The sixgill shark, known by its scientific name hexanchus griseus is a genus of sharks that consists of two separate species: the bluntnose sixgill shark and the bigeye sixgill shark. Its name is derived from the fact that it has six gill sets. Most sharks only have five gill sets. Also differentiating both species of the hexanchoid shark from other shark species is having singular dorsal fin, which rests near the tail of the fish. Most sharks have two dorsal fins, a back dorsal fin and one closer to their head. Other common names for both species are mud shark and cow shark.
The bluntnose sixgill shark is the more common of the two species and is occasionally fished both for game and for commercial purposes. It is usually found in deep sea waters, up to 6,150 feet (1,875 meters) deep, but it is occasionally seen in shallows. Its distribution is believed to be anywhere in the world with warm climates, and it is most often spotted along the coasts of Monterery Bay, Ca., South Africa and British Columbia. Not much is known about the reproductive nature of the animal, but it gives birth to live sharks, not eggs.
The litters of the shark can be as high as 20, which most likely serves to combat a high mortality rate. Once it matures it can reach up to 16 feet (4.8 meters) in length and weigh more than 800 pounds (362 kilograms). It is a long-living species of shark, with estimates suggesting it can live upwards of 80 years in the wild. The bluntnose sixgill is unique among many sharks of its size because it hunts live prey; typically smaller fish, rays and marine mammals. Many sharks of it size survive on a diet of plankton.
The bigeye sixgill shark is smaller than the bluntnose, usually about 5 feet (1.5 meters) in length and weighing around 40 pounds (18 kilograms). It is almost exclusively found in deep waters, up to 1,970 feet (600 meters) deep, so very little is known about the shark's feeding or breeding habits. It is believed that it feeds on small fish and small bottom dwelling animals. However, there is evidence to suggest that it does occasionally rise to feed on shallow water fish such as tuna and other small fish.
Aside from the size difference it is very similar in appearance to the bluntnose shark. Like its name implies, it has larger eyes than most sharks of similar size. Neither the bluntnose or the bigeye sixgill shark is of any danger to man.