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 species of shark, usually growing to between 2.4 and 7 m long and weighing between 400 and 1400 kg. 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.

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, and no recorded attacks have ever occurred.

Description
The Greenland shark is likely one of the largest living species of shark. It usually grows to between 2.4 and 7 m long and weighs between 400 and 1400 kg. The all-tackle International Game Fish Association (IGFA) record for this species is 775 kg. Males are typically smaller than females. It rivals the Pacific sleeper shark (possibly up to 7 m long) as the largest species in the family Somniosidae. 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.

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 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.

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.

Diet
The Greenland shark is an apex predator and eats primarily fish, although they have been observed actively hunting seals in Canada. The prey found in the stomachs of Greenland sharks is an indicator of the active hunting patterns of these predators. Recorded fish prey have included smaller sharks, skates, eels, herring, capelin, Arctic char, cod, rosefish, sculpins, lumpfish, wolffish, and flounder. Small Greenland sharks eat predominantly squid, while the larger sharks that are greater than 200 cm were discovered eating prey such as epibenthic and benthic fishes as well as seals. 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. 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.

Greenland sharks have also been found with remains of moose, polar bear and reindeer (in one case an entire reindeer body) in their stomachs. 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. They also scavenge on 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. No cases of predation on humans have been verified.

Movement and migration
The shark occupies what tends to be a very deep environment seeking its preferred cold water (-0.6 to 12 °C) habitat. As an ectotherm living 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. It swims at 0.76 mph, with its fastest cruising speed only reaching 1.6 mph. 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.

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 2200 m by a submersible investigating the wreck of the SS Central America that lies about 160 nmi east of Cape Hatteras, North Carolina. Daily vertical migration between shallower and deeper waters has also been recorded.

In August 2013, researchers from Florida State University caught a Greenland shark in the Gulf of Mexico at a depth of 1749 m, where the water temperature was 4.1 °C. Four previous records of Greenland shark were reported from Cuba and the northern Gulf of Mexico. A more typical depth range is 0-1500 m, 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 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.

"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."

Longevity
The Greenland shark has the longest known lifespan of all vertebrate species. 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 per year. In 2016, a study based on 28 specimens that ranged from 81 to(-) in length used radiocarbon dating of 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. The authors further concluded that the species reaches sexual maturity at about 150 years of age. Efforts to conserve Greenland sharks are particularly important due to their extreme longevity, long maturation periods, and the heightened sensitivity of large shark populations.

Reproduction
Greenland sharks are born alive (a process known as ovoviviparity) after an estimated gestation period of 8–18 years. About ten pups per litter is normal, each initially measuring some 38 - 42 cm in length. 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. It has been estimated that due to their extreme longevity, Greenland sharks can have 200 to 700 pups during their lifetime.

Physiological adaptations
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 $α$ 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.

As food


The flesh of the Greenland shark is toxic because of the presence of high concentrations of 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.

The meat can be treated for safe consumption by boiling in several changes of water, drying, or fermenting for several months to produce kæstur hákarl. Traditionally, this is done by burying the meat in boreal ground for 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. It is considered a delicacy in Iceland.

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.

Role in Arctic ecosystems
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.

Conservation and management
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. 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.

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 perceived as a problem for other fisheries in western Norway and the government subsidized a fishery to reduce the stock of the species.

Approximately 3,500 individuals are taken each year in the Northwest Atlantic. Of this number, 89% comes from the Barents Sea, Iceland, Greenland, and the Canadian Arctic.

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.