Structure and function Diagrammatic vertical section through the eye of teleost fish. Objects in water will only appear as their real colours near the surface where all wavelengths of light are still available, or if the other wavelengths of light are provided artificially, such as by illuminating the object with a dive light. A red object at depth will not appear red because there is no red light available to reflect off of the object. Blue is the only colour of light available at depth underwater, so it is the only colour that can be reflected back to the eye, and everything has a blue tinge under water. So the only colour reaching the eye is red. An object appears red to the eye because it reflects red light and absorbs other colours.
This is why things appear blue underwater: how colours are perceived by the eye depends on the wavelengths of light that are received by the eye. Shorter wavelengths penetrate further, with blue and green light reaching the deepest depths. In clear ocean waters red is absorbed in the upper 10 metres, orange by about 40 metres, and yellow disappears before 100 metres. The wavelengths at the extreme ends of the visible spectrum are attenuated faster than those wavelengths in the middle. In addition to overall attenuation, the oceans absorb the different wavelengths of light at different rates. At 10 metres depth only 16% of the light is still present, and only 1% of the original light is left at 100 metres. In clear ocean water, at one metre depth only 45% of the solar energy that falls on the ocean surface remains. Water is very effective at absorbing incoming light, so the amount of light penetrating the ocean declines rapidly (is attenuated) with depth. Besides these universal qualities of water, different bodies of water may absorb light of different wavelengths due to varying salt and/or chemical presence in the water. Ultraviolet light (even shorter wavelength than violet) can penetrate deeper than visual spectra. red, orange) is absorbed more in water than light of shorter wavelengths (green, blue). For example, visible light of long wavelengths (e.g. The optical properties of water also lead to different wavelengths of light being absorbed to different degrees. Water absorbs light so that with increasing depth the amount of light available decreases quickly. įish and other aquatic animals live in a different light environment than terrestrial species do. Water absorbs the warmer long wavelengths colours, like reds and oranges, and scatters the cooler short wavelength colours. Comparison of the depths which different colours of light penetrate clear open ocean waters and the murkier coastal waters. Shorter wavelengths are on the violet and ultraviolet end of the spectrum, and the longer wavelengths are at the red and infrared end. Fish vision shows evolutionary adaptation to their visual environment, for example deep sea fish have eyes suited to the dark environment.Įach colour of visible light has unique wavelengths between about 400-700 nm and together they make up white light. The ancestors of modern hagfish, thought to be the protovertebrate, were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. Some fish can see ultraviolet and some are sensitive to polarised light.Īmong jawless fishes, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Fish retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Birds and mammals (including humans) normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Vision is an important sensory system for most species of fish. An oscar, Astronotus ocellatus, surveys its environment
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