Science. Retrieved 14 December 2016.
- Pawluk, Rebecca J.; Ciampoli, Marco; Mariani, Stefano (14 April 2015). "Host size constrains growth patterns in both female and male Ceratothoa italica, a mouth-dwelling isopod". Marine and Freshwater Research. 66 (4): 381–384. doi:10.1071/MF14125. ISSN 1448-6059.
- Williams, Ernest H. Jr.; Bunkley-Williams, Lucy (2003). "New records of fish-parasitic isopods (Cymothoidae) in the Eastern Pacific (Galapagos and Costa Rica)" (PDF). Noticias de Galápagos (62): 21–23.
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- "Tongue-eating louse found on supermarket snapper". Practical Fishkeeping. 6 September 2005.
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- "Rare tongue-eating parasite found". BBC News. 9 September 2009.
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