A layer of elongate crystals embedded in cytoplasm will reflect some wavelengths of light and transmit others. In other words, the wavelength reflected is a function of the size and spacing of the layer of crystals. Arrangement of different crystal layers so they overlap will result in reflection of all wavelengths. This can be brought about experimentally by keeping the fish in a tank with a white background. The fish first lightens physiologically then gradually over a period of days becomes still lighter.
Examination reveals that the number of pigment containing melanophores has decreased.
Whether the cells have become less numerous or simply shed their pigment is not known. A flatfish kept in a glass-bottomed tank illuminated from the bottom will gradually develop a pigmented lower surface. Light in a marine or lake environment is quite directional. It is strong from the top. Back-scattered light from below is much weaker.