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Six-gilled Shark (HEXANCHUS GRISEUS)

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SHARK, SIX-GILLED / HEXANCHUS GRISEUS
24 50 143.00 315.4 El Hierro Canary Is. 160996 H. Otto
37 80 466.00 1027.5 Faial Azores 131187 W. Reece
60 130 485.00 1069.4 Faial Azores 181090 J. Reece
AT AT 485.00 1069.4 Faial Azores 181090 J. Reece

The six-gilled shark (HEXANCHUS GRISEUS)

occurs 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 2500m.

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)

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. (MacQuity and King, 2000)

six-gilled shark (hexanchus griseus)

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. (Musick and McMillan, 2002; Parker and Parker, 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)

Information supplied by http://animaldiversity.ummz.umich.edu/site/accounts/information/Hexanchus_griseus.html

 
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