Sharks are particularly sensitive to low frequency sounds, such as those made by wounded, thrashing animals. On each side of the shark's body runs a row of small holes connecting an internal canal called the lateral line which is sensitive to nearby movements. In their snout area sharks have pits, known as ampullae of Lorenzini, which detect the small electrical signals that all animals, including human beings, emit. This sense works best at close range and enables the shark to pinpoint an animal it may not even see in dark water. Metal objects also emit electrical pulses, a fact that may help explain why sharks sometimes attack boats, docks and steel cages.
Specialized eyes enable the shark to see in dim light.
Certain sharks, such as the reef and bull shark, have color vision that attracts them to bright orange and yellow hues. Sharks are the only fish that have eyelids, termed nictitating membranes . Several families, including the hammerheads, catsharks and smooth dogfish, possess these eyelids which roll up in the final seconds before it attacks, essentially leaving the shark blind. This action protects the shark's eyes from the flailing and scratching of its prey. Strangely, the aggressive great white shark lacks these nictitating membranes.