Sharks have evolved highly developed nervous and sensory systems that they use to find prey. Each species has adapted these senses to their own environment. The four main senses include: chemoreception (smell and taste), mechanoreception (touching and hearing), photoreception (vision), and electroreception (the ability to sense electric fields).
The nostril (seen in the photo to the right, on a lemon shark) is located on the underside of the snout and leads directly to the olfactory bulb in the shark's brain. The shark's ability to smell is perhaps its most important sense, and this is apparent by the fact that one-fifth of a shark's brain may be utilized to analyze scent information. Some species are able to detect as little as one part per million of blood in seawater. Sharks' tremendous ability to detect scents allows them to find food sources from long distances, often following a scent by crisscrossing an odour trail while swimming downstream from the odour. As they are stimulated by the scent of blood and food, sharks may become aggressive when they detect these substances; however, it is suggested that sharks are not stimulated by mammal blood (e.g. from humans) in the same way as they are by fish blood.
Sound and movement are detected by two sensory organs in sharks: the ears and the lateral line. Shark ears are two tiny pores located on the side of the animal's head, behind its eyes. As sharks' flesh has a similar density to that of seawater, vibrations of sound ripple through the water along the ear cannel to the inner ear, but also through the shark's head, thus being acoustically transmitted, and stimulating and producing nerve signals. A shark's hearing is related to its lateral line system.
The lateral line (right) is a series of fluid-filled canals just beneath the skin that run from the head down the side of its body. This system, which is very sensitive to low-frequency vibrations, reacts to variations in water currents and pressures from underwater sound and allows the shark to detect both direction and amount of movement from great distances. For example, a wounded animal sends vibrations through the water, indicating to predators that it is in trouble and thus is easy prey.
A shark's ability to be an efficient hunter depends heavily on its vision. Despite what most people think, sharks have excellent vision; however, only some sharks have colour vision. While sharks may have immoveable eyes, typically they have three eyelids for protection, one of which can move. The upper and lower eyelids protect the eye, but do not cover it completely. The third eyelid, known as the nictitating membrane (right), is a moveable eyelid. During feeding, the membrane closes over the eye momentarily for protection, rendering the shark temporarily blind while it strikes.
At close range, sharks are able to detect minute electrical currents generated by the nervous system of their prey. The electricity sensing devices are clusters of tiny pores in the skin around the shark's head called ampullae of Lorenzini (right). These pores or ampullae are small sensory organs, each containing a sensory hair cell filled with an electrically conductive jelly. The ampullae are remarkably sensitive and particularly useful in finding prey that is disguised or hiding.