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Greenland shark

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Greenland shark
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Squaliformes
Family: Somniosidae
Genus: Somniosus
Species:
S. microcephalus
Binomial name
Somniosus microcephalus
Range of the Greenland shark
Synonyms
  • Squalus squatina(nonLinnaeus,1758)
  • Squalus carcharis(Gunnerus,1776)
  • Squalus microcephalusBloch & 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)

TheGreenland shark(Somniosus microcephalus), also known as thegurry sharkorgrey shark,is a largesharkof the familySomniosidae( "sleeper sharks" ), closely related to thePacificandsouthern 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.

Greenland sharks have the longest lifespan of any knownvertebrate,estimated to be between 250 and 500 years.[3]They are among the largest extant species of shark, usually growing to between 2.4 and 7 m (7.9 and 23.0 ft) long and weighing between 400 and 1,400 kg (880 and 3,090 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 ageneralistfeeder, consuming a variety of available foods.[4]

Greenland shark meat is toxic to mammals due to its high levels oftrimethylamineN-oxide,[5]although a treated form of it is eaten inIcelandas a delicacy known askæstur hákarl.[6]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 likely one of the largest living species of shark. It usually grows to between 2.4 and 7 m (7.9 and 23.0 ft) long and weighs between 400 and 1,400 kg (880 and 3,090 lb).[7][8][9][10]The all-tackleInternational Game Fish Association(IGFA) record for this species is 775 kg (1,709 lb).[11]

Greenland shark atAdmiralty Inlet,Nunavut,with anOmmatokoita

Males are typically smaller than females. It rivals thePacific sleeper shark(possibly up to 7 m or 23 ft long) as the largest species in the familySomniosidae.The Greenland shark is a thickset species, with a short, rounded snout, small eyes, and very small dorsal and pectoral fins. The gill openings are very small for the species' great size.

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.[9]

The shark is often infested by thecopepodOmmatokoita elongata,a crustacean which attaches itself to the shark's eyes.[12]It was speculated that the copepod may displaybioluminescenceand thus attract prey for the shark in a mutualistic relationship, but this hypothesis has not been verified.[13]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.[12][14]

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.[9]Teeth in the two halves of the lower jaw are strongly pitched in opposite directions.[15]

Behavior

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Diet

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The Greenland shark is anapex predatorand eats primarily fish, although they have been observed actively huntingsealsin Canada.[4]The prey found in the stomachs of Greenland sharks is an indicator of the active hunting patterns of these predators.[16]Recorded fish prey have included smallersharks,skates,eels,herring,capelin,Arctic char,cod,rosefish,sculpins,lumpfish,wolffish,andflounder.[9]Small Greenland sharks eat predominantlysquid,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. The largest of these sharks were found having eatenredfish,as well as other highertrophic levelprey.[17]

It is proposed that, because of their slow speeds andlow 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.[16]

Greenland sharks have also been found with remains ofmoose[18],polar bearandreindeer(in one case an entire reindeer body) in their stomachs.[9][19]The Greenland shark is known to be ascavengerand is attracted by the smell of rotting meat in the water. The sharks have frequently been observed gathering around fishing boats.[9]They also scavenge on seals.[20]

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.[9]

Movement and migration

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The shark occupies what tends to be a very deep environment seeking its preferred cold water (−0.6 to 12 °C or 31 to 54 °F) habitat.[1]As anectothermliving in a just-above-freezing environment, the Greenland shark 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.[21]It swims at 1.22 km/h (0.76 mph), with its fastest cruising speed only reaching 2.6 km/h (1.6 mph).[22]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.[23]

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 to80° 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 theSSCentral Americathat lies about 160nautical miles(180 mi; 300 km) east of Cape Hatteras,North Carolina.[24]Daily vertical migration between shallower and deeper waters has also been recorded.[25]

In August 2013, researchers fromFlorida State Universitycaught a Greenland shark in theGulf of Mexicoat a depth of 1,749 m (5,738 ft), where the water temperature was 4.1 °C (39.4 °F).[26]Four previous records of Greenland shark were reported from Cuba and the northern Gulf of Mexico.[27]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.[28][29]

In April 2022, a largeSomniosusshark was caught and subsequently released onGlover's Reefoff the coast ofBelize.This shark was identified as being either a Greenland shark or a Greenland/Pacific sleeper sharkhybrid. This observation is notable for being the first possible record of a Greenland shark from theWestern Caribbean,and being caught on a nearshorecoral reef(the only other record of this species from the Caribbean was made from a deep-water habitat off the Caribbean coast ofColombia). The discovery indicates that Greenland sharks may have a wider distribution in the tropics, primarily at greater depths, than previously believed.[30]

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)[31]

