Many octopuses can change the color and pattern of their skin in a fraction of a second, vanishing against a rock or a patch of sand. This ability rests on thousands of tiny organs called chromatophores, each a sac of pigment ringed by muscles. When the muscles contract, the sac stretches and its color spreads across the skin; when they relax, the color shrinks to a point. Nerves run directly from the brain to these muscles, which is why the changes can be so fast.
What makes the feat more remarkable is that most octopuses are, by the usual tests, colorblind. Their eyes contain only one type of light-sensitive pigment, so in principle they should not be able to distinguish the hues they so precisely imitate. How they match a background they apparently cannot see in color remains an open puzzle.
One proposed answer is that octopus skin itself responds to light. Researchers have found the same light-sensitive pigment present in the eye scattered across the skin, raising the possibility that the body senses brightness and perhaps color without relying on the eyes alone. Whether this skin sensing actually guides camouflage, or merely accompanies it, has not been established, and the matching ability is still far from fully explained.
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