A great white shark attacks from A Seton and behind, taking its victim by
surprise. To do this it has to come up fast and keep its quarry in sight. Its
eves must react rapidly to the change in light if its hunt is to be successful.
Like all other sharks, the great white relies on its array of senses to locate
prey such as seals and sea lions. But its sight is crucial to pinpointing a
victim at the surface. Silhouetted against the light just as a human, entering
a brightly lit room from the darkness outside, can be briefly dazzled and take
2 few seconds to adjust to the change, so a shark, rising to the surface from
the murky depths must cope with a rapid increase in light. It cannot afford to
wait for its eves to adjust — a lost second and the prey could be gone.
The secret of this
ability lies in the tapetum, a laver of mirror-like plates at the back of the
eyeball. It is the tapetum that eerily lights up the cat’s eyes when it is caught
in Car headlamps at night. By reflecting light back through the retina, it
effectively doubles the amount of light for the eye to use. This is essential
for night hunters, like the cat. if they are to see with clarity, especially in the faint moonlight. In the day, though, too much light can be a problem. The cat
copes by narrowing its pupil, the gap that Jets in light, to 4 wafer-thin slits,
using the muscles in the iris.
The great white shark's
tapetum greatly enhances its vision in the murky depths, but it has no iris to
protect its eyes when it surfaces rapidly. In order to compensate, it has
developed a ‘curtain’ of cells containing pigment. As the shark moves into
bright light, these automatically expand over each tapetal plate and then
contract as it returns to the depths.
While it is swimming in shallow water, a shark needs to be able to see into the light above it and into the dark below. In this case, the tapetal curtain reacts differently in each half of the eye. The lower half-eye of a killer whale hunting deep in the ocean, a blue shark relies upon reflective plates behind its eyeballs to increase the amount of light striking the retina. of the tapetum, which reflects light from above, is covered to protect the retina. The upper half of the tapetum, which reflects light from below, is exposed to make the most of the light hitting the retina and so give as clear a picture as possible of the murky depths below.
