An octopus can change the color of its skin to blend in with its surroundings, and the machinery behind that trick is built from muscle and nerve. An octopus's skin has special pigment cells called chromatophores that let it change color. An octopus can have from tens of thousands to millions of chromatophores, each controlled by signals from the brain. Each chromatophore can be expanded by radial muscles, and those muscles are controlled by motor neurons. Tiny muscles around each color sac squeeze it open or let it close, so the color shows more or less.

Not all of the skin's color cells hold ordinary pigment. Iridophores are skin cells that make shiny, rainbow-like colors. They contain a protein called reflectin that stacks up inside the cells and works like a mirror. Leucophores are skin cells with white pigment that scatter light and help control the contrast and brightness of colors. Blue-ringed octopuses use their chromatophores and iridophores to make the iridescent blue rings appear when they are alarmed. The rings are an example of aposematism, bright colors used as a warning. Octopuses can change color faster than almost any other animal.

Octopuses are cephalopods, a word that means "head-foot" because the arms are attached to the head. Cephalopods use three main types of camouflage pattern: uniform and mottle patterns that match the background, and disruptive patterns. The skin is built in layers, because an octopus's skin has three layers of chromatophores, and the pigment in each layer reflects a different color. The skin can also change in texture, because an octopus can raise bumps called papillae, which are a muscular hydrostat, a structure made of muscle with no skeleton to support it, like the human tongue.

Even in sleep, the skin keeps showing patterns. In 2019 scientists filmed an octopus changing colors in her sleep, and some researchers wonder whether octopuses dream. A 2023 study found that sleeping octopuses have a quiet sleep stage broken up by bouts of about 60 seconds of body movement and rapid changes in skin patterning and texture. Computer analysis of sleeping octopuses found skin patterns that strongly resemble the patterns seen while they are awake. The recordings show that the skin keeps changing color and texture even when the octopus is asleep.

One big question remains. Octopuses have no hard shell or outside armor, so they had to evolve ways of not being seen. An octopus eye is built much like a human eye, but it has no blind spot because its optic nerve passes behind the retina. Octopus eyes have only one type of photoreceptor, which means they can detect differences in brightness but not color in the usual way. You may have heard that octopuses change color because they see the colors around them and copy them. In fact, most octopuses are technically color-blind, yet they can still match the colors around them. How octopuses match their skin color to their surroundings is still an unsolved mystery.

Words to know

• motor: movement-driving
• neurons: nerve cells
• hydrostat: muscle-only structure
• papillae: skin bumps
• cephalopods: head-foot animals