Cymothoa exigua on Trissocles

• Cymothoa exigua, commonly known as the tongue-eating louse, is a member of the parasitic isopod family Cymothoidae. It gains entry into its host fish through the gills. Once inside, the female louse attaches to the host's tongue, while the male attaches to the gill arches beneath and behind the female.

• Females typically measure between 8–29 mm (0.3–1.1 in) in length and 4–14 mm (0.16–0.55 in) in width, while males are around 7.5–15 mm (0.3–0.6 in) long and 3–7 mm (0.12–0.28 in) wide.

• Upon infestation, the parasite severs the blood vessels in the fish's tongue, causing it to detach. Subsequently, the parasite attaches itself to the remaining tongue stub, effectively functioning as the fish's new "tongue".

BEHAVIOR

• Using its front claws, C. exigua severs the blood vessels in the fish's tongue, leading to necrosis due to lack of blood flow.

• The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. Interestingly, the parasite does not appear to cause significant additional harm to the host fish. 

• Once C. exigua replaces the tongue, some individuals feed on the host's blood while others feed on fish mucus. When the host fish dies, C. exigua eventually detaches itself from the tongue stub after some time and exits the fish's oral cavity. It can then be observed clinging to the fish's head or body externally.

DISTRIBUTION

• Cymothoa 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.

REPRODUCTION

• The species exhibits sexual reproduction, beginning its life as a juvenile in a brief, free-living stage in the water column.

•  Initially, juveniles likely attach to the gills of a fish and develop into males. As they mature, they transition into females, with mating presumably taking place on the gills.

•  Fertilized eggs are housed in a marsupium, similar to that of a kangaroo.

•  In cases where no female is present among two males, one male can transform into a female once it reaches a length of 10 mm (0.4 in). The female then migrates to the fish's mouth, where it uses its front claws to attach to the fish's tongue.

INFLUENCE ON HUMANS

• Cymothoa exigua is not believed to be harmful to humans, except it may bite if separated from its host and handled.