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The Greenland shark, Somniosus microcephalus, is a huge cold-water shark that dwells in the Arctic and north Atlantic oceans, near the coast of Greenland, Canada and the extreme northern United States. It is generally regarded as the fourth largest shark in the world, reaching up to 7 meters in length and up to 1000 kilograms in mass - behind only the great white, basking and whale sharks.

The Greenland shark tends to prefer deep water, often as deep as 2 kilometers, but has also been filmed in very shallow water, less than three meters. Throughout the year, it migrates up and down the water column, following the coldest layer. During the summer, they are found in deep water, but in winter when the surface layers are colder, they come to shallower water to feed on seals and other marine mammals.

It is an active predator, though its movements are much slower and less aggressive than other large species like the great white, tiger or great hammerhead. Its feeding habits, like those of its kin, the pacific sleeper shark, are not fully understood. They are known to prey on fish and squid, and to scavenge whale carcasses, but they have also been observed to attack seals and dolphins, and even to swim up rivers and attack caribou and even polar bears as they drink, or fish, at the water's edge.

When not feeding, the shark is slow-moving and quite tolerant of divers and other disruptions. This languidness earns it the nickname of sleeper shark. That said, they are capable of surprising bursts of speed. There are no records of any attacks on humans, but due to the animal's size and strength, great care is advised if diving in their vicinity.

Like most sharks, they rely on their senses of smell and electroreception much more strongly than on vision. Indeed, many if not most Greenland sharks suffer from a parasitic copepod, Ommatokoita elongata, that affixes itself to the shark's cornea and feeds on it. The damage inflicted by this parasite leaves the shark with diminished vision, or in some cases, completely blind. Oddly, Greenland sharks in the St. Lawrence estuary are hardly ever affected by the copepod. The precise reason for this is unknown.

Due to high concentrations of trimethylamine, the flesh of the Greenland shark is toxic. It can be rendered edible by boiling in multiple changes of water, or by allowing it to ferment for several months. The latter technique is used in the Icelandic dish hakarl.

All in all, the Greenland shark is one of the most poorly understood species of shark, owing largely to its inhospitable habitat and infrequent interaction with humans. More studies are underway to determine the nature and habits of this strange, elusive king of Arctic fish.

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