Penguins are living on land and in water. A good eyesight is advantageous in both environments. Penguins could avoid predators and find their prey better in water, on land they could find their nest. The human's eyesight is different in both environments. Most people see sharply in air. But their vision is blurred under water without diving goggles. A blurred vision under water could be fatal for penguins. So, how the penguin's eyesight is under water and in air?
Researchers found out by retinoscopy and photorefraction that penguins are more or less emmetropic in both environments. In water the refractive power of the horny skin (cornea) gets lost, that is why we human beings cannot see sharply under water. But some special structures in the eyes of the penguins enable them a sharp vision unter water.
Penguins have a more flat horny skin compared to us or flying birds so that the loss of the refractive power is not that high. Nevertheless, this adaptation does not suffice for a sharp vision under water, approximately 30 dioptres have to be compensated by other eye structures.
The refractive power of the eye can be adapted by deformation of the lens, this process is called accomodation. The lens is formed rounder by muscle contraction. The refractive power of a round lens is higher than the refractive power of a flat lens. In general, the lens of penguins is rounder in comparison to flying birds. Additionally, the iris sphincter muscle and ciliary muscle of penguins are very strong so that the lens could be formed very round. These special structures and a strong accomodation suggest that the vision of penguins is sharp in water.
In the retina of penguins three different cones were detected that can be stimulated by green, blue or violet light (absorption maxima 403, 450 and 543 nm). So, penguins seem to be able to perceive light with short wavelenghts well.
Researchers suppose that the eyes of penguins were adapted to the life at sea. With increasing water depth light of long wavelenghts (e. g. red light) is absorbed. Penguins hunt in deeper areas of the sea were only light of longer wavelenghts (e. g. green or blue light) arrive. Due to their cones penguins can perceive these wavelenghts and are able to see well during hunting.
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