The Greenland shark (Somniosus microcephalus), also known as the grey shark or gurry shark, is a large shark of the family Somniosidae ("sleeper sharks"), closely related to the Pacific and southern sleeper sharks. Inhabiting the North Atlantic and Arctic Oceans, they are notable for their exceptional longevity, although they are poorly studied because of the depth and remoteness of their natural habitat.
Greenland sharks have the longest lifespan of any known vertebrate, estimated to be between 272 and 510 years. They are among the largest extant shark species, reaching a maximum confirmed length of and weighing more than . They reach sexual maturity around 150 years of age and their pups are born alive after an estimated gestation period of 8 to 18 years. The shark is a generalist feeder, consuming a variety of available foods, including carrion.
Greenland shark meat is made toxic to mammals by high levels of trimethylamine N-oxide, although a treated form of it is eaten in Iceland as the pungent delicacy kæstur hákarl. Because they live deep in remote parts of the northern oceans, Greenland sharks are not considered a threat to humans. A possible attack occurred in August 1936 on two British fishermen, but the species was never identified.
Description The Greenland shark is one of the largest known existing species of shark, with adults growing to around . The largest confirmed specimen measured up to long and weighed around . The all-tackle International Game Fish Association (IGFA) record for this species is . It rivals the Pacific sleeper shark (possibly up to long) for the largest species in the family Somniosidae. Genetic data indicate that Greenland sharks diverged from ancestral sleeper sharks in the Canadian Arctic approximately 1–2.34 million years ago during the Pleistocene epoch, likely influenced by glacial fluctuations that periodically isolated marine populations. These ecological challenges may have driven physiological and metabolic adaptations for cold, deep waters.
The Greenland shark is a thickset species, with a short, rounded snout, small eyes, and small dorsal and pectoral fins. The five gill openings are very small for the species's great size. Females are typically larger than males, with males reaching maturity at a smaller size than females.
Coloration can range from pale creamy-gray to blackish-brown and the body is typically uniform in color, though whitish spots or faint dark streaks are occasionally seen on the back.
The shark is often infested by the copepod Ommatokoita elongata, a crustacean that attaches itself to the shark's eyes. The copepod may display bioluminescence, thus attracting prey for the shark in a mutualistic relationship, but this hypothesis has not been verified. These parasites can cause multiple forms of damage to the sharks' eyes, such as ulceration, mineralization, and edema of the cornea, leading to almost complete blindness. This does not seem to reduce the life expectancy or predatory ability of Greenland sharks, due to their strong reliance on smell and hearing.
The genome of the Greenland shark was published in 2024. It is 6.45 billion base pairs in length.
Dentition
When feeding on large carcasses, the shark employs a rolling motion of its jaw. The 48 to 52 teeth of the upper jaw are very thin and pointed, lacking serrations. These upper jaw teeth act as an anchor, while the lower jaw proceeds to cut massive chunks out of the prey.
The 48 to 52 lower teeth interlock, are broad and square in shape, and contain short, smooth cusps that point outward. Teeth in the two halves of the lower jaw are strongly pitched in opposite directions.
Physiology Like other Elasmobranchii, Greenland sharks have high concentrations of the two nitrogenous compounds urea and trimethylamine N-oxide (TMAO) in their tissues, which increase their buoyancy and function as osmoprotectants. TMAO also counteracts the protein-destabilizing tendencies of urea and deep-water pressure. Its presence in the tissues of both elasmobranch and teleost fish has been found to increase with depth.
The blood of Greenland sharks contains three major types of hemoglobin, made up of two copies of globin combined with two copies of three very similar subunits. These three types show very similar oxygenation and carbonylation properties, which are unaffected by urea, an important compound in marine Elasmobranchii physiology. They display identical electronic absorption and resonance in Raman spectroscopy, indicating that their heme-pocket structures are identical or highly similar. The hemoglobins also have a lower affinity for oxygen compared to those of temperate sharks. These characteristics are believed to be adaptations to living at great depths.
Biology The Greenland shark prefers cold water temperatures () and deep water ().
As an ectotherm living in a just-above-freezing environment, this species is sluggish and slow moving, with the lowest swim speed and tail-beat frequency for its size across all fish species, which most likely correlates with its very slow metabolism and extreme longevity. It swims at an average of per second, with its fastest cruising speed reaching about per second. Because this speed is a fraction of the speed reached by seals (a well-established prey item), biologists are uncertain how the sharks are able to prey on the seals, though the sharks are thought to ambush while seals are asleep.
Like other sharks, the Greenland Shark has a well-developed lateral line system. This system is made up of fluid-filled canals that run beneath their skin, along the sides of their bodies, and over their heads. Small pores connect these canals to the water surrounding them, allowing them to detect vibrations and low-frequency pressure changes caused by movement in the water, such as prey swimming nearby. This is important, as Greenland Sharks live in deep, dark environments that provide no sunlight, where they cannot rely heavily on vision. Due to many Greenland Sharks experiencing eye damage from parasitic copepods, it is possible that vision is not their main, relied-upon sense. Instead, the lateral line, along with their sharp sense of smell, plays a key role in helping them locate prey and move through their environment, despite them being slow swimmers.
