Bluntnose sixgill shark
|Bluntnose sixgill shark|
|Bluntnose sixgill shark in the Gulf of Mexico.|
|Range of bluntnose sixgill shark (in blue)|
The bluntnose sixgill shark, Hexanchus griseus, often simply called the cow shark, is the largest hexanchoid shark, growing to 16 ft (4.9 m) in length. This shark is one of the commonly studied deep-sea sharks, due to students using bait (tuna or carrion) to perform tests to see how deep-sea fish find their food in the light-less world of the deep sea.
The bluntnose sixgill shark is a member of the Hexanchidae family. Many of its relatives are extinct – it has more close relatives in the fossil record than living relatives. The related living species include the dogfish, the Greenland shark, and other six- and sevengilled sharks. Some of the shark's relatives date back 200 million years. This shark is a notable species due to both its primitive and current physical characteristics.
Skin color ranges from tan, through brown, to black. It has a light-colored lateral line down the sides and on the fins' edges, and darker colored spots on the sides. The general shape is a heavy, powerful body with a broad head with large eyes. The pupils are black and the eye color is a fluorescent blue-green. The bluntnose sixgill shark can grow to 8 m (26 ft). Adult males generally average between 3.1 and 3.3 metres (10 and 11 ft), while adult females average between 3.5 and 4.2 m (11 and 14 ft).
The bluntnose sixgill shark resembles many of the fossil sharks from the Triassic period. A greater number of Hexanchus relatives occur in the fossil record than are alive today. They have one dorsal fin located near the caudal fin. The pectoral fins are broad, with rounded edges. The six gill slits give the shark its name. Most common sharks today only have five gill slits.
This species typically inhabits depths greater than 90 m (300 ft), and has been recorded as deep as 1,875 m (6,152 ft). Like many deep-sea creatures, the bluntnose sixgill shark is known to undertake nightly vertical migrations (travelling surfaceward at night, returning to the depths before dawn).
The bluntnose sixgill shark can be seen at depths of 30 m (98 ft) and shallower during parts of the year in some specific places, e.g. Flora Islet, near Hornby Island. These sharks have been seen as shallow as 12 m (40ft)in Barkley Sound on the West coast of Vancouver island. Sightings are made during shallow evening dives in Whytecliff Park West Vancouver in British Columbia, in Puget Sound, Monterey Canyon off Monterey, California, and fjords in Norway. The sharks are deep-sea sharks, but like most fish that prefer the deep, they come to the shallower depths to feed.
Although sluggish in nature, the bluntnose sixgill shark is capable of attaining high speeds for chasing and catching its prey. Because of their large and diverse range, they have a wide variety of prey items, including mollusks, crustaceans, hagfish and sea lampreys. They also dine on Cape anchovies, Pacific salmon, and various species of hake. Many other species are eaten depending on the shark's home range. In the BBC's The Blue Planet (in the episode "The Deep" and the special "The Abyss"), this shark was filmed eating the remains of a yellowfin tuna. Although the bluntnose sixgill shark can grow to a great size, it has not been known to attack any humans. In 2013, during filming for the Shark Week episode "Alien Sharks", bluntnose sixgill sharks were found to be territorial as one was filmed laying claim to the carcass of a sperm whale calf that was being used to lure deep-sea shark species within range of submarine cameras. The shark was recognized as being the same individual by the scars on its back and sides, and the bites it removed from the carcass allowed other animals, such as hagfish, to feed and further break down the dead whale. 
Very little is known about the reproductive process of bluntnose sixgill sharks. Many biologists believe that the male bluntnose sixgill shark's teeth are specially adapted for courtship. The male nips at the female's gill slits using its longer-cusped teeth. This action is thought to entice the female into mating. Evidence of this hypothesis is that female bluntnose sixgill sharks show up with seasonal scars around their gill slits, which apparently is from breeding with males. The female bluntnose sixgill shark reaches sexual maturity between 18 and 35 years old. Males usually reach sexual maturity much younger, between 11 and 14 years old. Males and females are thought to meet seasonally between May and November. The gestation period is unknown, but is probably more than two years. The bluntnose sixgill shark is ovoviviparous - the young are carried within the mother's body until the eggs hatch. They develop without a placenta to provide nourishment. The pups are born at a fairly large and developed stage at 65 to 74 cm. New pups are also born with a lighter belly color than adults. This form of cryptic coloration or camouflage is used to disguise the pup's appearance. The litter size ranges from 22 to 108 pups. A high mortality rate of the young pups is presumed, owing to the large litter size.
- "Boy, 9, reels in 8-foot sixgill shark near Burien". Seattle Times. August 10, 2009.
- Alien Sharks
- Shark Specialist Group (2000). "Hexanchus griseus". IUCN Red List of Threatened Species. Version 2006. International Union for Conservation of Nature. Retrieved 11 May 2006. Database entry includes justification for why this species is near threatened
- "Hexanchus griseus". Integrated Taxonomic Information System. Retrieved 23 January 2006.
- Froese, Rainer and Pauly, Daniel, eds. (2005). "Hexanchus griseus" in FishBase. 09 2005 version.