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. In effect the shark's field of vision can swap between monnocular and stereoscopic at any time.