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Greenland shark

From Wikipedia, the free encyclopedia

Greenland shark
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Order: Squaliformes
Family: Somniosidae
Genus: Somniosus
Species:
S. microcephalus
Binomial name
Somniosus microcephalus
Range of the Greenland shark
Synonyms
  • Squalus squatina (non Linnaeus, 1758)
  • Squalus carcharis (Gunnerus, 1776)
  • Squalus microcephalus Bloch & Schneider, 1801
  • Somniosus brevipinna (Lesueur, 1818)
  • Squalus borealis (Scoresby, 1820)
  • Squalus norvegianus (Blainville, 1825)
  • Scymnus gunneri (Thienemann, 1828)
  • Scymnus glacialis (Faber, 1829)
  • Scymnus micropterus (Valenciennes, 1832)
  • Leiodon echinatum (Wood, 1846)

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.[2] 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.[3]

Greenland sharks have the longest lifespan of any known vertebrate, estimated to be between 250 and 500 years.[4] They are among the largest extant species of shark, reaching a maximum confirmed length of 6.4 m (21 ft) long and weighing over 1,000 kg (2,200 lb). 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.[5]

Greenland shark meat is toxic to mammals due to its high levels of trimethylamine N-oxide,[6] although a treated form of it is eaten in Iceland as a delicacy known as kæstur hákarl.[7] Because they live deep in remote parts of the northern oceans, Greenland sharks are not considered a threat to humans, and no recorded attacks have ever occurred.

Description

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The Greenland shark is one of the largest known extant species of shark, with adults growing to around 400 to 500 cm.[8] The largest confirmed specimen measured up to 6.4 metres (21 ft) long and weighed around 1,023 kilograms (2,255 lb).[9][10][11] The all-tackle International Game Fish Association (IGFA) record for this species is 775 kg (1,709 lb).[12] It rivals the Pacific sleeper shark (possibly up to 7 m or 23 ft long) for the largest species in the family Somniosidae.

Greenland shark at Admiralty Inlet, Nunavut, with an Ommatokoita

The Greenland shark is a thickset species, with a short, rounded snout, small eyes, and small dorsal and pectoral fins.[9] The gill openings are very small for the species' great size. Female Greenland sharks are typically larger than males.[13]

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.[11]

The shark is often infested by the copepod Ommatokoita elongata, a crustacean which attaches itself to the shark's eyes.[14] 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.[15] These parasites also damage the eyeball in several ways, 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.[14][16]

Dentition

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The dentition of a Greenland shark

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.[17] Teeth in the two halves of the lower jaw are strongly pitched in opposite directions.[18]

Behavior

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Diet

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As both scavengers and active predators, Greenland sharks have established themselves as apex predators in Arctic ecosystems.[19][20] They primarily eat fish (cod, wolffish, haddock, and skates) and seal.[19][20] Some Greenland sharks have been found to also eat minke whale.[20] Small Greenland sharks eat predominantly squid, as well as sea birds, crabs, amphipods, marine snails, brittle stars, sea urchins, and jellyfish, while the larger sharks that are greater than 200 cm (79 in) were discovered eating prey such as epibenthic and benthic fishes, as well as seals and small cetaceans such as oceanic dolphins and porpoises.[21][22][23] The largest of these sharks were found having eaten redfish, as well as other higher trophic level prey.[24]

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. Regarding most other benthic species, they utilize their cryptic coloration, and approach prey undetected before closing the remaining distance, expanding their buccal cavity to create suction, drawing in prey. This is the likely explanation for why the gut contents of Greenland sharks are often whole prey specimens.[20]

Greenland sharks have also been found with remains of moose, polar bear, horse, and reindeer (in one case an entire reindeer body) in their stomachs.[25][26][27] 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 gathering around fishing boats.[25] They also scavenge on seals.[28]

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. No cases of predation on humans have been verified.[25]