Genetics The Greenland Shark genome has roughly 5.9 billion base pairs, which is one of the largest shark genomes ever sequenced. More than two-thirds of the genes found in the Greenland Shark genome are transposable elements, commonly referred to as jumping genes, which are genes that can self-replicate. These genes often cause issues with normal gene function and lead to mutations and other genetic complications. However, in Greenland Sharks, DNA repair genes are hypothesized to be able to ‘jump’ like transposable elements, helping mitigate the damage caused by the other jumping genes. 81 genes that are unique to the Greenland Shark genome are suggested to be linked to DNA repair.
Because of the parasitic copepods that attach to the Greenland Shark’s eyes, along with the dark and visually obstructed environment, their vision may appear to be useless or impaired. However, the Greenland Sharks’ optic tectum (the brain region responsible for processing visual information) is a similar size to that of other elasmobranchii. Greenland Sharks have a tapetum lucidum, which enhances photon capture in low-light conditions. Greenland Sharks also possess rod-dominated retinas and rely on rod-specific phototransduction genes to be able to see in low-light conditions. The genes responsible for this rod-specific phototransduction include the rh1, sag, gnat1, gucy2f, pde6a, pde6b, pde6d, pde6g, grk1, cnga1, gnb5, rcvrn and gngt1 genes. Furthermore, only one set of cone-specific phototransduction genes (grk7) is found in the Greenland Shark’s genome, confirming its reliance on rod-based vision.
Diet As both scavengers and active predators, Greenland sharks have established themselves as apex predators in Arctic ecosystems. They primarily eat fish (herring, salmon, smelt, cod, wolffish, haddock, halibut, redfish, sculpins, lumpfish, and skates) and seals.Somniosus microcephalus (Greenland shark). (n.d.). Animal Diversity Web. https://animaldiversity.org/accounts/Somniosus_microcephalus/ Some Greenland sharks have been found also to eat minke whale. Small Greenland sharks eat predominantly squid, as well as sea birds, crabs, amphipods, sea snails, brittle stars, sea urchins, and jellyfish. Large Greenland sharks (greater than 200 cm or 79 in) eat prey such as demersal fishes, as well as seals and small cetaceans such as oceanic dolphins and porpoises.Somniosus microcephalus summary page. (n.d.). FishBase. https://www.fishbase.se/summary/Somniosus-microcephalus?ref=biodaily.itSomniosus microcephalus. (n.d.). Discover Fishes. https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/somniosus-microcephalus/ The largest of these sharks were found having eaten redfish, as well as other higher trophic level prey.
Because of their slow speeds and low-twitch-speed muscle fiber, Greenland sharks are proposed to hunt marine mammals, such as seals and smaller cetaceans, that are asleep, injured, or sick, as well as scavenging. Regarding most benthic prey, they use their cryptic coloration, approaching prey undetected before closing the remaining distance. Once they get close to their prey, Greenland sharks expand their buccal cavity to create suction, drawing in prey. This suction mechanism is the likely explanation for why the gut contents of Greenland sharks are often of whole prey specimens.
Greenland sharks have also been found with remains of moose, horse, and reindeer (in one case an entire reindeer carcass) in their stomachs. Polar bear remains have also been recovered, though these are thought to be of bears that died from other causes. The Greenland shark is known to be a scavenger and is attracted by the smell of rotting meat in the water. The sharks have frequently been observed lurking around fishing boats and herds of seals.
Although such a large shark could easily consume a human swimmer, the frigid waters it typically inhabits make the likelihood of attacks on people very low. To date only one report exists of human predation by a Greenland shark. Around 1859 in Pond Inlet, Canada, a Greenland shark was caught reportedly containing a human leg in its stomach. This story, however, was never scientifically investigated and remains unverified.
Migration Greenland sharks migrate annually based on depth and temperature rather than distance, although some do travel. During the winter, the sharks congregate in the shallows (up to 80° north) for warmth, but migrate separately in summer to the deeps or even farther south. The species has been observed at a depth of by a submersible investigating the wreck of the SS Central America that lies about east of Cape Hatteras, North Carolina. Daily vertical migration between shallow and deep waters has also been recorded.
Genetic evidence suggests that Greenland sharks historically inhabited deep-sea environments, ranging across panoceanic regions at depths greater than 1,000 m. During the Quaternary period, global cooling influenced thickening sea ice and submerged Arctic landscapes, which likely severed connectivity between the local shark populations. Fluctuating glacial cycles, though, periodically opened opportunities for population mixing, impacting migration patterns and genetic diversity. Today, Greenland sharks are found not only in Arctic and sub-Arctic waters, but also in regions further south, such as the Mid-Atlantic Ridge and the Gulf of Mexico.Grubs, Dean (15 August 2013). " Deep-C Scientists Capture First Greenland Shark in the Gulf of Mexico". deep-c.org These findings indicate a wider ecological range than previously thought.