Hexanchus griseus

Geographic Range

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 .

• Biogeographic Regions
• indian ocean
• atlantic ocean atlantic ocean
• pacific ocean pacific ocean
• mediterranean sea

• Other Geographic Terms
• cosmopolitan cosmopolitan

Habitat

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.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• benthic benthic
• coastal coastal

Physical Description

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.

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.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

Development

Little is yet known about the life cycle and fetal development of Hexanchus griseus .

Reproduction

Very little is known about these sharks in terms of their social behavior and thus little is known about their mating systems. There are a few theories, however, attempting to explain how H. griseus mates. Researchers believe that the morphology of the teeth of H. griseus play an important role in mating. The male has a more erect primary cusp than do the females. The male is believed to nip the female's gills with this cusp in order to catch her attention and entice her into mating. Evidence supporting this idea of courtship is evident by the seasonal scars that appear on females every year presumably from being nipped by males. Bluntnose sixgill sharks are believed to be primarily solitary animals and there is no information indicating whether they prefer one or many mates.

There is not much information pertaining to the reproductive behavior of Hexanchus griseus ; however, there is some hypothetical information available. These sharks are believed to meet seasonally, moving to shallower depths in the May to November months. Scientists are unsure of the bluntnose sixgill shark's gestation period, but it is thought to be longer than 2 years. The means of reproduction for these sharks is ovoviviparity, meaning they carry their eggs internally until they hatch. Babies develop within the mother without a placenta to provide nourishment, and they are born at a fairly mature size (generally 70 cm at birth). Each litter can number from about 22 to 108 pups and this incredibly large litter size for H. griseus could suggest that mortality rates for the pups are very high. Little is known about their maturation because until recently determining their age was difficult as a result of their poorly calcified vertebrae. The pups of H. griseus , however, are speculated to mature around 11 to 14 years for males and 18 to 35 years for females. Little else is known about its reproductive system.

• Key Reproductive Features
• seasonal breeding seasonal breeding
• sexual sexual
• fertilization fertilization
  • internal internal
• ovoviviparous ovoviviparous

There is no information available pertaining to parental care for Hexanchus griseus . However, as with other sharks, it can be assumed that no parental care is given to the young, which can number up to 108.

• Parental Investment
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• natatorial natatorial
• nocturnal nocturnal
• motile motile
• migratory migratory
• solitary solitary

Communication and Perception

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).

• Communication Channels
• visual visual
• tactile tactile

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• vibrations vibrations
• chemical chemical
• electric electric

Food Habits

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.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • eats non-insect arthropods
  • molluscivore molluscivore

• Animal Foods
• mammals
• fish
• carrion carrion
• mollusks
• aquatic crustaceans

Predation

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.

Ecosystem Roles

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.

Economic Importance for Humans: Positive

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.

• Positive Impacts
• food food

Economic Importance for Humans: Negative

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.

Conservation Status

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.

Other Comments

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.

Additional Links

Encyclopedia of Life

Contributors

David Armitage (editor), Animal Diversity Web, George Hammond (editor), Animal Diversity Web.

Jessica Bauml (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

References

Bester, C. 2001. "Bluntnose Sixgill Shark" (On-line). Icthyology at the Florida Museum of Natural History. Accessed December 07, 2004 at http://www.flmnh.ufl.edu/fish/Gallery/Descript/BSixgill/Bsixgill.html .

Ebert, D. 2002. Some observations on the reproductive biology of the Sixgill Shark Hexanchus griseus . South African Journal of Marine Science , 24: 359-363.

MacQuity, M., D. King. 2000. SHARKS . New York: DK Publishing, Inc.

Martin, R. 2000. "Biology of the Bluntnose Sixgill" (On-line). Accessed July 28, 2004 at http://elasmo-research.org/research/sixgill.htm .

McFarlane , G., J. King, M. Saunders . 2002. Preliminary study on the use of neural arches in the age determination of bluntnose sixgill sharks. Fish Bulletin , 4: 861-864.

Musick, J. A., B. McMillan. 2002. The Shark Chronicles . New York: Henry Holt and Company, Inc.

Nevell, L. "British Marine Life Study Society: Six-gilled Shark" (On-line ). Accessed 3/17/03 at http://ourworld.compuserve.com/homepages/BMLSS/six-gill.htm .

Parker, S., J. Parker. 2002. The Encyclopedia of Sharks . Ontario: Firefly Books LTD.

Shark Foundation. "Hexanchus griseus" (On-line ). Accessed 3/17/03 at http://www.shark.ch/cgi-bin/Sharks/spec_conv.pl?E+Hexanchus.griseus .

IUCN. "IUCN Red List of Threatened Species: Hexanchus griseus" (On-line ). Accessed 3/19/03 at http://www.redlist.org/search/details.php?species=10030 .