Well, what we have here is a dynamic duo of sorts. A parasitical relationship, to be exact. The freaky photo above shows the gaping mouth of an unfortunate fish caught in the Atlantic bearing one of the freakiest fiends of the deep: Ceratothoa exigua, otherwise known as the sea louse. A parasitical crustacean, it bears a striking resemblance to the nasty movie creature that pursued actress Sigourney Weaver in Alien, film where a sort of sentient arthropod parasitized human beings. The sea louse is not nearly sentient, but is a peculiarly nasty arthropod.
This creepy parasite enters fish through the gills, and attaches itself at the base of the fish's tongue. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female.
(C. exigua in a red snapper)
Once the sea louse take hold of a hapless host, it seizes upon the tongue and eats it. Once it has eaten the fishy tongue, it remains attached to the mouth. It extracts blood through its front claws, thereby causing the tongue to atrophy from lack of blood. The louse then replaces the fish's tongue by attaching its own body to the muscles of the remaining tongue stub. The fish is able to use the parasite just like a normal tongue, while the parasite does not cause any other damage to the host fish.
Living nestled in a fish’s mouth, the C. exigua feeds on the host's blood, while other species might merely savor fish mucus. This is the only known case of a parasite actually replacing an organ of a host. There are numerous species of Cymothoa, but it is only C. exigua that is known to consume and replace its host's tongue.
The distribution of C. exigua is quite widespread. It can be found from the Gulf of California south to north of the Gulf of Guayaquil of Ecuador. Some have been found off the Atlantic shore of Central America, as well. Samples have been found 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 Perciformes, 3 snappers (Lutjanidae), 1 grunt (Haemulidae), 3 drums (Sciaenidae), and 1 grunion (Atherinidae)]. Fem ales of this isopod were found in the mouths of three species of snappers. New-found host fish from Costa Rica include the Colorado snapper, Lutjanus colorado and Jordan's snapper. One terrified diner ate a sea louse as part of an otherwise delicious dinner of red snapper. Terrified, the person believed that the creepy arthopod had poisoned him and actually sued in court for damages. Eventually, the suit was thrown out because the defending party could show that some people eat and actually thrive on a diet of arthopods!
C. exigua has been found off the shore of the United Kingdom, but this is probably a fluke. Nonetheless, the Old World has similar species, C. imbricata, which is sometimes found within the mouths of weaverfish. Presumably, no relation to the actress.
Cymothoa exigua – ‘Sea louse’
Species: C. exigua
Binomial name Cymothoa exigua