Movement and migration

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The Greenland shark prefers cold water temperatures (—1.1 to 7.4°C) and deep water (100 to 1,200m).[29] 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.[30] It swims at an average of 0.34 m·s-1, with its fastest cruising speed only reaching 0.74 m·s-1.[31][32] Because this top speed is a fraction of that of a typical seal in their diet, biologists are uncertain how the sharks are able to prey on the seals. It is hypothesized that they may ambush them while they sleep.[32][33]

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 2,200 metres (7,200 ft) by a submersible investigating the wreck of the SS Central America that lies about 160 nautical miles (180 mi; 300 km) east of Cape Hatteras, North Carolina.[34] Daily vertical migration between shallower and deeper waters has also been recorded.[35]

In August 2013, researchers from Florida State University caught a Greenland shark in the Gulf of Mexico at a depth of 1,749 m (5,738 ft), where the water temperature was 4.1 °C (39.4 °F).[36] Four previous records of Greenland shark were reported from Cuba and the northern Gulf of Mexico.[37] A more typical depth range is 0–1,500 m (0–4,900 ft), with the species often occurring in relatively shallow waters in the far north and deeper in the southern part of its range.[38][39]

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 a Greenland/Pacific sleeper shark hybrid. 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.[40]

When hoisted upon deck, it beats so violently with its tail, that it is dangerous to be near it, and the seamen generally dispatch it, without much loss of time. The pieces that are cut off exhibit a contraction of their muscular fibres for some time after life is extinct. It is, therefore, extremely difficult to kill, and unsafe to trust the hand within its mouth, even when the head is cut off. And, if we are to believe Crantz, this motion is to be observed three days after, if the part is trod on or struck.

— Henry William Dewhurst, The Natural History of the Order Cetacea (1834)[41]

Longevity

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The Greenland shark has the longest known lifespan of all vertebrate species.[42] 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.[43] Greenland sharks are estimated to reach sexual maturity at around 150 years of age at which point females measure around 4.19±0.04 and males measure around 2.84±0.06.[43][44][45] 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 0.5–1 cm (1⁄4–1⁄2 in) per year.[46] Efforts to conserve Greenland sharks are particularly important due to their extreme longevity, long maturation periods, and the heightened sensitivity of large shark populations.[47]

Reproduction

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Greenland sharks are born alive (a process known as ovoviviparity) after an estimated gestation period of 8–18 years.[48] 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 38–42 cm in length.[49] Based on these estimates, It is thought that, due to their extreme longevity, Greenland sharks can have between 200 to 700 pups during their lifetime.[48] 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.[50] Other studies, however, have estimated that Greenland sharks may produce between 200 to 324 pups per litter, measuring between 35–45 cm in length.[51]

Physiological adaptations

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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[52] and function as osmoprotectants. TMAO also counteracts the protein-destabilizing tendencies of urea[53][54] and deep-water pressure.[55][54] Its presence in the tissues of both elasmobranch and teleost fish has been found to increase with depth.[55][56]

The blood of Greenland sharks contains three major types of hemoglobin, made up of two copies of α globin combined with two copies of three very similar β 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.[57]

Threats

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Overfishing and climate change are the main driving factors of Greenland shark diminishing numbers even though studies have shown that their metabolic enzymes are more active in warmer temperatures.

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.[58] 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.[59] 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.[60]

The shark is likely affected by quantity, dynamics, and distribution of Arctic sea ice.[61] 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.[61] There is greater potential for new fisheries to develop as more productive and abundant southerly species invade the warming Arctic waters.[62]

Conservation and management

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Greenland sharks are recognized as the longest-lived 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. As a result of their low productivity and extreme longevity, this species is particularly susceptible to 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.[61]

Hákarl

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Greenland shark meat or kæstur hákarl in Iceland

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.[63][64]

Greenland shark meat produced and eaten in Iceland where, today, it is known as a delicacy called hákaral. 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. [65][66]

Inuit legends

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The Greenland shark's poisonous flesh has a high urea content, which gave rise to the Inuit legend of Skalugsuak, the first Greenland shark.[67] 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.[68] Another legend tells of Sedna whose father cut off her fingers while drowning her, with each finger turning into a sea creature, including Skalugsuak.[69]

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.[70]

See also

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Notes

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References

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Further reading

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