Sharks are commonly associated with warmer, tropical waters, far from the icy Arctic. However, the Arctic represents an extreme environment, characterized by sub-zero temperatures, extensive ice cover, and unique ecological challenges. The question of whether sharks can survive and thrive in such a harsh, cold habitat challenges conventional notions about these ancient fish. This exploration delves into the surprising presence of sharks in the Arctic and the remarkable adaptations that enable their survival.
Sharks of the Arctic
Sharks indeed inhabit the Arctic, with the Greenland Shark (Somniosus microcephalus) being the most prominent resident of these frigid waters. This species is primarily found in the North Atlantic and Arctic Oceans, ranging from Baffin Bay to the Barents Sea and as far south as the Gulf of Maine. Greenland Sharks are among the largest shark species, reaching lengths of up to 6.4 meters (21 feet) and weighing over 1,000 kilograms (2,200 pounds). They are also recognized as the longest-living vertebrates known to science, with estimated lifespans potentially exceeding 500 years.
While the Greenland Shark is the true Arctic specialist, other shark species occasionally venture into the peripheral Arctic Ocean. These include:
Porbeagle (Lamna nasus)
Salmon Shark (Lamna ditropis)
Basking Shark (Cetorhinus maximus)
Bluntnose Sixgill (Hexanchus griseus)
Spiny Dogfish (Squalus acanthias)
The Pacific Sleeper Shark (Somniosus pacificus), a close relative, inhabits the northern Pacific Ocean and shares similar cold-water adaptations. However, these other species are generally considered visitors or fringe inhabitants, with the Greenland Shark being the only one consistently found within the Arctic Circle. Its ability to live regularly in waters between -1.1 and 7.4 °C (30.0 and 45.3 °F) highlights its unique status as a cold-water resident.
Surviving the Extreme Cold
Arctic sharks, particularly the Greenland Shark, possess physiological and behavioral adaptations for survival in extreme cold. Their bodies contain high concentrations of nitrogenous compounds, like urea and trimethylamine N-oxide (TMAO), which act as natural cryoprotectants. These compounds prevent ice crystal formation within tissues and lower the freezing point of bodily fluids, effectively acting as an internal antifreeze. TMAO also stabilizes proteins against the denaturing effects of deep-water pressure and high urea concentrations.
The Greenland Shark exhibits a remarkably slow metabolism and growth rate, a direct consequence of its cold habitat. They grow at an estimated rate of only about 1 centimeter (0.4 inches) per year. This slow pace of life allows them to conserve energy in an environment where resources can be scarce and temperatures demand high energy expenditure for basic bodily functions. Their sluggish movement, averaging around 0.34 meters per second (1.1 feet per second), is the slowest cruising speed recorded for any fish of its size.
Greenland Sharks primarily inhabit deep waters, typically ranging from 100 to 1,200 meters (330 to 3,940 feet), though they have been observed at depths of 2,200 meters (7,200 feet). While they may venture into shallower waters during colder winter months, their preference for deeper, consistently cold environments helps them avoid significant temperature fluctuations. As opportunistic feeders and scavengers, their diet includes fish like cod and wolffish, as well as seals and carrion, such as the remains of whales, reindeer, or even polar bears that have fallen into the water. Their ability to consume a wide variety of prey, including surprisingly fast-moving animals, suggests they may employ ambush tactics, possibly preying on sleeping seals.