How Does an Octopus Change Colour? Skin That Thinks for Itself

The colour comes from three kinds of structure stacked in the skin. On top are the chromatophores: tiny elastic sacs of pigment, yellow, red, brown or black, each surrounded by a ring of muscle. When the muscles contract the sac is stretched flat into a disc of colour up to fifteen times its resting size; when they relax it shrinks to a dot and the colour vanishes. An octopus has around two million of them, and each is wired to the brain by its own nerve, so patterns can be painted onto the skin at the speed of a nerve impulse.

Below the chromatophores are the iridophores, layers of reflecting plates that produce iridescent blues and greens by interference, the way a soap bubble does, and below those the leucophores, which scatter all wavelengths and reflect back whatever colour of light is falling on them. An octopus in blue water looks blue at that layer, and in green light green, without doing anything. The pigment sacs above then paint the pattern on top.

Colour is only half of it. The skin also carries papillae, small bumps of muscle that can be raised into spikes, ridges or a rough rind in a second and smoothed away as fast, so that the animal's outline changes along with its colour. Cuttlefish and octopuses use a limited repertoire of patterns rather than reproducing what they see: a uniform tone, a mottle of light and dark blotches, and a disruptive pattern of large contrasting patches that breaks up the body's shape. Which one they choose depends on the size of the objects around them, and researchers have driven the choice by placing cuttlefish on chequerboards of different scales.

The same skin is used for signalling. A male octopus courting turns one colour and warns a rival with another, and a threatened one can flash dark and pale in waves. The mimic octopus of Indonesia goes further and impersonates lionfish, sea snakes and flatfish by combining colour, posture and movement.

The eye of an octopus contains one kind of light-sensitive pigment, where a human has three, and behavioural tests confirm that it cannot tell colours apart. Yet it matches coloured backgrounds well enough to fool predators with excellent colour vision. Several explanations are being tested:

• •The skin can see: octopus skin contains the same light-sensing proteins as the eye, and isolated patches of it respond to light by expanding chromatophores without any signal from the brain
• •The leucophores do the work: since they reflect the ambient light, they match the background's colour automatically and the animal only has to manage brightness and pattern
• •The pupil is a prism: the octopus's odd U-shaped pupil splits light by wavelength as a prism does, and by focusing at different depths the eye might read colour from blur
• •Colour matters less than it seems: most predators view the seabed from a distance where pattern, contrast and texture dominate and small colour errors go unnoticed

Two thirds of an octopus's half a billion neurons are not in its head but in its arms, and the skin's responses are partly managed locally. A severed arm will still change colour and grip. The animal's central brain sets the overall pattern and the arms and skin fill in the detail, an arrangement with no parallel among vertebrates and one reason the octopus is studied by engineers designing soft robots and adaptive materials. Sheets of artificial skin that change colour and texture under electrical control now exist, modelled directly on the chromatophore.

All of this is packed into an animal that lives one or two years, learns by observation, opens jars, and has a lineage that split from ours more than 500 million years ago. Its intelligence evolved entirely separately from a vertebrate's, and its skin is the most visible result.

The chameleon is the animal most people think of when they hear about colour change, but it works by a different mechanism. Much of a chameleon's brighter colour comes from a layer of skin cells packed with tiny crystals arranged in a regular lattice. When the animal relaxes or excites these cells, the spacing between the crystals changes, and that shifts which wavelengths of light are reflected, so the skin moves from green towards yellow or orange. It is closer to how iridophores work in an octopus than to how its pigment sacs work, and it is slower, taking seconds to minutes rather than a fraction of a second.

There is also a common misconception about why chameleons do it. They are often said to match whatever they are sitting on, but most of their dramatic changes are social signals, used when males compete or when a female rejects a suitor, and some are about temperature, since darker skin absorbs more heat on a cool morning. Their resting greens and browns already blend with leaves and branches. The octopus is the genuine specialist in background matching, which makes its colour blindness all the more puzzling.

Octopuses sleep, and their skin shows it. In ordinary quiet sleep an octopus turns pale and still, its pupils narrow and it is slow to react to a touch. Every so often, roughly once an hour in one well-studied species, this is interrupted by a brief burst of what researchers call active sleep, lasting about a minute, in which the skin flashes through dark and light patterns, the eyes and arms twitch and the breathing changes. Studies published in the 2020s have described this two-stage pattern in more than one species, and it has drawn comparisons with the alternation of dreaming and non-dreaming sleep in mammals and birds.

When researchers recorded the skin patterns of active sleep, many resembled the camouflage and display patterns the same animal produces when awake, and brain recordings during these episodes resembled waking activity too. Octopuses kept from entering active sleep made up for it later with more of it, which is one of the standard tests of whether a state is genuine sleep rather than mere rest. One interpretation is that the brain is replaying and consolidating waking experience, as sleeping mammals appear to do. Whether the octopus experiences anything like a dream cannot be tested, but its skin offers an unusually direct view of what its brain is doing.

Octopuses are famous for slipping out of tanks, and one escape became world news. In 2016 staff at the National Aquarium of New Zealand in Napier found that an octopus named Inky had disappeared. A gap had been left at the top of his enclosure, and wet tracks suggested he had climbed out, crossed the floor and squeezed into a drainpipe leading to the sea.

Because an octopus has no bones, it can fit through any opening larger than its hard beak, and its arms can sense and grip surfaces independently, which suits exploring. Aquarium keepers often describe octopuses as curious, quick to learn and easily bored, so many give them puzzles and toys to keep them occupied, which is good for the animals and harder for the escape artists.

An octopus changes colour with millions of nerve-driven pigment sacs that expand and contract in a fraction of a second, over reflecting layers that take on the ambient light, and it changes texture with muscular bumps in its skin. It does this while colour-blind, probably by a mix of light-sensing skin, automatic reflection and patterns that fool predators at a distance, and much of the control sits in its arms rather than its head.