Ampullae of Lorenzini

Electroreception:

At some point during the evolution of elasmobranchs the lateral line pores around the snout developed a sensitivity to fluctuations of the electrical fields in the sharks habitat. These modified sensory organs are known as the ampullae of Lorenzini. They consist of relatively large bulbous pores filled with a gelatinous substance. Connected to the pores are cylindrical canals in which the gelatinous secretions are stored. At the base of each pore is a sensory nerve which transports the electrical signals (which are collected by sensory cells lining the pore) to the brain. Actively hunting sharks may have as many as 1500 ampullae around their snout and head whilst more sedate species may only have a few hundred. The ampullae also react to a lesser degree to temperature and pressure changes.

The ability of sharks and rays to detect weak electrical signals in their surroundings may be one of the greatest factors relating to their survival through the millennia. The organs are sensitive enough for hammerheads and some other sharks to detect the small electrical signals put out by their prey whilst it hides motionless below the sand. In fact the ampullae are so sensitive that they can pick up voltage fluctuations of just 10 millionths of a volt or the equivalent of the electrical gradient of a AA battery with wires put into the sea 1 mile apart. It has been suggested that the widened heads of the hammerhead family may be an adaptation designed to increase the triangulation capabilities of their electroreception. 

When sharks are close to prey it appears that their electrical sense takes over from sight or smell. This would explain why sharks which have been chummed to a fishing or shark diving boat will sometimes attack the propellers and other metal objects rather than the bait which has been put in the water in front of them.