Sharks are excellent predators and are understandably feared by many. However, more often than not, humans are not the intended targets by sharks and attacks on humans are relatively rare.
As predators, sharks have an extremely advanced sensory system. They have two nasal sacs which are lined with cells. As sea water passes over these cells, they are able to detect traces of fish flesh at concentrations of one part per ten million parts water. They are able to pick up sounds and vibrations by either pores on their head, which detect water pressure variations, or through their ears. They have pores in the skin around the snout that are sensitive to electrical fields and so can hunt fish. They also have very good vision and are able to easily distinguish moving objects from backgrounds. A further feature is that they have mirrors behind the retinas which enables better vision in the dark, however in bright light, the mirrors cloud over with black granules to adjust. Sharks also have organs that look like taste buds around their lower jaw, which are thought to be a part of a sensory system but it is unknown what it is used for.
Sharks are cartilaginous fish, meaning that their skeleton is made of of cartilage. Their teeth are continuously replaced (about every 4-8 weeks in adults) thus remaining sharp. They have an asymmetrical tail with a large upper lobe to provide lift in the water, and a large liver which is rich in oil to help increase buoyancy. They also have small plates all over the body which are called denticles. These can severely lacerate anyone who lightly brushes them.
The Great White shark (Carcharodon carcharias), averages 4.6 metres long, although the longest ever measured was 9 metres and estimated to weigh more than 4.5 tonnes. They can bite with enormous force and are quite aggressive. Great Whites (or White Pointers as they are sometimes called) account for more than a third of human shark fatalities. Their life span is on average about 70 years or more and almost nothing is known about their methods of reproduction. One of the most famous Great White shark attacks in history was that of Rodney Fox in 1963. He was spearfishing when the Great White clamped its jaws over his body, from shoulder to stomach. Fox’s oxygen mask was ripped off, and he struggled with the shark, astonishingly, wrapping his arms and legs around the shark. When he returned to the surface, the shark followed but from a distance. The shark then went for the marker buoy tied to Fox’s waist. Fox could not find the clip to undo it, but eventually the lines parted, and Fox was pulled into a boat. With his intestines falling out onto the beach, he suffered a punctured diaphragm, ripped lungs, uncovered spleen, all ribs broken, all tendons, fingers and thumb cut in his right hand, huge loss of blood and a piece of shark tooth embedded in his wrist, which remains there today. He required 462 stitches, but miraculously he survived.
The tiger shark (Galeocerdo cuvieri), is another species which can be very dangerous. It is named a tiger shark due to the stripes on its back; they are more prominent in juveniles and fade with age. A well-known tiger shark attack was on Bethany Hamilton in 2003. She was aged 13 and was lying on her surfboard. A 4.2 metre tiger shark severed her left arm just below the shoulder. However, Hamilton returned to surfing just 3 weeks later and taught herself to surf one-armed.
The Shortfin Mako (Isurus oxyrinchus), is a species that usually stays out to sea so it is not as dangerous to swimmers, however it will ferociously attack boats by knocking the hulls, biting the propellers or even leaping 6 metres out of the water to land on the boat. In 1977, some South African fishermen reported this happening. With the Mako on board, one man got severely bitten on the thigh and threw himself overboard, so as to escape the shark.
There are other species of shark too, such as the Requiem shark and the Hammerhead, however the ones mentioned above are of the most danger to humans. Sharks are formidable predators, and extremely well adapted to life in the ocean, as seen by their highly advanced sensory systems, however, there is still much that is unknown. What are their mating rituals? Why is it so difficult for them to survive in captivity with even a mild restraint on their environment? What are the details of reproduction? No one has ever seen a Great White’s birth, but why? There are so many questions surrounding sharks, which makes them such fascinating creatures.