Greenland Shark

The Greenland shark (''Somniosus microcephalus''), also known as the gurry shark or grey 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 due to the depth and remoteness of their natural habitat.

Greenland sharks have the longest lifespan of any known vertebrate, estimated to be between 250 and 500 years. They are among the largest extant shark species, reaching a maximum confirmed length of long and weighing over . They reach sexual maturity at about 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 toxic to mammals due to its high levels of trimethylamine ''N''-oxide, although a treated form of it is eaten in Iceland as a delicacy known as '' 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 400 to 500 cm. 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 gill openings are very small for the species' great size. Female Greenland sharks 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 which attaches itself to the shark's eyes. It was speculated that the copepod may display bioluminescence and thus attract prey for the shark in a mutualistic relationship, but this hypothesis has not been verified. These parasites can cause several damages to the sharks' eyes, such as ulceration, mineralization, and oedema 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 Gb (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 are interlocking, broad and square in shape, containing 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 temperate sharks. These characteristics are interpreted as adaptations to living at great water 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 only reaching per second. Because this speed is a fraction of the speeds seals (a well-established prey item) can reach, biologists are uncertain how the sharks are able to prey on the seals, though it is hypothesized that they may ambush them while they sleep.

Diet

As both scavengers and active predators, Greenland sharks have established themselves as apex predators in Arctic ecosystems. They primarily eat fish (cod, wolffish, haddock, and skates) and seal. Some Greenland sharks have been found to also 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 (>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 (Greenland shark). (n.d.). Animal Diversity Web. https://animaldiversity.org/accounts/Somniosus_microcephalus/ The largest of these sharks were found having eaten redfish, as well as other higher trophic level prey.

It is proposed that, because of their slow speeds and low twitch speed muscle fiber, Greenland sharks hunt marine mammals, such as seals and smaller cetaceans, that are asleep, injured, or sick, as well as scavenging. Regarding most benthic prey, they utilize 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 seal.

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 there exists only one report of human predation by a Greenland shark. Around 1859 in Pond Inlet, Canada, it was reported that a Greenland shark was caught 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 shallower and deeper waters has also been recorded.

Genetic evidence suggests that Greenland sharks historically inhabited deep-sea environments, ranging across pan-oceanic regions at depths greater than 1,000 meters. During the Quaternary period, global cooling influenced thickening sea ice and submerged Arctic landscapes, which likely isolated connectivity between the sharks. However, fluctuating glacial cycles 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. These findings indicate a wider ecological range than previously thought.

In August 2013, researchers from Florida State University caught a Greenland shark in the Gulf of Mexico at a depth of , where the water temperature was . Four previous records of Greenland shark were reported from Cuba and the northern Gulf of Mexico. A more typical depth range is , with the species often occurring in relatively shallow waters in the far north and deeper in the southern part of its range.

In April 2022, a large ''Somniosus'' shark was caught and subsequently released on Glover's Reef off the coast of Belize. This shark was identified as being either a Greenland shark or hybrid; Greenland × Pacific sleeper shark. This observation is notable for being the first possible record of a Greenland shark from the Western Caribbean, and being caught on a nearshore coral reef (the only other record of this species from the Caribbean was made from a deep-water habitat off the Caribbean coast of Colombia). The discovery indicates that Greenland sharks may have a wider distribution in the tropics, primarily at greater depths, than previously believed.

Life history

The Greenland shark has the longest known lifespan of all vertebrate species. It is estimated that the species has a lifespan of at least 272 years, with the oldest individual estimated to be 392 ± 120 years of age. Estimates of age were made using radiocarbon dating of crystals within the lenses of their eyes. Greenland sharks are estimated to reach sexual maturity at around 150 years of age at which point females measure around 4.19 ± 0.04 meters and males measure around 2.84 ± 0.06 meters. One Greenland shark was tagged off the coast of Greenland in 1936 and recaptured in 1952. Its measurements suggest that Greenland sharks grow at a rate of per year. Efforts to conserve Greenland sharks are particularly important due to their extreme longevity, long maturation periods, and the heightened sensitivity of large shark populations. There are also efforts to understand exceptional Greenland shark longevity on the molecular level. Several published works suggest that expansion of transposable elements may play a crucial role. The longer duration and complexity of the female maturation reflect sex differences in reproductive biology. Females undergo seven distinct maturation stages: immature, developing, ripening, early gravid, midterm gravid, late gravid, and post-natal. Males undergo three stages: immature, maturing, and mature. These additional stages in females are necessary for their larger size, preparation for pregnancy, and the eventual birth of a large number of pups.

Reproduction

Greenland sharks are born alive (a process known as ovoviviparity) after an estimated gestation period of 8–18 years. This extremely long gestation rate is crucial to understanding effective conservation strategies around the Greenland shark. Given the ongoing fishing pressures on Greenland sharks, their prolonged long gestation period and slow reproductive rate may severely limit their ability to recover from overfishing.

