Senses
Sharks have many obvious advantages over people in the water, especially when it comes to senses. In addition to those we have – sight, hearing, touch, smell and taste – sharks have two other senses, mediated by specialized receptors: electroreceptors and lateral lines.
A shark’s most acute sense, the one it may use to detect prey from the greatest distance, is probably its sense of hearing. Sound travels faster and farther in water than in air. The general structure of a shark’s internal ear resembles that of humans, but a shark’s is much more sensitive. Sharks can hear disturbances in the water caused by struggling fishes from great distances. As in humans, the shark’s inner ear also controls balance.
Sharks seem to hear sounds best in the range of 20-300 Hz, and are especially attracted to irregular sounds at or below 40 Hz, which is about the range produced by struggling fish. The ability to detect sounds depends on the magnitude and distance of the source, and in some tests sharks have been attracted to sounds from over a mile away.
Sharks are sometimes called “swimming noses” because of their remarkable sense of smell. The two nares, or nostrils, are located under the snout, but do not connect with the throat. As sharks swim, water flows in one side of the nostril, through a nasal sac, and out the other side. Sensitive olfactory receptors inside the nasal sac detect smells in extremely small concentrations, and send nerve impulses to the brain.
How good is a shark’s sense of smell? Blacktip sharks have been reported to detect fish flesh diluted to one part per 10 billion parts of seawater. As with sound, a shark’s ability to detect smell depends on the magnitude of the source, its distance, and dispersal. In some tests sharks were able to detect smells at distances of several hundred yards.
The lateral line system consists of a series of fluid-filled canals running just under the skin along the sides of the body and over the entire head. Visible pores along the lateral line open to the outside, providing a direct connection to the surrounding water. The system detects flow of water over the skin, and low-frequency pressure changes stimulate sensory cells within the canals. These pressure changes, caused by water movement, are often created by swimming motions of nearby prey. The portion of the lateral line located on the head may function during feeding. Both the ears and the lateral line system detect different forms of vibrations, but they are sometimes referred to together as the acoustico-lateralis system.
Shark eyes are somewhat similar in structure to those of humans, but the mechanism for focusing is different because the lens does not change shape. The retina contains both rods and cones, and at least some sharks can apparently see color. How far sharks can see obviously depends on water quality, but in some tests sharks could see objects at 30 yards in clear water. How well sharks can discriminate between subtle differences in shape is not known, but reflective or high-contrast objects are easy for them to see.
As in humans, the shark’s pupils can open and close to control the amount of light entering the eye. Many sharks have a reflective layer behind the retina known as a tapetum. This enables sharks to make the most of low light conditions, especially when feeding at twilight and night. Many sharks are also equipped with a nictitating membrane on each eye; it works somewhat like an eyelid and is used to protect the eye during encounters with prey.
Sharks and their relatives have one sense that is almost unique to their group: the ability to detect weak electric fields produced by the bodies of all living organisms. The fields are detected by sensory organs known as ampullae of Lorenzini, which in some tests have been shown to function at distances of up to a few feet.
As in the lateral line system, the electroreceptor sensory cells connect with the outside seawater through gel-filled canals which open at pores on the skin. These pores are located mostly around the snout, jaws, and head.
Electroreception helps sharks locate prey just before they bite it, as their eyes cannot see the area near their mouths. Some sharks use this sense to locate buried prey. Other functions may include detection of weak electric fields produced by mates or other individuals, and in detecting changes in the earth’s magnetic field for navigation.
Sharks have well-developed touch receptors located in their skin, all over their body. They also have taste receptors on the tongue, in the lining of the mouth, and in the pharynx, and will reject items they find distasteful (perhaps including people).
As sharks attempt to locate prey items, they probably make use of their senses in about the order they’re described above. Hearing and smell are thought to be effective at great distances. As sharks get closer, they may be able to see the prey. In the final approach, the shark may use its lateral line to detect prey movement, then its ampullae of Lorenzini to detect the prey’s electric field. Finally, touch and taste are used during the process of capture and ingestion.