Sixgill Sharks, Hexanchus griseus
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Description & Behavior
Sixgill or bluntnose sixgill sharks, Hexanchus griseus (Bonnaterre, 1788), have a number of common names. They are heavy-bodied, broad-headed sharks with broad, rounded snouts and ventral mouths containing 6 rows of blade-like (saw-like), comb-shaped teeth. Their anal fins are smaller than their dorsal fins. Brown or gray above, paler below, with a light stripe along their sides. Fins have white edges. Fluorescent green eyes. Six gill slits are very long. Body lengths ranges from 1.5-8 m long.
Sixgill sharks are members of the most ancient frill and cow sharks order, Hexanchiformes. Hexanchiform sharks have a single dorsal fin, either six or seven gill slits (versus the 5 found in all other existing sharks), and no nictitating membrane. Frilled sharks, Chlamydoselachus sp., are very different from the cow sharks and are likely to be moved to their own order Chlamydoselachiformes in the near future.
There are currently four known species of cow sharks:
- Sixgill sharks, Hexanchus griseus,
- Bigeyed sixgill sharks, Hexanchus nakamurai,
- Sevengill sharks, Notorynchus cepedianus, and
- Sharpnose sevengill sharks, Heptranchias perlo.
World Range & Habitat
A deepwater species (surface hunting at night to depths to 2,500 m during the day) of the outer continental and insular shelves and upper slopes. Juveniles may be found close inshore.
Circumglobal in tropical and temperate seas. Western Atlantic: North Carolina to Florida (USA) and northern Gulf of Mexico to northern Argentina. Eastern Atlantic: Iceland and Norway to Namibia, including the Mediterranean. Indian Ocean: Madagascar, Mozambique, and South Africa. Western Pacific: eastern Japan to New Zealand and Hawaii. Eastern Pacific: Aleutian Islands, Alaska to Baja California, Mexico; also Chile. Highly migratory species.
Feeding Behavior (Ecology)
Bluntnose sixgill sharks, Hexanchus griseus, feed on a wide range of marine species, including other sharks, rays, chimaeras, bony fish, squids, crabs, shrimps, carrion and even seals. They are probably nocturnal hunters.
Dioecism, internal (oviduct) fertilization, ovoviviparous. The pups eat any unfertilized eggs and even each other, a behavior known as oviphagy. With other species very few pups in a litter survive until birth due to this form of sibling cannibalism. Litters are very large containing 22-108 pups. Size at birth 60-75 cm. Distinct pairing with embrace during mating.
Ovoviviparous: eggs are retained within the body of the female in a brood chamber where the embryo develops, receiving nourishment from a yolk sac. This is the method of reproduction for the "live-bearing" fishes where pups hatch from egg capsules inside the mother's uterus and are born soon afterward. Also known as aplacental viviparous.
Conservation Status & Comments
A valuable food and sports fish, this species seems unable to sustain target fisheries and is als0 taken as bycatch (e.g., in Centrophorus liver oil fisheries now underway over large areas of the Indo-Pacific, etc.). Fisheries activity in parts of its range, including the Northeast Pacific, has led to the depletion of regional populations. Their liver is reported to be poisonous to eat. Marketed fresh, frozen, or dried salted; this species is also utilized as a source of oil and fishmeal.
Bluntnose sixgill sharks are not known to have attacked people without provocation though this large shark has been reported to scare a few deep divers.
Listed as Near Threatened on the IUCN Red List of Threatened Species:
NEAR THREATENED (NT) - A taxon is Near Threatened when it has been evaluated against the criteria but does not qualify for Critically Endangered, Endangered or Vulnerable now, but is close to qualifying for or is likely to qualify for a threatened category in the near future.
References & Further Research
Research Hexanchus griseus » Barcode of Life ~ BioOne ~ Biodiversity Heritage Library ~ CITES ~ Cornell Macaulay Library [audio / video] ~ Encyclopedia of Life (EOL) ~ ESA Online Journals ~ FishBase ~ Florida Museum of Natural History Ichthyology Department ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList (Threatened Status) ~ Marine Species Identification Portal ~ NCBI (PubMed, GenBank, etc.) ~ Ocean Biogeographic Information System ~ PLOS ~ SIRIS ~ Tree of Life Web Project ~ UNEP-WCMC Species Database ~ WoRMS
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