Vision and a Carpet of Light
The eyes of the White Shark are moderately large, suggesting that vision is fairly important to this species' way of life. In structure and function, the eyes of the Great White and other sharks are remarkably similar to our own - with a few interesting and important differences mixed in. Perhaps because we are such visually dominated creatures ourselves, sight is perhaps the most studied and best understood of shark senses. Until recently, however, we knew next to nothing about the visual capabilities of the White Shark. What little we did know had to be gleaned from studies on other species.
The shark eye is essentially a hollow ball equipped with all the features one would expect in that a typical vertebrate: cornea, iris, lens, and retina. The cornea is the clear, outer covering of the vertebrate eye. In humans, it is responsible for about 81 percent of the eye's total focusing power (which is why even small irregularities in the cornea can cause such serious visual impairment in humans). However, since seawater has an optical density almost identical to that of corneal tissue, sharks must rely on the spherical lens to handle the task of focusing light within the eye. In sharks and humans, the amount of light entering the eye is controlled by the iris. The iris is contractile sheet of muscle perforated by an opening called a pupil. Under conditions of low light, iris muscles contract to dilate the pupils (those of many deep-sea sharks are permanently dilated, to capture what little light flickers in their realm of perpetual darkness); under high light conditions, the iris muscles relax and the pupil contracts. As unimpressive as this may seem to humans, accustomed to having their pupils dilate and contract continually, realize that no teleost fish can do this - which explains why most bony fishes always seem to be staring blankly into next week. Pupil contraction can also increase a shark's visual depth of field (range of depths that are in focus), in much the same way as we can extend our focal range under harsh light conditions by squinting.
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Although many sharks have eyes with slit-like pupils, those of the Great White appear dark as buttons of onyx. Yet if one looks into the eye of a living individual from up close, internal structure can be discerned. The pupil is circular, the iris dark and ringed with a spectral hint of midnight blue. Although the eyes can be rotated within their sockets, when relaxed and not obviously tracking an object, a White Shark's eyes seem to be oriented forward and down. This orientation may explain why these animals often cruise past a caged diver slightly above his or her eye-level: the better to visually inspect the strange, bubbling biped.
The shark lens is crystalline and roughly spherical in shape, giving it terrific refractive power. However, unlike the lenses of humans - which change focal length by changing shape - those of sharks focus by changing position. To focus on distant objects, the muscles controlling the shark lens relax, allowing the lens to settle some distance from the cornea; to focus on near objects, the shark's lens controlling muscles contract, pulling the lens toward the cornea. Curiously, this is the same way that amphibians and snakes (but not other reptiles) focus yet opposite to the way that teleosts move the lens to focus on distant and near objects. Studies by shark biologist Samuel Gruber have revealed that most sharks are somewhat far-sighted, having a focal length of about 9 inches (23 centimetres) to optical infinity. This dramatic range of focal lengths testifies eloquently to the refractive qualities of the shark lens. At distances of 50 feet (15 metres) or less, vision is probably a shark's dominant sense.
The retina is a light-sensitive sheet of tissue lining the inside surface of the eyeball. The retina detects light energy by virtue of a vitamin A-based pigment called rhodopsin. When struck by a photon (packet) of visible light, rhodopsin undergoes a temporary conformational change. This, in turn, generates an electrical signal that is transmitted via the optic nerve to the brain, where the stimulus is interpreted as vision. Because this conformational change requires time and the speed of nerve transmission is finite, there are sharp limits to the frequency of images that an eye can separate. Persistence of images on the human retina is what allows film to provide us the illusion of movement: successive still frames are projected at a frequency faster than our eyes can transmit them. The minimum frequency of flashes or images at which an eye can no longer separate them is termed flicker fusion frequency. Shark vision studies by Gruber have revealed that juvenile Lemon Sharks (Negaprion brevirostris) experience flicker fusion at a frequency of about 45 flashes per second - nearly twice the frequency at which humans cease to see discrete flashes. So, to a young Lemon Shark, one of our movies would appear like a rapid-fire slide show, made up of a series of discrete still images.
