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Vision
It has long been the subject of debate, whether if sharks have
poor eyesight or not. Experimental studies carried out the past
10-20 years have completely dispelled that notion. Sight is extremely
important for sharks in detecting prey, especially at distances
of 15m or less.
Shark's eyes vary greatly from species to species, in size and
shape. But the general structure is a somewhat flattened version
of the same eye most vertebrates have, with an iris, lens and retina.
Generally the lens is spherical and held in place by ligaments on
the top and muscles from below. The pupil in the eyes of some sharks
(requiem sharks such as tiger sharks and reef sharks), contrary
to popular belief, may open and shut quite rapidly. In contrast
to mammals the lens cannot change in shape to focus. To what extent
sharks are actually capable of focusing is not yet known, but recent
studies are showing that at least a few sharks do have the ability
to move their lens backward and forward to better focus on an object.
What is known though is that sharks can maintain a constant visual
field and nearly panoramic vision even when they twist and turn,
due to prominent muscles that can effectively rotate the eyes.
Another special feature of the shark's eye is a structure found
behind the retina called the tapetum lucidum. It consists of two
parts: a series of silvery plates containing the pigment guanine,
and mobile cells containing the dark pigment melanin. This structure
greatly enhances vision in dim light, since light that enters the
eye is reflected back to the retina by the tapetum lucidum to enhance
the picture. For sharks that hunt during the day the extra light
reflected by the tapetum back to the retina could be harmful and
so the reflecting plates of the tapetum are covered by the dark
pigments contained by the melanoblasts.
Water absorbs light differently
according to the wavelength of the light. Red light has a long wavelength
and is absorbed the most, whilst blue and green light are absorbed
less. Experiments have shown that the eyes of sharks are very sensitive
to blue and green light and barely register red light, and therefore
show great adaptation to living in the sea. It is now known for
sure that sharks see in dim light, possibly discern colour, adapt
quickly to the dark and are capable of both nocturnal and daytime
activity, but it is still not clear to what those capabilities are
important to the sharks way of life in the ocean. Because seawater
absorbs more colours rapidly and is relatively opaque and clouded
with suspended particles, vision and sensitivity to colour may not
be as important to sharks as their senses of smell and hearing.
Ref: Springer VG and Gold JP (1989),
Sharks in quetion: the Smithsonian answear book, Smithsonian
Institution Press, Washington, D.C. London
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