Greenland shark

• 1. Introduction

  ›

  The Greenland shark, Somniosus microcephalus, was tagged off the coast of Greenland in 1936 and caught again in 1952. In those sixteen years its growth was tiny. Measurements from the two captures suggest the species adds only half a centimetre to a centimetre of length each year. That crawl is the first clue to a very strange animal. It is also known as the grey shark or the gurry shark. It lives in the North Atlantic and Arctic Oceans, so deep and so remote that it is still poorly studied. Several puzzles follow from its pace. How long can an animal live when it moves through life this slowly? How does something so sluggish catch a meal? And how does a species whose flesh makes mammals ill end up as a dish on Icelandic tables?

  111030141

• 2. A Very Long Life

  ›

  Radiocarbon dating of crystals inside the lenses of the eyes gave researchers their first real handle on the Greenland shark's age. The method points to a lifespan of at least 272 years. The oldest individual was put at 392 years, give or take 120. Across the estimates, the range runs from 272 to 510 years, longer than any other known vertebrate.

  A genome published in 2024 offers hints about how the animal manages it. It is the largest genome of any shark sequenced so far. More than two-thirds of the genes found in it are transposable elements, often called jumping genes. These self-copying stretches of DNA usually cause trouble, producing mutations and other genetic complications. In the Greenland shark, DNA repair genes are hypothesized to jump in the same way, helping to contain the damage. A set of 81 genes unique to this shark is suggested to be linked to DNA repair.

  Researchers point to other candidates too. Immune enhancement and resistance to cancer are both on the list. So are unusual amino acid substitutions in a protein called linker histone H1.0, which might keep chromatin stable. Several published works disagree on emphasis, and efforts to explain the longevity at the molecular level are still under way.

  1050513456

• 3. Body, Teeth and Chemistry

  ›

  Adult Greenland sharks usually reach 4 to 5 metres. The largest confirmed specimen measured 6.4 metres and weighed about 1023 kilograms. The all-tackle record from the International Game Fish Association stands at 775 kilograms. Only the Pacific sleeper shark, possibly up to 7 metres long, rivals it for the title of largest in the sleeper shark family, Somniosidae. Genetic data place the split from ancestral sleeper sharks in the Canadian Arctic, roughly 1 to 2.34 million years ago, during the Pleistocene. Glacial swings that periodically isolated marine populations probably shaped that divergence.

  The body is thickset, with a short rounded snout, small eyes, and small dorsal and pectoral fins. Its five gill openings are tiny for an animal of such size. Females tend to be larger than males, and males mature at a smaller size. Colour runs from pale creamy-gray to blackish-brown, usually uniform, with the occasional whitish spot or faint dark streak on the back.

  The jaws work as a two-part tool. The 48 to 52 teeth of the upper jaw are thin, pointed and unserrated, and they anchor the shark to a large carcass. The lower jaw carries another 48 to 52 teeth, broad and square, with short smooth cusps pointing outward. Teeth in the two halves of the lower jaw are pitched in opposite directions. While the upper teeth hold, a rolling motion of the jaw cuts out massive chunks.

  Inside the tissues, urea and trimethylamine N-oxide, known as TMAO, build up in high concentrations, as in other sharks and their relatives. They add buoyancy and protect cells as osmoprotectants. TMAO also counteracts the protein-destabilizing effects of urea and deep-water pressure, and fish tissues hold more of it at greater depth. The blood carries three major hemoglobin types, each built from two alpha globin chains and two of three nearly identical beta subunits. All three hold oxygen less tightly than the hemoglobin of temperate sharks, which is believed to be an adaptation to great depths.

  One old description of the shark's death throes is vivid. Hoisted onto a deck, it thrashed its tail so violently that sailors dispatched it quickly. Cut pieces kept twitching for some time, and the head was unsafe to touch even after removal. The writer adds that, if Crantz is to be believed, movement could still be seen three days later when a piece was trodden on or struck.

  1123252715

• 4. A Slow Hunter

  ›

  The Greenland shark averages 0.34 metres per second, and its fastest cruising speed reaches only about 0.74. For its size, it has the lowest swim speed and tail-beat frequency of any fish. As an ectotherm in water just above freezing, it prefers temperatures from -1.1 to 7.4 degrees Celsius and depths from 100 to 1200 metres. Its very slow metabolism most likely goes hand in hand with its extreme longevity.

  Seals are a well-established prey item, and they swim far faster. Biologists are therefore uncertain how the shark catches them. The favoured idea is an ambush while the seals are asleep. Low-twitch muscle fibre and slow speed also lead researchers to propose that it targets marine mammals that are injured or sick. For most bottom-dwelling prey, it relies on cryptic coloration to approach unseen. Then it expands its mouth cavity to create suction and draws the victim in. That suction is the likely reason its stomach so often holds whole prey.

  Fish dominate the menu, including herring, salmon, smelt, cod, wolffish, haddock, halibut, redfish, sculpins, lumpfish and skates. Seals are on it too, and some sharks have eaten minke whale. Small sharks eat mostly squid, along with sea birds, crabs, amphipods, sea snails, brittle stars, sea urchins and jellyfish. Sharks longer than 200 centimetres, or 79 inches, take bottom-dwelling fishes, seals and small cetaceans such as oceanic dolphins and porpoises.

  Stomach contents show how much the animal scavenges. Moose, horse and reindeer remains have turned up, in one case an entire reindeer carcass. Polar bear remains have also been recovered, though they are thought to come from bears that died of other causes. The shark is drawn by the smell of rotting meat, and it has often been seen lurking around fishing boats and herds of seals.

  People rarely meet it, because the frigid waters make attacks very unlikely. A possible attack in August 1936 on two British fishermen was never tied to a species. Around 1859 at Pond Inlet, Canada, a Greenland shark was reportedly caught with a human leg in its stomach. That story was never scientifically investigated and remains unverified.

