Hexanchus griseus
bluntnose sixgill shark
(Also: sixgill shark)

Hexanchus griseus occur globally in all oceans. These sharks live and thrive in the most widespread distribution of all known sharks, with the possible exception of white sharks. (MacQuity and King, 2000)

Hexanchus griseus is mainly a deep water shark, rarely found at depths of less than 100 m. The species seems to usually stay close to the bottom, near rocky reefs or soft sediments. The deepest one has been found was about 2500 m.

These sharks are diel vertical migrators; they are nocturnal and remain in the deep oceans during the day but rise towards the surface at night. Hexanchus griseus also seasonally migrates to shallower coastal waters. During the warmer months of the year, these sharks can occasionally be found in shallower waters at depths of 23 to 39 m during the day and as shallow as 3 m at night. (Martin, 2000; Musick and McMillan, 2002)

Hexanchus griseus is characteristically a large shark species with a heavy build. These sharks have a short, blunt snout, a broadly rounded mouth, and six pairs of gill slits (from which its common name, the bluntnose sixgill, is derived). They have large, green eyes and broad comb-like teeth on each side of the lower jaw arranged in 6 rows. Their coloring shades varies from grayish-black to chocolate brown on the dorsal surface and lightens to grayish-white on its belly. There is an anal fin, and one dorsal fin located on the back end of the body. The caudal fin is slightly raised so that the lower lobe is lined up with the body axis. The pelvic fins are located to the anterior of the anal fin and are a bit larger. Like many benthic sharks, the caudal fin of Hexanchus griseus has a weakly developed lower lobe. However, the bluntnose sixgill shark is still a very strong swimmer. (MacQuity and King, 2000; Martin, 2000)

There exist size differences between male and female sharks. Females tend to be slightly larger than males, averaging around 4.3 m in length while males tend to stay near 3.4 m. There is little or no color difference between the sexes; however, the seasonal scars appearing on the fins of females, which are believed to be a result of mating, are commonly used for sex identification. Sex can be easily determined by the presence of elongate claspers on the pelvic fins of male sharks. The bluntnose sixgilled shark is classified under the genus Hexanchus with only one other species, Hexanchus vitulus, or the bigeyed sixgill shark. Both sharks are similar in all aspects aside from their unmistakable size difference. While H. vitulus reaches only about 2.3 m in length, H. griseus reaches lengths of 4.8 m. (Parker and Parker, 2002)

Little is yet known about the life cycle and fetal development of Hexanchus griseus. (Ebert, 2002)

There is little information available about the lifespan of Hexanchus griseus. These sharks have a life expectancy no longer than 80 years in the wild. There is some suggestion that because they have such high infant birth rates, mortality rates could be very high as well. There is no known record for the oldest bluntnose sixgill shark in the wild, and this species has not been excessively studied or maintained in captivity, so there is no information on its lifespan in captivity. A new study is available, however, regarding the age determination of H. griseus. Previous techniques used in determining the age of H. griseus have been unsuccessful because of its poorly calcified vertebral centra (a characteristic of deep-water species and of primative families). This new study indicates that examining the neural arches on the fins of H. griseus can be useful in determining the age of this particular shark. (McFarlane, King, and Saunders, 2002; Musick and McMillan, 2002)

Researchers have had very little opportunity to observe the bluntnose sixgill shark's behavior in the wild. These sharks are mainly solitary animals and most likely never swim in schools or seek out interaction. Some researchers, however, have speculated that H. griseus return to shallow waters year after year (during the months of May to November) in order to interact socially with other bluntnose sharks, most likely for the sole purpose of mating. Richard Martin suggests that these light-sensitive sharks are drawn up to the shallower depths due to the yearly summer algae bloom which greatly increases light attenuation, allowing the sharks comfort while providing them with more prey items. During these seasonal interactions, males are believed to nip at the gills of females to court the females. During non-mating season, H. griseus remains at the lower depths rising only to feed at night. (Martin, 2000; Parker and Parker, 2002)

Hexanchus griseus are believed to have few forms of communication, as they seem to be solitary animals for the most part. Yet any social forms of communication that do exist between these animals are unknown. The only known form of communication to occur in H. griseus is during mating. The males are believed to use their teeth to entice the females into mating. These sharks are equipped with highly sensitive scent and visual organs, which are useful for perceiving the dark environment they live in. H. griseus is also able to detect other organisms by means of its lateral line system (used for detecting vibrations), and its ampullae of Lorenzini (which detect faint electric signals). (MacQuity and King, 2000; Musick and McMillan, 2002)

Hexanchus griseus is a skilled predator and is solely carnivorous, feeding on such animals as fishes, rays, and other sharks. Although they have been reported as being sluggish in nature, their body structure enables them to reach remarkable speeds for chasing and effectively capturing prey. Aside from feeding on molluscs and marine mammals, they eat crustaceans (crabs and shrimp), agnathans (Hagfish and sea lampreys), chondrichthyans (ratfish) and teleosts (dolphinfish and lingcod). A subspecies of H. griseus living in Cuban waters is also a skilled scavenger that feeds on carcasses of mammals. (Parker and Parker, 2002)

Hexanchus griseus has no known evolved anti-predator adaptations. These sharks, however, are equipped with very sensitive perception organs, which may allow them to detect potential predators. The retinas are comprised of mostly rods and, therefore, do not function well in even moderately lit areas but are well suited for the dark conditions of the deep oceans. Being such a large-bodied shark, its only real predators would be other big sharks, such as whites, or possibly orca whales, which are known to prey on adult sharks. Young H. griseus have been taken by sharks, whales, dolphins, and sea lions. (Martin, 2000)

This species is a large, deep-water predator, but we have little information on its ecological effects. There is some evidence that Hexanchus griseus has an important impact on the white sharks' population off the coast of South Africa. Researchers there believe that H. griseus will eventually outcompete Carcharodon carcharias in that area. H. griseus is not known to participate in any symbiotic relationships. (Martin, 2000; Musick and McMillan, 2002)

Despite their size, these sharks are not considered much of a direct threat to humans. They are described as shy, nonagressive animals that pose no threat to humans unless physically provoked. Also, their preference for deep water and darkness makes human encounters with this species relatively rare.

Some medical professionals consider the liver of Hexanchus griseus to be toxic, as its ingestion has been known to cause painful sickness for up to 10 days. The skin of H. griseus has also been known to cause such sickness.

This species is killed for food, harvested with line gear, gill nets, and other equipment. It is also caught by game fishermen.

Since they are large and widespread animals, these sharkes they may have a significant role in deep-water fisheries, but we have no information on this. (Bester, 2001; MacQuity and King, 2000; Martin, 2000; McFarlane, King, and Saunders, 2002)

Fishermen are killing H. griseus for sport and for food (as they are being more frequently spotted in fishing areas) faster than ever before. Because of their low reproductive rate, sixgill sharks can easily be over-harvested. There are new regulations being enacted prohibiting the recreational killing of these sharks. The IUCN rates this species as "Lower Risk/Near Threatened", and notes that the lack of population data means that this species could be in more trouble than we know. (Bester, 2001)

Hexanchus griseus are mainly deepwater sharks with shy demeanors. Opportunities to study live specimens are few and far between. Bluntnose sixgill sharks kept in captivity suffer from stress due to their light-sensitive eyes and their large size.