Greenland Sharks: The Elusive Grandpa of the Deep
The Greenland shark (Somniosus microcephalus) is one of the most interesting creatures inhabiting our most Northern seas. While being a slight misnomer (they aren’t often found in Greenland) its Twitter-given nickname of the “pee shark” is by no means the most interesting thing about this deep-sea dweller. Found only in the most northern waters of the Atlantic Ocean and the Arctic Ocean, these cartilaginous fish dwell at a depth of at least 1,200 meters in temperatures of only 1-2 degrees Celsius. These two extreme environments have led to some striking adaptations, both inside and outside its body.
It’s the Arctic’s largest fish, weighing 900kg on average and growing to over 6 metres long. But perhaps the most interesting figure, and the Greenland shark’s claim to fame, is its longevity. They’re the world’s longest living vertebrate. One shark found by scientists was aged at least 272 years old, but could have been as old as 500. Living half a millennia, growing almost as big as a car, and living in two of the Earth’s harshest environments?
Getting the cold shoulder
In the deepest parts the Atlantic and Arctic Oceans, life is much different to warm coastal waters, and the adaptations of the Greenland shark reflect this. Unlike most sharks they lack an anal fin and have very small, stunted dorsal fins; their bodies are stocky and rounded and their snouts are short and blunt. This is because while most pelagic sharks depend on being able to move swiftly through the water, S. microcephalus does the exact opposite! Greenland sharks are the slowest moving fish on our planet, swimming at a slow 0.34 m·s− 1 – in comparison, the average human’s walking speed is 1.4m·s− 1). Being a member of the family sleeper sharks, they are unique among elasmobranchs in that they don’t need to keep moving in order to draw water over their gills to stay alive, which allows them to function despite their sedentary lifestyles.
But it’s not just their movements that are slow. They’re slow moving, slow growing, they mature slowly, hence their huge size and longevity. Every part of their body functions at as slow of a pace as possible to help the shark save energy. Every process in an organism’s body releases energy, and in such cold waters energy is scarce. To reserve these energy stores, S. microcephalus only hunts, breeds and even moves when it’s absolutely necessary.
As apex predators, Greenland sharks feed on just about anything: both deep-water organisms and larger creatures, too, and have been known to devour the carcasses of whales and even seals. But how can such a slow-moving fish catch something as swift as a seal? It has been theorised that S. microcephalus waits until nightfall when the seals retreat to the ocean to sleep, which they do to hide from polar bears. It is then that the shark rises near the surface and attacks – a low-effort but very effective form of predation.
We have to go deeper
When they’re not dining on seal, the Greenland shark will use a very unique adaptation to feed on small fish and crustaceans on the very bottom of the sea floor. But how can it see? Light attenuation occurs in oceans all over the world, meaning that light can only penetrate so far. At 10 meters deep, 50% of all visible light has already been absorbed.
In many photographs of S. microcephalus, a pale string can be seen coming from their eyes. This is a copepod parasite called Ommatokoita elongata. They latch onto the shark’s eyes and burrow into the cornea, which they will slowly eat, eventually blinding the shark. Luckily for the Greenland shark, they don’t rely heavily on vision and their olfactory senses contribute to most of their hunting tactics. However, in an interesting twist, it has been proposed that the relationship between shark and parasite may actually be symbiotic, meaning they benefit one another. The copepod gains nutrients from the shark, and in return will emit a bioluminescence which will attract prey to the shark for it to eat.
There is still much to be learned about the Greenland shark, but if the rest of its life history is as fascinating as this, it may be a good thing that scientists have only just begun to detail the inner workings of the world’s oldest, coldest shark.