The Six-Gilled Shark (Hexanchus griseus), in the family Hexanchidae (the cowsharks), is one of many "living fossils" having roamed the oceans practically unchanged over the past 150 million years.
Ranging in colour from dark brown to dark silvery grey with large, opalescent eyes, Hexanchus griseus possesses six gill slits which distinguish it from the majority of sharks which have only five. Its single dorsal fin is set near the tail making the shark distinctive.
Mature specimens range from 3.5 metres to nearly 5 metres from the tip of the snout to the end of the tail. They have stout bodies and round, bluntly pointed heads. They live in deep ocean waters between 500 metres and 2,000 metres below trhe surface. It is possible that mature specimens make their way to feed in shallower waters at night to feed but very little is known about their behaviour because of the inacessability of their deep ocean habitat.
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Mature females can reach a length of 4.82 m (15.4 feet) and more than 650 kg (1432.6 lbs) in weight.
Hexanchus griseus ranks amongst the largest of the predatory sharks; the Great White Shark reaches lengths of 7 metres and the and the deep water (it has been filmed at a depth of 2,200 metres) Sleeper Shark (Somniosus microcephalus) grows up to 6.4 metres in length.
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Specimens caught in coastal waters up to depths of 500 metres tend to be juveniles and immature adults. These tend to be small, rarely exceeding 2.5 metres (8 feet) in length and 90 kg (200 lb) in weight.
The species is ovoviviparous, carrying the eggs until they hatch inside the body of the mother which thus gives birth to live young which are fully formed. The species has a remarkable high fecundity (possibly the result of their large size and isolation in the deep ocean making it difficult to find a mate) with litters of 22 to 108 pups which are boren about 0.65 metres in length and weighing about 1.2 kg.
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A female 4.21 m which was captured off Church Rock, California, was found to contain 51 near term embryos within her body.
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Hexanchus griseus preys on large and fast-moving species including numerous sharks and rays, especially the spurdog and on bony fish with large deep water hake (Merluccius merluccius) a major prey item. Also on crabs, shrimps, squid, swordfish, cetaceans and other marine mammals.
The range Hexanchus griseus in the deep ocean waters of the abyssal plain off the edge of the continental shelf is very large but there are few details due to the inaccessibility of its environment. It is a rare visitor to the coastal waters of the British Isles.
The sharks have Found when feeding on the large stocks of deep water fish in commercial fishing grounds around the British Isles. A few specimens ranging up to 54.3 kg have been caught from boats by offshore anglers off South West Ireland around Dingle Bay and Kinsale Bay (County Kerry).
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The UK record for a Six-Gilled Shark caught on rod and line stands at 69.8 kg (154 lb). It was caught on the Irish coast by Andrew Ball of Essex in 1968.
A much larger specimen weighing 142.8 kg (315 lb) was caught off Baltimore in Southern Ireland late in 1993 by Stefano but, as the rod was touched by two people (it was handed to Keith Walker after 45 minutes who brought it to the surface), Andrew Ball's 1968 still stands.
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Commercial fishermen are not required to keep records of this shark which is returned to the sea if hauled up in their nets so Hexanchus griseus may be much commoner in the waters off the British Isles than the few records indicate.
Information wanted on the Six-Gilled Shark, Hexanchus griseus;-
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