Longevity

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The Greenland shark has thelongest known lifespanof all vertebrate species.[32]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 of0.5–1 cm (1⁄4–1⁄2in) per year.[33]In 2016, a study based on 28 specimens that ranged from 81 to 502 cm (2 ft 8 in – 16 ft 6 in) in length usedradiocarbon datingof crystals within the lenses of their eyes to determine their approximate ages. The oldest of the animals sampled, which was also the largest, had lived for 392 ± 120 years, and was consequently born between 1504 and 1744.[a]The authors further concluded that the species reachessexual maturityat about 150 years of age.[32][34][35]Efforts toconserveGreenland sharks are particularly important due to their extreme longevity, long maturation periods, and the heightened sensitivity of large shark populations.[36]

Reproduction

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Greenland sharks are born alive (a process known asovoviviparity) after an estimated gestation period of 8–18 years.[21]About ten pups per litter is normal, each initially measuring some38–42 cm (15–16+1⁄2in) in length.[34][37][self-published source?]Within a Greenland shark's uterus, villi serve a key function in supplying oxygen to embryos. It is speculated that due to embryonic metabolism dealing with reproduction, this only allows for a limited litter size of around 10 pups.[38]It has been estimated that due to their extreme longevity, Greenland sharks can have 200 to 700 pups during their lifetime.[21]

Physiological adaptations

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Like otherelasmobranchii,Greenland sharks have high concentrations of the two nitrogenous compoundsureaandtrimethylamineN-oxide(TMAO) in their tissues, which increase theirbuoyancy[39]and function asosmoprotectants.TMAO also counteracts theprotein-destabilizingtendencies of urea[40][41]and deep-water pressure.[42][41]Its presence in the tissues of both elasmobranch andteleostfish has been found to increase with depth.[42][43]

The blood of Greenland sharks contains three major types ofhemoglobin,made up of two copies ofαglobin combined with two copies of three very similarβsubunits. These three types show very similar oxygenation and carbonylation properties, which are unaffected by urea, an important compound in marineelasmobranchiiphysiology. They display identical electronic absorption and resonance inRaman spectroscopy,indicating that theirheme-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.[44]

As food

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Greenland shark meat orkæstur hákarlinIceland

The flesh of the Greenland shark is toxic because of the presence of high concentrations oftrimethylamine oxide(TMAO). If the meat is eaten without pretreatment, the ingested TMAO is metabolized intotrimethylamine,which may be auremic toxin.Occasionally,sled dogsthat eat the flesh are unable to stand up because of this effect. Similar toxic effects occur with the relatedPacific sleeper shark,but not in most other shark species.[45][46]

The meat can be treated for safe consumption by boiling in several changes of water, drying, orfermentingfor several months to producekæstur hákarl.Traditionally, this is done by burying the meat inboreal groundfor 6–8 weeks, which presses the TMAO out of the meat and also results in partial fermentation. The meat is then excavated and hung in strips to dry for several more months.[47]It is considered a delicacy inIceland.[48][self-published source?][49][50]

Inuit legends

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The Greenland shark's poisonous flesh has a highureacontent, which gave rise to theInuitlegend of Skalugsuak, the first Greenland shark.[51]The legend says that an old woman washed her hair in urine (a common practice to killhead lice) and dried it with a cloth. The cloth blew into the ocean to become Skalugsuak.[52]Another legend tells ofSednawhose father cut off her fingers while drowning her, with each finger turning into a sea creature, including Skalugsuak.[53]

The Greenland shark plays a role incosmologiesof the Inuit from the Canadian Eastern Arctic and Greenland.IgloolikInuit believe that the shark lives within the urine pot ofSedna,goddess of the sea, and consequently, its flesh has a urine-like smell and acts as a helping spirit to shamans.[54]

Ecological importance

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Role in Arctic ecosystems

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As both scavengers and active predators, Greenland sharks have established themselves as apex predators in Arctic ecosystems. They eat a wide variety of fish, seals, and other prey within these ecosystems and have an important role in the intricate food web.[16][17]

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 lowfecundity,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 tooverfishing.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.[55]

Threats

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Overfishingand 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 itsliver oilup until the development ofsynthetic oilsand cessation of export of liver oil and skin from Greenland in the 1960s.[56]In the 1970s, the species was perceived as a problem for other fisheries in western Norway and the government subsidized a fishery to reduce the stock of the species.[57]

Approximately 3,500 individuals are taken each year in the Northwest Atlantic. Of this number, 89% comes from theBarents Sea,Iceland,Greenland,and theCanadian Arctic.[1]

The shark is likely affected by quantity, dynamics, and distribution of Arctic sea ice.[55]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.[55]There is greater potential for new fisheries to develop as more productive and abundant southerly species invade the warming Arctic waters.[58]

See also

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Notes

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  1. ^Lower-age estimage392 − 120 = 272 years,and2016 − 272 = 1744;higher age estimate392 + 120 = 512 years,and2016 − 512 = 1504.

References

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  58. ^Tai, Travis C.; Steiner, Nadja S.; Hoover, Carie; Cheung, William W.L.; Sumaila, U. Rashid (2019)."Evaluating present and future potential of arctic fisheries in Canada".Marine Policy.108:103637.doi:10.1016/j.marpol.2019.103637.

Further reading

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