The colouration of marine creatures is obviously related to the illumination of their surroundings. Many creatures of shallow water are protectively coloured, usually dark on the upper surface and whitish underneath. The upper parts are often mottled or patterned in a way which makes them very inconspicuous against their normal background, and in some cases they can rapidly change colour to match different surroundings. Some pelagic fish are difficult to see in water because their dorsal surfaces are darkly pigmented and their scales have a structure and arrangement which reflect an amount of light closely equivalent to the background illumination from almost any angle of view (Denton, 1971). In most cephalopods the skin contains both chromatophores and reflecting elements which together enable the animal to match the background. Deep-water creatures which at times come close to the surface may have a reflecting surface or be almost transparent. At middle depths, where only very faint blue light penetrates, there are many highly pigmented species, usually black, red, or brown. By reducing reflection these colours must in the dim blue illumination give virtual invisibility below 500 m. In the total darkness of great depths there are numerous non-pigmented forms, light or buff in colour.
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