Most daylight fish have eyes that are well developed and can see in full colour just like humans. Their eye is made up of mostly the same components as humans however, fish have to deal with the low light levels in the water, the fact that the light is refracted when it hits the water and also the light reflecting off particles floating in the water. Therefore their eyes have a few adaptations that human eyes don’t have in order to make their vision more efficient.

As the cornea of the fish is unable to magnify light within the water, the lens (which is large and spherical) is responsible for magnifying the whole image onto the retina. The shape and size of the lens creates a gradient for refraction which allows a sharp focus of light.

Fishes lenses are incapable of shape change unlike humans. The distance between the lens and the retina is important in determining the quality of focus and the visual field range. During close- up vision, the lens rests against the cornea, and is drawn back by the retractor muscle for long distance vision. Accommodation of light onto the retina is achieved by ‘swinging’ the lens in and out of focus.

In humans, the iris and the pupil control the amount of light allowed into the eye. Most of the time, in fish, this is not necessary as there is little light in the water in which they live. There is minimum change in the size of their pupil and iris to allow the maximum amount of light in.

The front of a fish’s eye is covered by a cornea, which refracts (bends) the light to focus it more. It is thick and transparent.

Some fish have a more complex cornea, divided into two parts, the true cornea and the spectacle. This is for fish that live in shallow waters, or those that are bottom dwelling. This spectacle is an additional covering to the eye that protects the eye from debris and silt, and for shallow water fish, protects the eye from bright sunlight which could cause damage (in this case the spectacle is usually coloured yellow).