#cold waters

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Greenland shark

Somniosus microcephalus

Family: Somniosidae

The Greenland shark ━an apex predator of the frigid deep sea━ also known as the "sleeper shark," is a massive, mysterious predator of the North Atlantic and Arctic Oceans. It's best known for being the longest-living vertebrate animal on Earth, with a lifespan that can estimated between 250 and 500 years.

Physical Description and Size

It's a heavy-set species with a body that is cylindrical and lacks an anal fin with a colour range from pale creamy-gray to dark brown or blackish-brown. The body is typically uniform in color, though some individuals may have whitish spots or faint dark streaks on their backs.

The distinctive features of this species include a short, rounded snout, very small eyes, and relatively small dorsal and pectoral fins. Notably, the gill openings are small relative to the shark's massive size. Females are typically larger than males.

But, one of their most distinguishing characteristics is the prevalence of the parasite Ommatokoita elongata, which commonly causes significant vision impairment. It attaches to the shark's eyes causing corneal lesions and leaving scar tissue which leads to impaired vision, even partial blindness. But this doesn't necessarily affect the shark, since it relies heavily on its other senses.

Size and Weight: ◦ Length: Adults generally grow to 4 to 5 meters, but the largest confirmed specimen measured 6.4 meters long. ◦ Weight: They can weigh over 1,000 kg. Though measuring the Greenland shark’s growth rate is challenging because individuals are rarely recaptured.

Distribution and Habitat

As a true cold-water species, the Greenland shark primarily inhabits the North Atlantic and Arctic Oceans.

They can be found from the icy waters around Greenland, Iceland, and Canada (Nunavut, Labrador) eastward to the Barents Sea and Norway. But its range also extends southward to the North Sea and the waters off the Eastern Seaboard of the United States (Cape Cod and North Carolina), and they have been spotted as far south as the Gulf of Mexico and even the Caribbean.

This species is found at depths ranging from the surface down to at least 2,200 meters, possibly deeper, and preferring water temperatures from -1.8°C to 7.4°C, but can tolerate up to 17°C.

They migrate to shallower waters in winter, when surface temperatures drop to their preferred range, and retreat to the depths in summer.

Behavior and Diet

Even though its behavior is defined by its incredibly slow metabolism and sluggish speed, the Greenland shark is an apex predator.

It's the slowest swimming shark for its size, cruises at an average of 0.34 m/s and has a maximum speed of only 0.74 m/s. Because it is far slower than its prey, it led scientists to believe it ambushes sleeping seals or they may be opportunistic hunters. That last belief is supported by stomach contents that have included polar bears, horses, and reindeer.

Diet: As both a scavenger and an active predator, it is known to be a generalist feeder. ◦ Fish (Herring, salmon, cod, halibut, redfish, skates, and even other smaller sharks) ◦ Marine mammals (Seals [ringed and bearded], and small cetaceans like porpoises and dolphins) ◦ As scavenging (minke whale, polar bears, reindeer [entire carcass], moose, and horse remains)

Reproduction and Life Cycle

The Greenland shark is known to have the slowest life cycle of any vertebrate, which makes it extremely vulnerable to overfishing.

It is aplacental viviparous. The pups develop inside the mother's uterus, nourished by their yolk sac rather than through a placenta, until gestation is complete. The gestation period is incredibly long, estimated at between 8 and 18 years with 10 pups per litter.

This shark is the longest-lived vertebrate, scientists discovered Greenland sharks have a life expectancy of at least 272 years and could reach 392 years old. In addition, they do not reach sexual maturity until they are approximately 150 years old.

Relationship with Humans

Greenland sharks are not considered a threat to humans because of the frigid waters they inhabit, people typically don't swim or dive near them, making the encounters extremely rare.

The fresh flesh of the Greenland shark is highly toxic to mammals due to high concentrations of trimethylamine N-oxide (TMAO), which acts as a neurotoxin; if eaten raw, can be fatal. Even so, the meat is eaten in Iceland as a national delicacy called kæstur hákarl. To make it safe, the meat is buried in a shallow pit for 2-3 months to ferment and then hung to dry for several months, which breaks down the toxin.

Also, Inuit used to use its skin as leather for boots and their sharp lower teeth as knives for cutting hair, and historically, their massive livers were rendered for valuable oil.

⚠️Conservation Status

The International Union for Conservation of Nature (IUCN) classifies the Greenland shark as Vulnerable globally as of 2020 (uplisted from Near Threatened in 2006).

One of the current threats is bycatch, an estimated 1,700 to 2,200 sharks are caught incidentally each year in Canadian trawl, longline, and gillnet fisheries alone, with global estimates over 3,500. The other is climate change, as an Arctic species, it is vulnerable to the rapid loss of sea ice and warming waters in its habitat.

Conservation Efforts: Retention is prohibited in many fisheries (e.g., Canada, NAFO regulatory areas), and they must be released alive. However, post-release survival rates are unknown.

🦈Fascinating Facts

It's the undisputed longest-living vertebrate animal on the planet. One studied individual was estimated to have been born between 1504 and 1744, meaning it was swimming in the ocean during the time of Shakespeare and Galileo.

Their tissues contain high concentrations of urea (TMAO being the primary compound responsible for their toxicity). This acts as a natural antifreeze and pressure regulator, allowing them to survive in freezing temperatures and at crushing depths of over 2,000 meters.

In 2024, scientists sequenced the Greenland shark's genome, which is 6.45 billion base pairs long (twice the size of the human genome). They found multiple copies of genes involved in DNA repair and inflammation control, which may hold the key to their extraordinary lifespan and resistance to age-related diseases.

Chris Burkard Photo.

Iceland