Sunday, November 26, 2000		Nature

Meet the ultimate killing machine!
By Nutan Shukla

SHARKS have highly developed senses of hearing as well as smell. Often called the ‘ultimate killing machine’, it might be first alerted to the sounds of a struggling fish. They take full advantage of their environment — the water — which is an ideal medium for carrying low frequency sounds (40Hz and below) to long distances and sharks can hear these sounds from over a mile away. At a quarter of a mile, it can smell blood or body fluids in the water. Its olfactory organs are so developed that it can detect one part of fish extract in 25 million parts of water; in other words, equivalent of 10 drops in an average-sized swimming-pool. Once it gets the smell it follows it to head upcurrent, taking a zig-zag course, sampling the water for the areas of greater concentration and gradually follows the odour trail towards the victim.

As the shark reaches about 100 metres from the prey its lateral line system comes into play. It consists of a row of fluid-filled sensory canals running along either side of the head and body. Tiny hairs in the canals are sensitive to vibrations, pressure changes and movements, so the shark can almost ‘feel’ the presence and location of something moving in the water. It is also able to compare minute differences in the current flow on either side of the body. Together with the information it receives from its olfactory system, the shark can more accurately locate a target upstream.

As this predator reaches about 75 feet from its prey, even in the dark and gloomy surroundings of the deep sea, it begins to see the movement of its potential victim, that too in colour. It is possible because these members of the group of cartilaginous fish have very acute eyesight, ten times more sensitive to dim light than that of humans. As far as colours are concerned, they can make out the difference between blue, blue-green and yellow. When a shark attacks its prey from beneath and rushes towards the surface it switches off its dark-adapted system and functions normally in bright light.

As this killing machine closes on for the final assault the eyes are protected. In some species, a membrane slides across and closes the eyes; in others, like the great white, eye-balls recede into the safety of the eye-sockets. In both the conditions it becomes blind when it is about to attack, but it has one more remarkable sensory system to bring into play — one that detects electricity. Sensory organs, located in small, jelly-filled pits in the snout, can detect minute electric currents associated with muscle activity, such as the beating heart or gill movements, in the prey. The sensitivity to the electric field surrounding a prey fish is so great that the shark can detect a change in the intensity of a hundred-millionth of a volt per centimetre, the equivalent of a flashlight battery creating a field between two electrodes 1,000 miles apart.

The hammerhead sharks, with their curious T-shaped heads, make full use of this system. By spreading the electroreceptors along the width of the head, they are able to scan a wider area and more accurately detect prey, such as flatfish, skates and rays buried in the sand.

And if that’s not enough, sharks are thought to be able to detect the lines of force in the earth’s magnetic field, so that a wandering great white can be in the right place, at the right time, in order to be ready to grab a meal.

Contrary to popular belief, these ultimate predators are not primitive creatures. The two-and-a-half-hundred known and living species are the product of the evolutionary process of the last 350 million years, a time-span enough for any killing machine to be developed and perfected. Some of the outstanding examples are the great white, mako, porbeagle, the hammerheads, and the aptly named requiem sharks, including the notorious bull sharks, the oceanic whitetip, whalers and reef sharks. They are programmed to seek out and eat, and are aided in this task by the most diverse array of sensory mechanisms used any predator.