Bluntnose Sixgill Shark

Overview

The Bluntnose Sixgill Shark (Hexanchus griseus), historically celebrated by deep-sea oceanographers as the cow shark or mud shark, is a massive, primitive apex predator inhabiting the cold, pitch-black abyssal trenches and continental slopes of the world’s oceans. Belonging to the cow shark family Hexanchidae within the ancient order Hexanchiformes, this deep-sea titan represents one of the most archaic living shark lineages on Earth, with direct fossil relatives dating back over two hundred million years to the Jurassic Period. Growing up to eighteen feet in length and weighing over sixteen hundred pounds, it is the largest primitive shark alive today.

Ecologically, Hexanchus griseus operates as a dominant apex predator and primary deep-sea scavenger in bathypelagic and demersal marine food webs. Cruising along continental shelves, seamounts, and submarine drop-offs at depths down to 2,500 meters (8,200 feet), it preys on other sharks, skates, chimaeras, giant squids, and marine mammals, while consuming sunken whale carcasses (whale falls). In doing so, it recycles vast amounts of organic carbon across abyssal ocean floors and regulates populations of deep-sea demersal fishes.

Biology and Physical Characteristics

Hexanchus griseus displays primitive anatomical traits that starkly contrast with modern lamnid and carcharhinid sharks. Most famously, while virtually all modern sharks possess five gill slits, the bluntnose sixgill retains six long, conspicuous gill slits on each side of its broad neck. Its snout is short, blunt, and rounded, housing two large, mesmerizing, fluorescent emerald-green eyes that lack a nictitating membrane; packed with light-amplifying rods and a reflective tapetum lucidum, these glowing eyes provide exceptional night vision in pitch-black deep-sea depths. Unlike modern sharks that possess two dorsal fins, it has only a single, small dorsal fin set far back near the caudal peduncle, functioning in tandem with an elongated, sweeping upper tail lobe to deliver sudden, stealthy bursts of speed. Its dentition is extraordinary: the upper jaw bears sharp, curved spike teeth for impaling slippery prey, while the lower jaw features broad, comb-like, multi-cusped teeth engineered to saw cleanly through thick seal blubber and shark flesh.

Behavior and Ecology

Bluntnose sixgill sharks are sluggish, energy-conserving nocturnal hunters that undertake massive diel vertical migrations: spending daylight hours resting along freezing ocean floors at depths exceeding a thousand meters, they ascend into shallower coastal waters (under two hundred meters) under cover of night to ambush seals, spiny dogfish, and salmon. Reproductively, they are ovoviviparous (aplacental viviparous), with embryos developing inside egg cases within the female’s body; fecundity is colossal among sharks, with females giving birth to staggering litters of forty to over one hundred fully formed pups measuring nearly two feet in length.

Conservation and Human Impact

The Bluntnose Sixgill Shark is classified as Vulnerable on the IUCN Red List due to its exceptionally slow growth, late sexual maturity (females do not mature until thirty years of age), and susceptibility to commercial deep-water bottom trawls and longlines. Because deep-sea sharks accumulate high concentrations of heavy metals like mercury in their tissues, they are vulnerable to bioaccumulation. Marine protected areas encompassing deep submarine canyons—such as Monterey Canyon in California and the deep fjords of Puget Sound and British Columbia—provide critical research sanctuaries where marine biologists use deep-submersible cameras to study this elusive Jurassic giant.

Interesting Factoid

The sixgill shark has glowing emerald-green eyes like night-vision goggles: deep in the pitch-black ocean trenches, its eyes shine with a spooky, fluorescent green glow that captures every single photon of bioluminescent light, allowing it to hunt giant squids and other sharks in total darkness.