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 4.8 m (16 ft) in length. This shark is one of the commonly studied deep-sea sharks due to students using bait (tuna or carrion) to do tests to see how deep-sea fish find their food in the lightless world of the deep sea.
The bluntnose sixgill shark is a member of the Hexanchidae family. Many of its relatives are extinct - there are more closely related relatives in the fossil record than there are living species. The living species that are closest genetically include the dogfish, the Greenland shark, as well as other six- and sevengilled sharks. Some of the shark's relatives date back to 200 million years ago. This shark is a notable species due to both its primitive and current physical characteristics.
Skin color ranges from tan to brown to black. It has a light colored lateral line down the sides and on the fins' edges. There are darker colored spots on the sides. The general shape is a heavy, powerful body that has a broad head with small eyes. The pupils are black and the eye color is a fluorescent blue green. The bluntnose sixgill shark can grow to 4.8 m (16 ft). Adult males generally average between 3.1–3.3 m (10.2–10.8 ft), while adult females average between 3.5–4.2 m (11.5–13.8 ft).
The bluntnose sixgill shark resembles many of the fossil sharks from the Triassic period. This could be because there are a greater number of Hexicanus relatives in the fossil record than there are left alive today. They have one dorsal fin located near the caudal fin. The pectoral fins are broad, with rounded edges. There are six gill slits, which gives 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 meters (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 metres (98 ft) and shallower during parts of the year in some specific places e.g. Flora Islet, near Hornby Island, Sightings during shallow evening dives in Whytecliff Park West Vancouver in British Columbia, in Puget Sound, Monterey Canyon off Monterey, California and in fjords in Norway. The sharks are deepsea 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 the bluntnose sixgill shark's large and diverse range, they have a wide variety of prey items. Their diet consists of a variety of mollusks, crustaceans, and Agnathans (hagfish and sea lampreys). They also dine on Cape anchovies, Pacific salmon, and various species of hake. Many other species are eaten depending upon 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 hasn't been known to attack any humans. In 2013 during filming for the Shark Week episode "Alien Sharks", It was revealed that bluntnose sixgill sharks are 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 it was found that 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 will nip 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 the ages of 18 and 35 years. Males usually reach sexual maturity much younger, between the ages of 11 and 14 years old. Scientists are unsure how the bluntnose sixgill shark reproduces, but it is thought that males and females meet seasonally between the months of May and November. The gestation period is unknown, but scientists believe that it is more than two years. The bluntnose sixgill shark is ovoviviparous, which means that 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 than adults. This is a form of cryptic coloration or camouflage that is used to disguise the pup's appearance. The litter size ranges from 22 to 108 pups. It is presumed that there is a high mortality rate of the young pups, 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. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 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.
- Information on H. griseus from ReefQuest Center for Shark Research
- YouTube video (1:17) - 18' specimen filmed at 1000m in Molokai, HI.