- Education and Science»
- Geography, Nature & Weather
Greenland Sharks - the Slowest Sharks in the World
Greenland sharks (Somniosus microcephalus) are known for being the slowest sharks in the world, cruising at the dizzy speeds of .34m/s (less than 1mph).
They can put on bursts of speed when chasing prey, reaching 0.7m/s (1.6mph), but of course can only actually catch creatures that travel slower than this speed.
In the harsh marine environments of the Arctic Sea and the North Atlantic Ocean, most fish and other sea creatures travel slower than in warmer seas, in order to conserve energy.
Practically none travel as slowly as the lumbering Greenland shark, which would probably starve to death were it not a scavenger, eating practically everything it comes across on its journeys, dead or alive.
It is only recently that scientists discovered just how slowly these sleeper sharks travel, through a data logging tagging program carried out by the Norwegian Polar Institute.
Seals are a favorite food of Greenland sharks, but they swim at a rate of 1m/s (3mph) and are capable of much faster speeds.
So how do the sharks catch them?
It is now believed that the massive, lumbering Greenland sharks sneak up on sleeping seals, which in the Arctic sleep in the water to avoid predation by polar bears.
They then take a single, round bite out of the seal with their razor sharp, pointed teeth.
In order to release the flesh from the seal, Greenland sharks then have to (slowly) move their heads from side to side, until the flesh is disengaged from the unfortunate mammal.
Not all seals die from this bite. Bigger seals may find themselves with nothing more than an irritating wound in their flesh that will heal in time.
- sleeper shark
- ground shark
- gray shark
- gurry shark
- Greenland shark
Greenland Shark facts
The Latin name of the Greenland shark, Somniosus microcephalus, tells us a lot about it.
'Somniosus' refers to sleep, while 'microcephalus' means small head.
Despite its large, rounded body, it has a comparatively small head, which also results in a fairly small bite.
Its teeth are pointed and sharp, but not serrated. The lower teeth, which number 50 - 52, are designed for slicing while the 48 - 52 upper teeth are designed for pinning the prey into position.
Members of the dogfish family, sleeper sharks have no spines on their two dorsal fins.
Their coloring varies from individual to individual, and can be black, brown, grey or a mixture of all three.
They frequently have a white snout, which scientists believe is due to abrasive foraging on the ocean floor.
Greenland sharks are frequently blinded by small copepods of the genus Ommatokoita elongata.
These attach themselves to the small eyeballs of the sharks, and feed off the tissue.
Luckily for the sharks, they have highly tuned olfactory senses, and can detect food by its smell - the smellier the better!
Often considered to be scavengers, Greenland sharks feed quite happily off dead and decomposing bodies.
Their stomach contents have been found to contain fish, sea birds and kelp, cephalods, jellyfish, octopus, squid, whelks, snails, crustaceans, porpoises, seals and perhaps not so oddly, the remains of dogs, reindeer, polar bears and even a horse.
Anything that falls into the cold Arctic or Antarctic waters may find themselves inside the stomach of Greenland sharks, who are opportunistic sea cleansers, like most other sharks.
Greenland sharks inhabit the colder seas of the northern hemisphere, and are to be found in the waters off North-west America, Canada, Greenland and northern Europe.
There have been sightings of them in Antarctica, as well as in the mid-Atlantic Ocean.
They are a type of sleeper shark, of which only two are known to exist. The other, very similar shark is the Pacific sleeper shark, which inhabits the temperate waters of the North Pacific Ocean and down as far as Chile.
Slow, lumbering and designed not to expend energy, Greenland sharks prefer the colder waters that range from -6C to 10C (22F - 50F).
In summer when the surface waters warm up, these sharks dive to deeper waters, and in winter when the the surface temperatures drop, Greenland sharks come to the surface.
They are primarily bathybenthic, which means that they inhabit the bottom of the deepest oceans, deeper than man can physically go because of the tremendous water pressure at great depths.
In the warmer waters of the Gulf of Mexico, and even off the Brazilian coast, Greenland sleeper sharks have been recorded by scientists at depths of greater than 2,500m (8,200ft).
What size do Greenland sharks grow to?
Greenland sharks are strong contenders too for the title of being the biggest predatory sharks in the world, as well as the slowest.
They can reach a massive 24' long, and have a similar cylindrical body shape to the great white shark whose dimensions are not generally reckoned to go much beyond 22'.
There have been reported cases of bigger white pointers in the seas and oceans of the world, but none have been scientifically confirmed.
Most sleeper sharks (as they are also known) are smaller, with average lengths of 8' - 15'.
Greenland sharks are quite ugly, and have been compared to giant cigars.
What is their lifespan? Do they really live as long as 200 years?
