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What Do Fish See?

Vision is very important to fish. Like all vertebrates they have well developed eyes and the optic lobe is the largest part of the fish brain.

Light travels in different ways through water and air. Our curved cornea (the surface of our eye) functions well in the air but when we are in the water we need an artificial flat cornea - a mask - to see. Fish corneas are flat. This means that the cornea gathers less light than ours but the fish lens, inside the eye is particularly rounded to compensate for this. Most land animals focus by changing the shape of the lens within the eye but fish achieve the same result by moving their lens in and out - rather like a camera. Nevertheless, they are generally short sighted.

Different fish have different visions of their world depending on where they live and on the proportions of rods and cones in their eyes. Reef fish living in shallow water where colours are similar to those on land probably have good colour vision and may see a broader spectrum than many mammals. They can probably see ultra violet and infrared light. These fish are commonly coloured yellow, the best contrast with the predominantly blue background.

Water is a poor transmitter of light, which means that even in the clearest water the wavelengths (reds) are excluded beyond the first 10-15 meters. Only short wavelengths of light - the blues - can penetrate deep into the ocean. Many fish have eyes which react to light of blue and green wavelengths thus concentrating their vision on the blue end of the light spectrum.

With depth, rods become more important than cones. Compared with the colourful shallow reef fish, predatory fish such as bream, emperor, snapper and trevally have less colour. Nocturnal fish have a larger proportion of rods so they can see well in low light conditions. Sharks, Rays and Mantas have almost all rods and probably cannot see colour which may be the reason for their dull skins.

Fish also do not need eyelids. Their eyes are constantly bathed in water whereas we need our tears to keep moist. The position of the eyes of most fish, at the sides of the head, is usual in predatory animals, which generally concentrate on forward vision. However most fish are also potential prey so wide-angle vision is presumably more important.

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Diving Cairns - The ultimate guide to a diving holiday on North Queensland's Great Barrier Reef