Estimates of litter size have varied across studies. Some studies suggest that this species produce up to 10 pups per litter, each initially measuring some in length. Based on these estimates, it is thought that Greenland sharks can have between 200 and 700 pups during their lifetime. Within a Greenland shark's uterus, villi serve a key function in supplying oxygen to embryos. It is speculated that oxygen supply is a major limiting factor in the size of litters.

Other studies, however, which take into account ovarian data and analysis of other Squaliform shark species, have estimated that Greenland sharks may produce from 200 to 324 pups per litter, each pup measuring between in length. The same study also confirms that Greenland shark embyros develop inside the uterus without a placenta.

Threats

The shark has historically been hunted for its liver oil up until the development of synthetic oils and cessation of export of liver oil and skin from Greenland in the 1960s. In the 1970s, the species was seen as a problem for other fisheries in western Norway and the government subsidized a fishery to reduce the stock of the species. Today, the Greenland shark is primarily caught as bycatch in industrial fisheries. While about 25 Greenland sharks are caught per year by artisanal fisheries targeting the species in Iceland, 3,500 are caught annually as bycatch in the Arctic and Atlantic Oceans.

The shark is likely affected by quantity, dynamics, and distribution of Arctic sea ice. The rate of projected loss of sea ice will continue to negatively influence the abundance, distribution and availability of prey, while, at the same time, providing greater access for fishing fleets. There is greater potential for new fisheries to develop as more productive and abundant southerly species invade the warming Arctic waters.

Relation to humans

Conservation and management

Greenland sharks are recognized as the longest-living vertebrates on earth. They have a slow growth rate, late maturity period, and low fecundity, making the management and conservation of this species very important. Understanding their exceptionally long gestation period, along with other reproductive and developmental characteristics, is crucial to developing effective conservation strategies for the Greenland shark. Given the ongoing fishing pressures on Greenland sharks, their low productivity and extreme longevity may severely limit their ability to recover from overfishing and bycatch. Therefore, Greenland sharks' longevity and conservative life history traits, in tandem with their vulnerability to accidental catching and commercial fishing, promotes a growing concern for the sustainability of this species.

Hákarl

The flesh of the Greenland shark is toxic because of the presence of high concentrations of urea and trimethylamine oxide (TMAO). If the meat is eaten without pretreatment, the ingested TMAO is metabolized into trimethylamine, which may be a uremic toxin. Occasionally, sled dogs that eat the flesh are unable to stand up because of this effect. Similar toxic effects occur with the related Pacific sleeper shark, but not in most other shark species.

Greenland shark meat is produced and eaten in Iceland where, today, it is known as a delicacy called ''hákarl''. To make the shark safe for human consumption, it is first fermented and then dried in a process that can take multiple months. The shark was traditionally fermented by burying the meat in gravel pits near the ocean for at least several weeks. In the present day, shark cuts are typically fermented in containers that are perforated to allow liquid to drain. The fermentation process converts urea into ammonia and TMAO into TMA, which then drains as liquid from the meat. The meat is then excavated and hung in strips to dry for several more months.

Inuit legends

The Greenland shark's poisonous flesh has a high urea content, which gave rise to the Inuit legend of Skalugsuak, the first Greenland shark. The legend says that an old woman washed her hair in urine (a common practice to kill head lice) and dried it with a cloth. The cloth blew into the ocean to become Skalugsuak. Another legend tells of Sedna, whose father cut off her fingers while drowning her, with each finger turning into a sea creature, including Skalugsuak.

The Greenland shark plays a role in cosmologies of the Inuit from the Canadian Eastern Arctic and Greenland. Igloolik Inuit believe that the shark lives within the urine pot of Sedna, goddess of the sea, and consequently, its flesh has a urine-like smell and acts as a helping spirit to shamans.

See also

* Pacific sleeper shark
* Southern sleeper shark
* List of sharks

References

Further reading

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External links

Greenland Shark and Elasmobranch Education and Research Group


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"Greenland Shark" on ''As It Happens'' 6 May 2008; CBC Radio 1
(WMV file)

Greenland shark – Video on Check123

Old and Cold: Biology of the Greenland shark - project at Univ Copenhagen (bioold.science.ku.dk)

Old and Cold: Biology of the Greenland shark - project at Univ Copenhagen (mbl.ku.dk)

The Greenland shark (Somniosus microcephalus) genome provides insights into extreme longevity - BioArchives Sept 2024 - the first report of the Greenland shark genome

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Greenland shark
Fish of the Arctic Ocean
Fish of the North Atlantic
Fish of Greenland
Ovoviviparous fish
Species that are or were threatened by use as food
Greenlandic cuisine
Taxa named by Marcus Elieser Bloch
Taxa named by Johann Gottlob Theaenus Schneider
Greenland shark