  293140176

• 5. Senses in the Dark

  ›

  A crustacean called Ommatokoita elongata, a copepod, often clamps onto the shark's eyes. Its presence can cause ulceration, mineralization and swelling of the cornea, leaving the shark almost completely blind. One hypothesis says the copepod may glow and lure prey toward the shark, in a mutual arrangement, but nobody has verified it. The damage does not seem to shorten the shark's life or weaken its hunting, because it leans so heavily on smell and hearing.

  Beneath the skin, fluid-filled canals of the lateral line system run along the sides of the body and over the head. Small pores link those canals to the surrounding water. Through them, the shark detects vibrations and low-frequency pressure changes, such as those made by prey swimming nearby. In a dark habitat with no sunlight, that sense and a sharp sense of smell help a slow swimmer find its way.

  Yet the genome argues against writing off its eyes. The optic tectum, the brain region that processes visual information, is about the size seen in other sharks and their relatives. The shark has a tapetum lucidum, a layer that boosts photon capture in dim light. Its retinas are dominated by rods, and a long list of rod-specific phototransduction genes is present, including rh1 and gnat1. Only a single cone-specific gene, grk7, appears in the genome, which confirms how firmly the eye is tuned to low light.

  1718351932

• 6. Deep Wanderer

  ›

  In winter, Greenland sharks gather in the shallows as far north as 80 degrees for warmth. In summer they scatter, some heading to the deeps and some farther south. Their annual migration follows depth and temperature more than distance, and daily vertical trips between shallow and deep water have also been recorded.

  A submersible investigating the wreck of the SS Central America, about 160 nautical miles east of Cape Hatteras, North Carolina, saw one at 2200 metres. Genetic evidence suggests the species once ranged across the oceans at depths below 1,000 metres. During the Quaternary period, global cooling thickened the sea ice and submerged Arctic landscapes, which likely cut off local populations from one another. Fluctuating glacial cycles periodically allowed them to mix again, shaping migration patterns and genetic diversity.

  Today the sharks turn up well beyond Arctic and sub-Arctic waters, including the Mid-Atlantic Ridge and the Gulf of Mexico. In August 2013, researchers from Florida State University caught one in the Gulf of Mexico at 1749 metres, where the water was 4.1 degrees Celsius. Four earlier records came from Cuba and the northern Gulf.

  The most surprising record is from April 2022. A large Somniosus shark was caught and released on Glover's Reef, off the coast of Belize. It was identified as either a Greenland shark or a hybrid of a Greenland and a Pacific sleeper shark. It was the first possible record from the Western Caribbean, and it came from a nearshore coral reef. The only other Caribbean record was a deep-water catch off the coast of Colombia.

  4645491547

• 7. Slow Breeding, Heavy Pressure

  ›

  An estimated 8 to 18 years pass between conception and birth in a Greenland shark. The pups are born alive, a process called ovoviviparity, and the embryos develop in the uterus without a placenta. Villi in the uterus supply oxygen to the embryos, and oxygen supply is proposed as a major limit on litter size. Studies disagree on the details. Some suggest up to 10 pups per litter, each 38 to 42 centimetres long, which would mean 200 to 700 pups in a lifetime. Another study, using ovarian data and comparisons with related sharks, gives pup lengths of 35 to 45 centimetres.

  Sexual maturity arrives around 150 years of age. At that point females measure around 4.19 metres and males around 2.84. Females pass through seven maturation stages: immature, developing, ripening, early gravid, midterm gravid, late gravid, and postnatal. Males pass through only three: immature, maturing and mature.

  People have hunted the shark for its liver oil, until synthetic oils arrived and Greenland stopped exporting liver oil and skin in the 1960s. In the 1970s, fishermen in western Norway saw the species as a problem for other fisheries. The government subsidized a fishery to reduce its numbers. Now the main threat is bycatch. Artisanal fisheries in Iceland that target the shark take about 25 a year, while industrial fisheries catch roughly 3,500 a year by accident across the Arctic and Atlantic Oceans.

  Slow growth, late maturity and low fecundity mean the species may struggle to recover from overfishing and bycatch. Sea ice adds another worry. Projected ice loss will keep changing the abundance, distribution and availability of prey, while giving fishing fleets greater access. As warming Arctic waters draw in more productive southerly species, new fisheries could follow.

  1657546162

• 8. Poison, Delicacy and Myth

  ›

  In Iceland, the shark is made into kæstur hákarl, a pungent delicacy, even though its fresh meat is toxic. If eaten without pretreatment, the TMAO is metabolized into trimethylamine, which may act as a uremic toxin. Sled dogs that eat the flesh are sometimes unable to stand. The related Pacific sleeper shark has the same effect, but most other sharks do not.

  Making the meat safe takes months. Traditionally, the meat was buried in gravel pits near the ocean for at least several weeks. Today, cuts are usually fermented in perforated containers that let liquid drain away. Fermentation turns urea into ammonia and TMAO into trimethylamine, and that liquid drains from the meat. The meat is then dug out and hung in strips to dry for several more months.

  The urea-rich flesh also gave rise to the Inuit legend of Skalugsuak, the first Greenland shark. An old woman washed her hair in urine, a common way to kill head lice, and dried it with a cloth. The cloth blew into the ocean and became Skalugsuak. In another legend, Sedna's father cut off her fingers while drowning her, and each finger turned into a sea creature, Skalugsuak among them.

  The shark plays a role in the cosmologies of Inuit in the Canadian Eastern Arctic and Greenland. The Igloolik Inuit believe it lives inside the urine pot of Sedna, goddess of the sea. That is why its flesh smells like urine, and why it acts as a helping spirit to shamans.

  766656770