How can an octopus change a patch of skin so quickly?

An octopus settles beside a rock, and a patch of its pale skin becomes dark. No new coat has grown, and no paint has flowed across its body. Tiny structures already in the skin have changed how much colored material they show.

One of these structures is a chromatophore. At its center is an elastic sac filled with pigment, the material that gives it color. Small muscles reach outward from the sac like spokes from the center of a wheel. Nerves carry signals that control those muscles.

When the muscles contract, they shorten and pull the sac's edges outward. The sac spreads into a wider, flatter patch, so more of that color is visible from outside. When the muscles relax, the elastic sac becomes smaller again. The important change is the area covered by existing pigment, rather than the sudden creation of more pigment.

A single sac makes only a tiny patch. Many chromatophores changing together can produce larger spots, stripes or areas of shading. Different patterns can break up an animal's outline against its surroundings or make it stand out. Visual information about edges, brightness and contrast helps guide the nervous system's control of these patterns.

Pigment sacs are only part of the skin's toolkit. Other structures reflect light, and changes in skin texture can help an octopus resemble a rough surface. The speed comes from adjusting structures that are already there. A moving muscle can change the visible area of a small colored sac, and thousands of those local changes can transform the appearance of the whole animal.