An octopus can change the color of its skin to blend in with its surroundings, and that simple fact hides a paradox. 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, because octopus eyes have only one type of photoreceptor, which means they can detect differences in brightness but not color in the usual way. One proposal 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. That pupil-and-lens explanation is consistent with the data from cephalopod studies and offers a possible solution to the apparent paradox of vivid color behavior in color-blind animals.

To see what the pupil would have to explain, look at the skin itself. 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. Researchers have tracked the states of tens of thousands of chromatophores in camouflaging cephalopods at sixty frames per second, at single-cell resolution, and over weeks.

What do the cells have to produce? 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. In separate work, researchers found that size, contrast and edges of background objects are key visual cues that guide cephalopod camouflage patterning. Octopuses have no hard shell or outside armor, so they had to evolve ways of not being seen, and they can change color faster than almost any other animal.

Sleep offers another window onto the system. Octopuses are marine invertebrates that diverged evolutionarily from vertebrates roughly 550 million years ago. In octopuses, electrical recordings from the central brain during active sleep resemble the brain activity of waking. Computer analysis of sleeping octopuses found skin patterns that strongly resemble the patterns seen while they are awake. 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. The similarities between octopus and vertebrate sleep indicate that aspects of two-stage sleep may be convergent features of complex cognition.

A second line of evidence moves the question out of the eye and into the skin. In pieces of skin cut from Octopus bimaculoides, light made the chromatophores expand, a behavior the researchers named light-activated chromatophore expansion, or LACE. The expansion happened most quickly in response to blue light. Octopus skin is intrinsically light sensitive, and this spread-out sense of light might contribute to their patterning abilities. The researchers suggest that a molecular mechanism for detecting light in eyes may have been co-opted for light sensing in octopus skin. Even so, how octopuses match their skin color to their surroundings is still an unsolved mystery.

Words to know

• paradox: apparent contradiction
• photoreceptor: light-sensing cell
• chromatophores: color cells
• convergent: separately arising
• invertebrates: no-backbone animals