How Does an Octopus Change Colour? Skin That Thinks for Itself
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An octopus settling onto a patch of seabed changes from smooth and pale to mottled brown and knobbly in about a third of a second, faster than a chameleon by a factor of a hundred, and it can do it in the dark. It manages this with skin that is, in effect, a display screen driven directly by nerves, and it manages it while being, by every test yet devised, colour-blind. How it knows what to match is one of the open questions in animal biology.
Three layers of skin
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.
Texture and pattern
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 colour-blindness problem
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
A distributed brain
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 takeaway
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.