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'Increase your Knowledge'  Series:

Shark Senses

Over the course of hundreds of millions of years, sharks have evolved a remarkable array of prey-detection systems, probably the most diverse of any predator. A shark can home in on its prey from more than 2 kms. away. This is because its ears are tuned in to low-frequency vibrations, such as those produced by an injured fish, which can travel great distances.

     At a distance of 0.5 Kms., a shark can smell blood or body fluids in the water and follow the trail back towards its source. It can detect one part blood in 100 million parts water.

     At 100 mtrs. from its prey, the shark's lateral line - a row of fluid filled sensory canals running along either side of its head and body - picks up changes of pressure in the water produced by the prey as it moves or struggles. From about 25 mtrs., depending upon the clarity of the water, a shark can see its prey, and some sharks see in colour, discriminating between blue, blue-green and yellow. Shark eyes are ten times more sensitive to dim light than those of humans. This is due to a layer of reflective plates behind the retina that bounces light back onto the light-sensitive cells, enabling the shark to make use of every photon of available. When a shark needs to rise rapidly from the murky depths to attach prey swimming in the brightly lit surface water, it avoids being blinded by covering up the reflective plates. Cells filled with the dark pigment melanin quickly move into channels  on the reflective plates, preventing them from reflecting light. In dim light the cells migrate away.

     When some species of shark attack, they protect their eyes with a third eyelid known as a nictitating membrane, or by rolling them back into protective sockets in the head. At this point, a shark is swimming blind and it brings into play yet another remarkable sensory system. Jelly-filled pits in the shark's nose contain cells that are sensitive to electricity. They are so sensitive that they can detect a change of a hundred-millionth of a volt per centimeter. This means that a shark can pick up the minute electrical currents produced by a fish's beating heart or the contraction of muscles in its tail.

     The hammerhead shark, with its curious T-shaped head, is a prime exponent of this method of detection. It swings its head from side to side over the sea bed, like a person with a metal detector, searching for tiny electrical currents. In this way it can locate the position of flatfishes, skates and rays buried under the sea.