The world’s oceans are home to about 440 species of shark, from the tiny 17-centimetre dwarf lanternshark of South America to the gargantuan whale shark, which grows to about 12 metres in length. Just a few are dangerous to humans, with only four species responsible for fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger and bull sharks.
In a bad year, as many as 15 people are killed by sharks, yet humans dispatch more than 100 million hapless victims over the same period. Many are killed just for their fins, which are destined for bowls of shark fin soup. This begs the question: who should be scared of who? Dig a little deeper and one thing is clear: there’s a lot more to these amazing creatures of the deep than the horror stories we see in films and read about in headlines.
Sharks are armed with astute surveillance skills and finely honed senses, which make them mean hunters – primarily of fish. Many species are very particular, rarely varying their diet. Mealtime favourites include squid, crustaceans, fish and plankton.
Incredibly, two-thirds of a shark’s brain is dedicated to smell and they’re able to detect blood diluted in water at strengths of one to a million – that’s equivalent to half a medicine measure in an Olympic size swimming pool – from 400 metres. Not that their prey has to be bleeding to be detected; many shoaling fish exude chemicals to warn their mates when danger looms and sharks can intercept these nervous signals.
Sharks also have an incredible ability to detect low-frequency sounds – they can hear the heartbeats of fish from far away – and pressure-sensitive receptors allow sharks to ‘feel’ the remote movements of their prey through the water.
Possibly the best known of these legendary sensory abilities is the sharks’ ability to ‘see’ electrical currents (compliments of receptors on their snouts called the ampullae of Lorenzini). For example, we know that hammerheads sweep their heads from side to side over the sea floor like a metal detector while swimming and can pick up signals as feint as half a billionth of a volt.
As an aside, how’s this for incredible? The female frilled shark holds the world record for the longest pregnancy in nature (more than three years); the largest egg belongs not to the ostrich, but to the female whale shark; and the embryos of female sand tiger sharks kill and eat one another in the womb, which gives a whole new meaning to ‘survival of the fittest’.
Source: The Book of Animals by John Lloyd and John Mitchinson (Faber and Faber)
A beach holiday isn’t complete without a seafood meal or two. Stay environmentally conscious when ordering your coastal feast and make sure that the fish isn’t on the SASSI red data list. SMS the name of the fish to 079-499-8795 to see if it’s a species that’s sustainable and plentiful.
Every piece of plastic on the beach could kill a marine animal. If everyone started picking up the rubbish on our beaches, we could make a difference. This summer, don’t go to the beach without a garbage bag and do your bit for the environment.
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