The Greenland Shark: A Cold Water Conundrum
For us humans, one of the most extreme habitats on this planet are the polar ice caps. It is much the same for marine animals, with polar species having to contend with freezing temperatures, low light levels and sea ice cover fluctuating year round. The Greenland shark (Somniosus microcephalus) is an elusive resident of Arctic waters that is well adapted to living in this extreme environment.
Yet very little is known about Greenland sharks, most likely because of the harsh and remote environment they inhabit, their tendency to dive to up to 1816m and the fact there has been almost no commercial interest in this species in over 50 years. What little is known about them usually comes from specimens caught as fishery by-catch.
Coping With the Cold
What we currently understand about these sharks is astonishing. Looking like a friendly torpedo, they can grow up to 7m long, and scientists recently discovered that they can live for an incredible 400 years – making them the longest-lived vertebrate currently known to man. The previous record was held by a Bowhead whale believed to be around 200 years old. Due to their extremely long lifespan, they don’t reach sexual maturity until they are around 150 years old, and they are thought to give birth to small litters of just 10 live pups.
The Greenland shark has such a long lifespan due to its incredibly slow metabolism, which allows it to conserve energy in the freezing waters it inhabits. They grow at a rate of less than 1 cm per year, and move at a glacial pace of around 0.8 km/h, making them one of the slowest swimming fishes currently known to man. So what is stopping these sluggish sharks from freezing over?
The answer lies in the flesh of the Greenland shark, in the form of a chemical called TMAO (trimethylamine N-oxide). TMAO helps prevent ice crystals from forming within the shark’s cells and stabilises proteins in its body, allowing them to continue to function at extremely low temperatures. TMAO is also thought to stabilise cell proteins against the effect of pressure – an important property for these sharks, which at their deepest dives will experience an astounding 200 atmospheres of pressure on their bodies.
Unless treated properly, all that TMAO makes the shark’s flesh highly toxic. About 50 years ago, a team of sled dogs that were fed raw Greenland shark came down with some nasty symptoms, including: stiff movements, vomiting, explosive diarrhoea, respiratory distress and convulsions. Some even died. And yet still, there is actually a delicacy in Iceland called kæstur hákarl (“fermented shark” in Icelandic), made from Greenland shark flesh. To remove the deadly TMAO, the meat is prepared by burying the shark and fermenting it in its own bodily fluids for a few weeks, before being hung out to dry for a few months. To be sure, this “delicacy” has an overwhelming stench of ammonia and has been described by many as being the worst tasting food in the world.
Catching a Bite to Eat
As juveniles, the Greenland shark feeds mostly on octopus and squid. Once larger than around 2m, however, fish and marine mammals make up the majority of its diet. Surprisingly, freshly dead seals are frequently found in the stomach contents of these sharks, and there have also been reports of whales with chunks taken out of them that match the shape of the Greenland shark’s mouth. This has stumped scientists, as the Greenland shark swims far too slowly to be able to go after this sort of prey, even at its fastest swim speeds.
Scientists believe that the only way these sharks could successfully hunt such swift and agile marine mammals is by ambushing them when they are at their most vulnerable. It is thought that they lurk in the darkness below holes in the ice, waiting for unsuspecting seals and whales that have to surface at the holes to breathe (although there is no evidence that this is how they actually hunt). Seals are also known to sleep at the surface of/under the water in an effort to escape predation from polar bears, and it may be that Greenland sharks can slowly, quietly sneak up on them and catch them unawares.
Despite being perfectly capable of catching live prey, Greenland sharks are also very happy to scavenge any meal that might fall in off the ice (literally), such as reindeer, horses and even polar bears. They’ve even been documented scavenging around Norwegian minke whaling operations, which they visit for a quick and easy snack on the blubber strips that get (illegally) thrown overboard by the whalers.
Considering many people have never even heard of the Greenland shark, it may be unclear if and why these sharks actually matter. Somewhat surprisingly, these huge, lumbering sharks play an important role in Arctic ecosystems as apex predators, and may be responsible for keeping populations of large mammals under check. Even when scavenging, they help other animals out as they rip large chunks of flesh out of a corpses, leaving the soft insides exposed and readily available to any creatures looking for an easy meal.
Although they may not be under direct threat by commercial fishing, there are many reasons the Greenland shark needs protecting. Thanks to climate change, Arctic sea ice is retreating year on year, which means more of its habitat is becoming accessible for commercial fishing. There is also a general move northwards of commercial fish species as the oceans warm, and so the Greenland shark could become a frequent by-catch species as polar seas become more and more exploited.
The IUCN has listed this species as Near Threatened because its slow growth rates, late sexual maturity and small litter sizes means that this species would struggle to recover from high fishing mortalities. Unfortunately, until we know more about this enigmatic species, it will be difficult to put any effective management plans in place to protect them.