Shark Week: Sharks, the most misunderstood species on Earth?
Nearly every shark stereotype is wrong. They aren’t primitive animals, they aren’t mindless killing machines and they don’t prey on humans. Sharks are elasmobranchs, meaning that they are fish that have cartilage skeletons instead of bones. Far from being an evolutionary leftover, however, this evolved to save weight so that large sharks could still be fast and agile swimmers. Cartilage also lets sharks conserve calcium and phosphorous, which is important because they need both for making teeth. And they get through a lot of those.
What surprises many people is that sharks actually have individual personalities. They are known to socialise, have “best friends” and create social networks that are unusually complex for a lone hunter. They can be trained by humans to complete tasks much more quickly than rabbits or cats, and retain the knowledge for much longer. Sharks can also teach each other new tricks: how to find food, identify predators and attract potential mates. One common misconception is that they must keep swimming to survive, which isn’t actually true, and rather than being near-blind and dependant on their superb sense of smell, they in fact have advanced sight. They feel pain. And the boldest sharks face a greater risk of dying before adulthood.
The razor sharp teeth of the Great White are clearly visible here as it tries to snack on a passing bird
Having said all that, sharks are incredibly efficient hunters. They treat their teeth as disposable weapons and can lose a couple with every bite, in fact sharks get through so many teeth in their lifetime that the availability of the mineral phosphorous was historically the biggest constraint on their spread around the world.
Sharks haven’t always been objects of fear. Victorian sailors regarded them as entirely benign – like dolphins. Then in 1916, along the coast of New Jersey, four people were killed and one badly injured in a spate of unprovoked attacks by sharks. This sparked a media frenzy that inspired the 1974 novel Jaws by Peter Benchley, and sharks have been the villains of the sea ever since. Despite the fact that you are roughly twice as likely to die from a coconut landing on your head than a shark bite, we still have a grossly inflated sense of the threat these creatures pose.
Sharks are apex predators and very well adapted for hunting in the sea, but the fact remains that for every human eaten by a shark, 20 million sharks are eaten by humans.
How powerful are shark jaws?
A six-metre shark, such as a great white, can exert more than 18,000 newtons of force with a bite. That’s a huge force – twice as much as the largest alligators, which have the strongest bite of any land animal, and more even than current estimates of the bite of the T-rex. But it doesn’t end there. An alligator only exerts maximum bite force when its jaws are almost shut. The wider it opens its mouth, the less leverage is available for the jaw muscles and the weaker the bite becomes. Great white sharks have a unique arrangement of muscles in their jaw that enables them to exert maximum bite force, regardless of how wide their mouth is opened.
Top 5 Facts: Sharks
1) Huge liver- The liver of a shark can comprise up to 30 per cent of its body mass and performs an incredible number of tasks, including keeping it afloat.
2) No reverse gear- Sharks can’t use their fins to paddle, like most other fish. This means that they are unable to swim directly backwards.
3) Familiar eyes- Sharks have eyelids, although they never blink, but they can contract and dilate their pupils, like humans, something no bony fish can do.
4) Power napping- The spiny dogfish uses its spinal cord to co-ordinate swimming, rather than its brain, meaning it can swim while sleeping.
5) Fishy barometer- It is possible sharks may be able to use their lateral line to detect approaching frontal pressure systems and swim deeper to avoid hurricanes.
What’s the biggest shark in the world?
The biggest shark in the world, reaching lengths of over 12m (40ft) is the whale shark (rhincodon typus) but it’s not the most dangerous. It feeds by filtering very small animals such as shrimps out of the water via its gills and has incredibly tiny teeth. The second largest shark, the basking shark (cetorhinus maximus), is also harmless to man and feeds in the same way.
Basking sharks are usually around 7.5m (25ft). The largest, most dangerous shark is the great white shark (carcharodon carcharias) which at its maximum size is less than half as long as a whale shark but has much bigger teeth, over 5cm (2 inches) long. Great whites normally like to eat seals and other marine mammals but have been implicated in hundreds of attacks on humans.
How does a shark navigate?
Sharks have a special electric sense that enables them to home in on small electric signals generated by prey. But the special electroreceptors in their skin also appear to be important in the long trans-oceanic migrations made by some species.
The receptors develop small electrical currents in response to temperature gradients, and this may help those species that follow warm-water currents. They are also sensitive enough to measure the voltage gradients (or ‘electrical signals’) produced by the movement of oceanic water through the Earth’s magnetic field, as well as those generated by the shark’s movements through that field. It’s thought that this allows sharks to navigate as if they had their own magnetic compass.
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