Shark of the month: Greenland shark

The Greenland shark (Somniosus microcephalus), also known as the Grey shark, Gurry shark or Ealussuaq, has gotten a lot of publicity lately. This is because recently a Greenland shark was found that was estimated to be about 400 years old! That is 200 years older than their average lifespan and it makes the Greenland shark the longest living vertebrate on earth. Moreover Greenland sharks are incredibly large; they can grow up to about 6.5 meters and they weigh about 900 kilos. These subarctic giants move slowly through the cold deep waters of the North Atlantic. The flesh of Greenland sharks holds high concentrations of TMAO (Trimethylamine Oxide) which causes the flesh to be extremely toxic. Their flesh is so toxic that it could kill a human if consumed.

Somniosus_microcephalus_okeanos

Anatomy and appearance

Greenland sharks are one of the largest existing sharks. They can grow up to about 6.5 meters, however the average Greenland shark is about 5 meters in length. In addition, Greenland sharks are probably the slowest sharks and are therefore also known as sleeper sharks. Their average pace is 0.3 meters per second. However, they are capable of increasing their speed temporarily. The colours of adult Greenland sharks vary from slate grey, purplish grey, violet, brown to black. Greenland sharks have a short round snout and, in comparison with their length, they have extremely small dorsal and pectoral fins. The eyes of Greenland sharks are rather unique because of their small size. Moreover, most Greenland sharks are (partly) blind because of the Copepod (Ommatokoita elongata) parasite. These parasites attach themselves to the cornea of the eyes of the sharks. Some researchers believe that that these parasites are bioluminescent, however this has never been proven. Anyway Greenland sharks survive easily without their sight because they use their other senses to detect prey.

Habitat

The Greenland shark lives in the cold waters of the North Atlantic Ocean and the Artic Ocean. The water temperature that these sharks live in often varies from -1.6 to 10 degrees Celsius. Greenland sharks are the only known shark species that constantly live in these cold environments. These sharks can be found near the surface as well as extreme depths to about 2.200 meters. Generally Greenland sharks can be found in these extreme ocean depths during the summer months and closer to the surface during the winter months.

 habitat

By Chrishuh – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2828227

Diet

The diet of a Greenland shark is quite extensive, they will eat almost anything that crosses their path. Greenland sharks are primarily scavengers and fish eaters. However they also eat large sea mammals like seals. In addition Greenland sharks are known to ambush large animals that walk too close to the edge of the ice. For example the remains of polar bears, horses, moose and reindeers have been found in the stomachs of Greenland sharks.

Population

Because Greenland sharks are rare to spot and not much research has been conducted, the extent of their population is unknown. However, it is estimated that their population has decreased over the past years. Greenland sharks have the longest known lifespan of all vertebrate species and they (sexually) mature late, somewhere around 100 years! Moreover, these sharks grow at an incredible slow rate. Research shows that Greenland sharks only grow about 0.5 to 1 centimeter a year. So if Greenland sharks are not able to reach sexual maturity, for example due to fishing, it is almost impossible to recover the population. However, researchers say that there currently is a large amount of young Greenland sharks but it will take another 60 to 80 years for them to become sexually mature and reproduce.

Conservation

The official IUCN Red List conservation status of the Greenland shark is ‘Near Threatened’ (http://www.iucnredlist.org/details/60213/0). Greenland sharks used to be popular prey for fisherman in the North Atlantic Ocean and the Arctic Ocean. They were often hunted for their liver oil which is used for example for machine oil. Despite the toxicity of their meat the processed meat of Greenland sharks used to be served in Iceland as a specialty called hákarl or kæstur hákarl. Fortunately, today the Greenland shark is no longer commercially harvested.

Relationship to humans

Greenland sharks are large and strong enough to kill humans if they would feel threatened, however there has never been one verified case of an attack on humans by Greenland sharks.

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