An octopus can change the color of its skin to blend in with its surroundings. Octopuses are cephalopods, a word that means "head-foot" because the arms are attached to the head. Their skin has special pigment cells called chromatophores that let them 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.

Studying a skin with so many cells takes patient work. Researchers have followed individual chromatophores in camouflaging cephalopods, tracking the states of tens of thousands of them at sixty frames per second and over weeks. Other work shows that size, contrast and edges of background objects are key visual cues that guide cephalopod camouflage patterning. Cephalopods use three main types of camouflage pattern: uniform and mottle patterns that match the background, and disruptive patterns. Octopuses have no hard shell or outside armor, so they had to evolve ways of not being seen.

Most octopuses are technically color-blind, yet they can still match the colors around them. You may have heard that octopuses copy the colors they see. One experiment suggests that the answer may lie elsewhere. In pieces of skin cut from Octopus bimaculoides, light made the chromatophores expand, a behavior the researchers named light-activated chromatophore expansion, or LACE. Octopus skin is intrinsically light sensitive, and this spread-out sense of light might contribute to their patterning abilities.

The eyes remain a puzzle of their own. An octopus eye is built much like a human eye, but octopus eyes have only one type of photoreceptor, which means they can detect differences in brightness but not color in the usual way. One idea is that animals with a single photoreceptor type could tell colors apart if they combine an off-axis pupil with chromatic aberration, the way a lens focuses different colors at different distances. How octopuses match their skin color to their surroundings is still an unsolved mystery.

Words to know

• chromatophores: color cells
• cephalopods: head-foot animals
• photoreceptor: light-sensing cell
• intrinsically: by nature
• aberration: focus error