Shark senses

Sense of smell

Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater, up to a quarter of a mile away. They are attracted to the chemicals found in the guts of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks, have external barbels that greatly increase their ability to sense prey.

Sharks generally rely on their superior sense of smell to find prey, but at closer range they also use the lateral lines running along their sides to sense movement in the water, and also employ special sensory pores on their heads (Ampullae of Lorenzini) to detect electrical fields created by prey and the ambient electric fields of the ocean.

Sense of sight

Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to the marine environment with the help of a tissue called tapetum lucidum. This tissue is behind the retina and reflects light back to the retina, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some have nictitating membranes. This membrane covers the eyes during predation, and when the shark is being attacked. However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey. The importance of sight in shark hunting behavior is debated. Some believe that electro and chemoreception are more significant, while others point to the nictating membrane as evidence that sight is important. (Presumably, the shark would not protect its eyes were they unimportant.) The degree to which sight is used probably varies with species and water conditions.
Sense of hearing

Sharks also have a sharp sense of hearing and can hear prey many miles away. A small opening on each side of their heads (not to be confused with the spiracle) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, as it is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fishes and tetrapods the external opening into the inner ear has been lost.

Electroreception

Electroreceptors (Ampullae of Lorenzini) and lateral line canals in the head of a shark.

The Ampullae of Lorenzini are the electroreceptor organs of the shark, and they vary in number from a couple of hundred to thousands in an individual. The shark has the greatest electricity sensitivity known in all animals. This sense is used to find prey hidden in sand by detecting the electric fields inadvertently produced by all fish. It is this sense that sometimes confuses a shark into attacking a boat: when the metal interacts with salt water, the electrochemical potentials generated by the rusting metal are similar to the weak fields of prey, or in some cases, much stronger than the prey's electrical fields: strong enough to attract sharks from miles away. The oceanic currents moving in the magnetic field of the Earth also generate electric fields that can be used by the sharks for orientation and navigation.

Lateral line

This system is found in most fish, including sharks. It is used to detect motion or vibrations in the water. The shark uses this to detect the movements of other organisms, especially wounded fish. The shark can sense frequencies in the range of 25 to 50 Hz.[22