The bigeyed sixgill shark (''Hexanchus nakamurai'') is a

cow shark Cow sharks are a shark family, the Hexanchidae, characterized by an additional pair or pairs of gill slits. Its species are placed within the 11 genera: '' Gladioserratus'', '' Heptranchias'', '' Hexanchus'', ''Notidanodon'', '' Notorynchus'', '' ...

of the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Hexanchidae. Its

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

surface has a brownish-gray color, and is sharply separated from the light coloring of its

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

surface. The eyes are a fluorescent green while the shark is alive. The body of this shark is small, slim, and fusiform in shape. As the name suggests, this shark has six gill slits, unusual among most shark species. The head is narrow and somewhat flattened, and the mouth contains 5 rows of large, comb-shaped teeth. This shark's single dorsal fin is pushed back towards the

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

, and is behind the pelvic fins. The upper caudal fin is much longer than the lower, with a deep notch near the tip. All fins have thin white margins on the edge. In juveniles, the upper caudal fin has a black tip.

Measurements

Length at birth for this species is 34–45 cm; adults average 1.2 m for both sexes, with a maximum of 1.8 m. Mature adults weigh around 20 kg.

Range and habitat

Bigeyed sixgills are found in the Indian Ocean and the western Pacific Ocean. Populations in the western

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

formerly grouped within this species are now considered their own species, the Atlantic sixgill shark (''Hexanchus vitulus''). These sharks live near the sea floor between 90 and 600 m deep, but may move closer to the surface at night to take advantage of prey there. They make their home on continental shelves, insular shelves, and upper slopes. These sharks do not inhabit abyssal environments.

Parasites

As with other sharks, bigeyed sixgill sharks harbour a number of

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

species.

Monogenea Monogeneans, members of the class Monogenea, are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they ...

ns of the genus '' Protocotyle'' (family

Hexabothriidae Hexabothriidae is a family of monogenean parasites. The family name was proposed by Emmett W. Price in 1942.Price, E. W. 1942: North American monogenetic trematodes. V. The family Hexabothriidae n. n. (Polystomatoidea). Proceedings of the Helmint ...

) have been found only on the gills of species of ''Hexanchus''. The species ''

Protocotyle euzetmaillardi ''Protocotyle euzetmaillardi'' is a species of monogenean of the family Hexabothriidae. It is the third described species of the genus '' Protocotyle'', after '' Protocotyle grisea'' (Cerfontaine, 1899) Euzet & Maillard, 1974Euzet, L. & Maillar ...

'' Justine, 2011 was recently described from the gills of ''Hexanchus nakamurai'' off

New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...

, South Pacific Ocean.

Behavior

Little is known about the feeding habits of the bigeyed sixgill. Based on the few stomach examinations of these sharks, they are thought to feed mainly on small to moderately sized bony fish and bottom dwelling crustaceans. Like all sharks of the order Hexanchiformes found so far, this shark is

ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparity, oviparous and live-bearing viviparity, viviparous reproduction. Ovoviviparous animals possess embryos that develo ...

, meaning the eggs are retained inside the female's body until hatching. The bigeyed sixgill, after an unknown gestation period, gives birth to litters of up to 13 pups.

Conservation

The bigeyed sixgill is listed as near threatened by the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological ...

. Based on what little is known of the species, these sharks are not suffering major declines to their population, but pressure is growing from deepwater fisheries. According to the International Shark Attack File, the bigeyed sixgill is harmless, as its natural environment is far from people, and no attacks on humans by the species have been reported.Compagno, Leonardo, Dando, Marc and Fowler, Sarah (2005). ''Sharks of the World''. Princeton University Press. pp. 67–68. Hexanchus nakamurai JNC2615 body 2.JPG Hexanchus nakamurai jaws.jpg, Jaws Hexanchus nakamurai upper teeth.jpg, Upper teeth Hexanchus nakamurai lower teeth.jpg, Lower teeth Hexanchus nakamurai JNC2615 upper teeth.JPG, ''H. nakamurai'', upper teeth Hexanchus nakamurai JNC2615 6 gills.JPG, ''H. nakamurai'', six gill openings Hexanchus nakamurai JNC2615 Eye.JPG, ''H. nakamurai'', eye Hexanchus nakamurai JNC2615 head.JPG, Front part of body

References

External links

* FAO Species Catalogue Volume 4 Parts 1 and 2 Sharks of the World {{DEFAULTSORT:Bigeyed Sixgill Shark Hexanchus Fish described in 1962