The marine environment is incredibly diverse, with so many creatures unlike anything you will see on land. One such creature is the Greenland shark (Somniosus microcephalus), which is one of nature's oddballs, with poisonous flesh, an astounding life expectancy, and the ability to thrive in one of Earth's harshest environments.
Description
The Greenland shark is a species of sleeper shark belonging to the family Somniosidae. They are the largest Arctic fish and one of the largest shark species, with a length of around 6.4 to 7.3 m (21 to 24 ft) and a weight of around 1,000 to 1,400 kg (1.1 to 1.5 US tons). They live a long life of near solitude in the deep sea, but the darkness down there doesn't bother them as almost all Greenland sharks are blind.
It is a very unusual looking shark with a short, blunt snout, small eyes, and small fins relative to its body length. Even the gill openings are incredibly small for such a large shark. Unlike most other sharks, they don't have countershading and, instead, the body is uniform in colour, usually a blackish-brown with a mottled appearance.
Even more strangely, a Greenland shark's flesh is extremely poisonous. It contains high levels of urea and trimethylamine N-oxide (TMAO) which, when ingested, is metabolised into trimethylamine (TMA). This toxic compound causes unpleasant intestinal issues, as well as effects similar to extreme drunkenness. Eating too much Greenland shark flesh can even be fatal. However, TMAO is helpful to the shark. It increases buoyancy, acts as a natural antifreeze, and counteracts deep water pressure.
Habitat
Greenland Sharks are native to the Arctic and North Atlantic oceans. They live at incredible depths and have been observed as far down as 2,200 m (1.4 miles). However, the more common depth range for these sharks is between 0 and 1,500 m.
They also migrate each year and their preferred depth will vary with the temperature and the season. In the winter, Greenland sharks can be found congregating in shallower waters in the far north. On the other hand, in the summer, they move to deeper waters towards the south of their natural range.
Diet
Like most sharks, the Greenland shark is an apex predator. They mostly feed on fish, such as cod and herring, though other prey items recorded in the past have included smaller sharks, skates, eels, and squid. It has also been theorised that they may occasionally ambush sleeping seals and other marine mammals.
Their teeth are highly specialised and are designed to cut plugs out of flesh. The upper teeth are narrow and pointed and are used for gripping. Meanwhile, while the lower teeth are broad and they curve to the side. By swinging its head in a circular motion, a Greenland shark is able to cut out a round plug from its prey.
Life Cycle
Greenland sharks can live for up to 400 years, making them the longest living vertebrate in the world. They mature at around 150 years of age, so it is a long time before they are capable of reproducing.
Females are ovoviviparous, meaning that, instead of laying eggs, the eggs remain in the mother’s body as the embryos develop and she will then give birth to live young. The gestation period of a Greenland shark is not currently known.
Around 10 pups are born per litter, with each pup measuring around 38 to 42 cm (15 to 16.5 inches) in length. The sharks then grow at a rate of just 0.5 to 1 cm a year, meaning it takes them a very long time for them to reach their adult size.
How fast are they?
As Arctic natives, Greenland sharks live in very cold waters that can range from -2 to 12°C (28 to 54°F). As a result, they have a slow metabolism and are a very slow-moving species. In fact, they are the slowest moving of all known shark species and they have both the lowest swim speed and frequency of tail beats for their size across all fish species.
They have a cruising speed of just 1.22 km/h (0.76 mph) and a maximum speed of 2.5 km/h (1.6 mph), which they are only capable of maintaining for short bursts. This sluggish nature is how the sleeper shark family gets its name.
Why are they blind?
The vast majority of Greenland Sharks are blind. This is because they become colonised by a parasitic copepod, called Ommatokoita elongata, which attaches to the shark’s eye and feeds on the surface of the cornea. It has been suggested that the copepod may be bioluminescent, which would attract prey for the shark and make this a mutually beneficial arrangement. However, this has not yet been proven.
Luckily, in the dark ocean depths, Greenland sharks do not rely on their vision for survival.
Biggest Threats
Greenland sharks are currently listed as Near Threatened on the IUCN Redlist of Threatened Species. Fishing and the harvesting of aquatic resources cause the biggest impact to Greenland shark populations.
Despite the fact that their flesh is toxic, Greenland sharks are hunted, or are purchased as bycatch from fishing ships, and used for food as the meat can be treated to make it safe for consumption. This is done by boiling it in several water changes or by drying the meat.
Alternatively, it can be used to produce a traditional dish, Hákarl, which is considered a delicacy in Iceland. For this, the carcass is fermented in a pit for several weeks with stones pressing and draining the toxic fluids from the body. The meat is then hung up in strips and cured for several months.
Though demand for the species has decreased over time, it is estimated that around 1,300 are caught each year, mostly as accidental bycatch. The number of mature individuals continues to decline, which makes it harder for the population to recover with fewer reproductively active animals to help boost numbers. So, while the Greenland shark is not yet threatened, it's certainly something we have to keep an eye on if we want this fascinating creature to stick around.
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