eyes
Sharks have a basic vertebrate eye, but it is laterally compressed. The lens is large and spherical.
Although the threshold of a shark's visual acuity has not been demonstrated, it is apparent that they are well-suited for seeing in dim light. Their eyes are particularly sensitive to moving objects.
Sharks have a large proportion of rods, which are highly sensitive to changes in light intensity, making sharks sensitive to contrasts of light and shadow.
The eye has a layer of reflecting plates called the tapetum lacidum behind the retina. These plates act as mirrors to reflect light back through the retina a second time. The tapetum lucidum of a shark is twice as effective as that of a cat.
Unlike those of other fishes, a shark's pupil can dilate and contract.
Cone cells are present, indicating that sharks may have some sort of color vision.
In clear water, a shark's vision is effective at a distance up to about 15 m (50 ft.).
Some species have an eyelidlike structure called a nictitating membrane. The nictitating membraneprotects the eye from being injured by thrashing prey while the shark is feeding.
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Eye size and position vary, depending on the particular habitat or behavior of the species. In general, deep-water sharks have bigger eyes than shallow-water sharks.
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