Scientists don't actually know the lifespan of the Greenland shark, but ongoing studies are now being carried out by GEERG (Greenland Shark and Elasmobranch Education and Research Group).
It is suspected that they can live for an incredible 200 years or more.
There is no way of ageing a captured shark, so the only way to know how long they live is to capture and tag a newborn pup, and follow its growth stages.
If they do indeed live for 200 years, it is not something we will know for certain in our lifetime, but it is not unreasonable for an apex predator with very few natural enemies to live this long, when their natural environment is so cold, thus slowing down cell ageing.
Greenland sharks do not survive when kept in captivity.
In 1936, a specimen was tagged after being captured off Greenland.
It was re-captured 16 years later, and measured. It was found to have grown only 6cm (2.4") in that time, so barring unusual growth spurts, it is easy to imagine a 24' specimen to be very old indeed.
Are Greenland sharks dangerous to humans?
The answer to this would be "probably", but they seldom meet humans in their natural environment.
We do not bathe in waters so cold, nor so deep, as is preferred by Greenland sharks.
Should we fall overboard in such cold seas, we would die of exposure long before the sharks could take us.
Divers have photographed Greenland sharks underwater, and not come to any harm. I would imagine humans even in cumbersome dive suits could move faster than the incredibly slow Greenland shark.
Should be die by any natural means in the waters frequented by sleeper sharks, then no doubt we would become a meal for them at some point, if the fishes don't eat us first, but that's not going to matter to us, is it?
Are Greenland sharks edible?
Not really, and this is why they do not end up being fished out of existence, the same as many other species of shark.
Greenland sharks have evolved a unique system among fish to deal with both osmosis (the transference of salts and water through tissue) and the extreme cold conditions to which they have adapted.
As well as producing high quantities of urea, they also retain high levels of trimethylamine oxide (a by-product of the metabolic breakdown of proteins and amino acids) which helps counteract the toxic effects of the urea.
As trimethylamine oxide is also a salt, the body of the Greenland shark becomes saltier than the surrounding sea-water.
This in turn means that the shark does not have to expend energy by constantly getting rid of water, like most other fish.
Everything about the sleeper shark is designed to maintain energy without actually doing very much at all.
Trimethylamine oxide, however, turns into trimethylamine when eaten, which is a substance that is toxic to most animals.
It smells like rotting fish and causes neurological damage to the person or animal that consumes it.
While the effects are temporary, they can be fatal if too much is consumed.
The native people of Greenland and Iceland have learned over the generations how to cook Greenland sharks safely.
The process involves either boiling repeatedly in multiple water changes, or by burying the carcass in the ice for a period of time to allow the poisonous substance to dissipate.
Nowadays, sleeper shark meat is only served as delicacies during festivals.
Not much is known about the reproductive methods of Greenland sharks, except that they are ovoviviparous (their eggs are retained within their body, and they give birth to live young), and that each litter normally contains at least 10 pups.
Newborn pups are an average of 40cm (15.7") in length.
If they have nursery grounds, no-one yet knows, nor if the mother sticks around to look after the kids or goes off to do her own thing, like many other shark species are guilty of.
Greenland sharks are listed as Near Threatened by the IUCN Red List (International Union for the Conservation of Nature and Natural Resources).
Although they are mainly inedible and are not fished for their fins like so many other sharks, they have historically been targetted by fishermen wishing to save their nets and fish catch, both of which Greenland sharks have damaged or destroyed.
For this reason, traditional fishermen have finned them and thrown them overboard to die.
New rules and regulations have been brought in to protect the Greenland shark in many areas, including the St. Lawrence River estuary and Saguenay Fjord in Canada.
This lumbering grandfather of the seas should be respected but left in peace.
Like all sharks, Greenland sharks play a massive role in the ecology of the oceans, by cleaning up dead and decaying bodies, and keeping numbers of other species down.
A human leg was once found inside a Greenland shark, and no doubt would have kept its slow metabolism busy for at least a year.
We can only assume this person was already dead.
If you see a shark in the water, can you swim faster than 1.6mph? That is all you need to do to avoid predation by the Greenland shark, the slowest shark in the world.
Greenland sharks are now being found in shallow waters
Recent studies into many losses in the seal populations in the Northern Atlantic have been linked to the predatory nature of Greenland sharks.
In a break to their previous habits, Greenland sharks are now predating on seals as many as larger fish in shallow waters.
Scientists believe this is caused by overfishing of their normal diet of deep water fish.
It is now believed that Greenland sharks, almost blind because of the copepods that attach to their eyeballs, have a highly developed sense of smell, on top of the electrical field senses common to all sharks.
This allows them to creep up on sleeping prey, silently and unseen in murky waters.