36 Sharks: The Animal Answer Guide
ers know from trying to locate a buddy who is banging on a scuba tank to
alert other divers (perhaps to the presence of a shark in the area?). Local-
izing a sound requires your brain to be able to detect the slight difference
in the sound’s arrival time at either ear. Because human hearing evolved on
land, we have little trouble localizing terrestrial sounds, which travel at 340
m (1,115 ft) per second. Sound travels faster—in fact, four times faster—
in water. Nothing in our evolutionary history prepares us for that kind
of speed. Sharks are exquisitely adapted to the aquatic environment; we
are not.
How well can sharks see?
Although sharks live and can feed in the dark or in very turbid water,
they also have good vision because of well-developed eyes and large optic
lobes, the part of the brain devoted to vision. Sharks tend to be a little far-
sighted, meaning they are able to focus best on objects more than 23 cm
(9 in) away. But sharks don’t have to read textbooks, which is good because
their overhanging snouts would block their view of objects that close. For
locating objects close to their mouths, they probably rely more on electri-
cal input (see “How do sharks detect electric fields?” below).
A shark’s eyes are built on the same plan as the eyes of all vertebrates.
Moving along the path of light, the eye consists of an outer, clear cornea;
a lens (spherical rather than disk-shaped as in bony fishes and mammals); a
slit-like pupil surrounded by an iris diaphragm; and a retina made up of re-
ceptor cells called rods and cones. Light enters through the cornea. Under
differing conditions of brightness, the iris squeezes closed or opens up to
regulate the amount of light entering, something bony fishes can’t do. The
lens is moved back and forth for focusing. Cone cells in the retina operate
best under daytime, bright light conditions and provide acute vision (high
resolving power for smaller objects moving short distances). Cone cells are
also tuned to differing wavelengths of light and are responsible for color
vision in those species that can discriminate color (see “Can sharks see
color?” below). Rod cells are more sensitive to light and operate chiefly un-
der low light conditions such as twilight and nighttime, providing a black,
white, and gray view of objects and their movements.
Many sharks can rotate their eyeballs to follow an object, another thing
bony fishes can’t do. Some sharks and many rays have eyelids that pro-
tect the eye from physical harm or block out bright light. In some sharks
such as the evolutionarily advanced carcharhinid reef sharks, the eyelid is
an opaque tissue called a nictitating membrane that is pulled over the eye
to protect it. White Sharks lack any kind of eyelid but instead rotate the
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