As in humans, sharks have two basic types of retinal cell: rods and cones. As their name implies, rods are slightly elongated, while cones are gently tapered and relatively squat. Rods are particularly adept at detecting contrast and movement - even operating under relatively low light levels - but are not very good at discerning fine detail. In opposition, cones excel at discerning fine details - including color - but require well illuminated operating conditions. Many of the earliest studies of shark retinas seemed to indicate that these fishes have rods only. It was therefore concluded that shark vision is poor, providing at best a grainy and monochromatic view of the world. Later technologies and approaches have revealed that this is not at all the case. The visual pigment rhodopsin is most responsive to green light, which corresponds well to the greenish light which bathes shallow coastal waters. In deeper waters where blue is the predominant color of ambient light, sharks possess a secondary visual pigment, a golden photosensitive compound that is most responsive to light at the blue end of the spectrum. An analogous mix of rhodopsin and secondary pigments is what grants us color vision. Thanks to decades of careful, dedicated work by Samuel Gruber and his co-workers, we now know that many sharks see in color, too. In a revealing 1985 paper, shark biologist Samuel Gruber and anatomist Joel Cohen studied the retina of the White Shark. Gruber and Cohen demonstrated that the Great White retina has both rods and cones, but at a significantly different ratio from most sharks. The small, moderately deep-dwelling Spiny Dogfish has a rod-to-cone ratio of about 50:1, while in the larger, more shallow-water Sandbar Shark (Carcharhinus plumbeus) the rod-to-cone ratio is about 13:1. But in the White Shark, the rod-to-cone ratio is about 4:1 - roughly the same as in human beings. From these results, Gruber and Cohen concluded that the White Shark has the retinal mechanisms necessary for acute, bright-light, color vision. Further, they speculated that - while the Great White has different parts of the retina specialized for day and night vision - this species probably does not have the extended period of visual activity that the nocturnally predaceous Lemon Shark does.
Perhaps the most ecologically significant discovery about shark retinas was revealed in a fascinating 1991 paper by Robert Hueter. Hueter discovered that the Lemon Shark has a broad, horizontal band that lies across the equator of the retina and is disproportionally rich in cones. Based on a similar retinal band in the Lion (Panthera leo), Cheetah (Acinonyx jubatus), Thompson's Gazelle (Gazella rufifrons), Wildebeast (Connochaetes taurinus), and other mammals of the African plains, this so-called 'visual streak' probably grants the Lemon and other sharks a particularly clear view of the underwater horizon. As a shark's potential prey, rivals, and mates are most likely to first appear on the horizon at the limit of visibility, the adaptive (survival) value of the visual streak is easy to imagine. Gruber and Cohen's 1985 study of the White Shark retina has revealed a higher cone concentration in the "central" retina, suggesting that this species may also have some manner of visual streak. Human eyes also have a cone-rich retinal feature, the fovea, restricted to a small circular patch at the back of the eyeball. The fovea has been demonstrated to be the single most sensitive part of the human retina. Thus, if the White Shark does have a visual streak, this species may be able to use this hyper-sensitive retinal region to extend its period of visual activity well into crepuscular (dawn and dusk) or nocturnal periods.
Shark eyes exhibit at least one other particularly marvelous feature.
Like cats and other nocturnal animals renowned for their eyeshine, the eyes
of sharks have a reflective layer behind the retina. Called a tapetum
lucidum - literally, "carpet of light" - in sharks this structure
is composed of a series of plates silvered with guanine (a metabolic waste
product that is also responsible for the intense silveriness of herrings and
other pelagic teleosts). These plates are angled in such a way to reflect
light back through the retina at precisely the same angle as they had
originally passed through. Under low light conditions, this arrangement
allows a shark efficient re-use of each quanta of available light, greatly
increasing its photosensitivity. The eyes of humans - which lack this
tapetal system - are remarkably sensitive; on a clear, moonless night, the
unaided human eye can detect the light from a single match up to 10 miles
(16 kilometres) away. The tapetum-equipped eyes of sharks probably grants
them at least 10 times the light sensitivity we have. Under a new moon, many
shark species can probably hunt by starlight alone. During daylight hours,
however, such a light sensitivity-enhancing mechanism may be deleterious.
Sharks have evolved an elegant solution to the blinding problems of too much
ambient light. Many species of sharks have migratory pigment cells that can
occlude the tapetal plates under bright-light conditions - functioning much
like built-in sunglasses! Gruber & Cohen's 1985 study of the White Shark
retina has verified that it, too, has a well developed system of tapetal
plates.
The Great White thus has a highly developed visual system, even when compared with that of other sharks. The White Shark's retinas are sensitive to contrast and motion, an ability that may be particularly useful when stalking seals or sea lions by tracking their animated dark silhouettes against the glaring, sundappled surface. The White Shark retina is particularly well equipped to discern spatial relationships and detect fine details, which may be particularly important in ascertaining the relative size of other Great Whites or subtle aspects of their body language. The White Shark retina is also equipped with tapetal plates that are occlusible, enabling this species to protect its delicate retina from bright light - as when this species raises its eyes above water level, as though to survey what is happening above the surface. For all its wonderful adaptations, the White Shark's eyes lack a protective nictitating ('winking') membrane as found in many so-called 'higher' carcharhinoids (such as the whaler sharks, family Carcharhinidae). To protect its eyes from injury due to prey flailing desperately in its jaws, the White Shark has no recourse but to roll its eyes tailward in their sockets, exposing the tough, fibrous sclerotic coat. Despite this low-tech solution to a very real predatory problem, it is clear that the Great White - like ourselves - relies heavily on vision to provide it information about the world in which it must earn a living.
