Sixgill Shark

The Bluntnose Sixgill Shark (Hexanchus griseus) is a massive, primitive-lineage shark in the family Hexanchidae, whose members retain six gill slits rather than the five found in most modern sharks, a trait reflecting an ancient evolutionary lineage. Widely distributed in deep waters around the globe, it is one of the largest predatory sharks, rivaling great whites in length. Normally confined to deep continental slopes and canyons during the day, it undertakes vertical migrations into much shallower water at night, particularly in high-latitude fjords, making it occasionally accessible to divers. It is considered Near Threatened due to susceptibility to bycatch and slow reproduction.

Key field marks:

• Six gill slits per side, versus five in most sharks
• Single small dorsal fin set far back near the tail
• Long lower caudal lobe, comb-like lower teeth
• Uniform dark gray-brown to olive coloration, paler below
• Blunt, broadly rounded snout, large greenish eyes

The six gill slits are diagnostic and immediately separate this species from virtually all other large sharks.

Found in deep waters worldwide, typically along continental and insular slopes at depths from 100 to over 2,000 m, though it moves into much shallower water, under 20 m, at night or in cold, food-rich fjords and high-latitude coastal areas such as Puget Sound and Norwegian fjords. Prefers cool, dark deep-sea environments over soft sediment or rocky slope habitat, undertaking pronounced diel vertical migrations tied to feeding and light avoidance.

Sixgill Sharks are slow-moving, opportunistic predators and scavengers, feeding on a wide range of prey including bony fish, other sharks and rays, and carrion from marine mammal falls on the deep sea floor. They undertake extensive diel vertical migrations, remaining in deep, dark water during the day and rising toward the surface at night to forage, likely to avoid visual predators and exploit nocturnal prey activity. Reproduction is ovoviviparous, with females producing very large litters, sometimes over 100 pups, after a lengthy gestation. As one of the largest deep-sea predators, they play an important scavenging role in continental slope ecosystems.