How Does an Octopus Change Colour?

Step 1 of 5

Its skin is full of colour bags.

Just under the surface sit millions of tiny sacs, each holding one colour. A ring of muscle around each sac pulls it wide open to show the colour, or lets it shrink to a dot to hide it.

Step 2 of 5

Muscle is faster than growing.

A chameleon takes seconds to change. An octopus does it in a blink, because nothing has to grow or move — a muscle just pulls, the same way you make a fist.

Step 3 of 5

It changes its shape too.

Underneath the colour bags are muscles that can pull the skin up into spikes and lumps. So it does not just match a rock's colour — it matches the rock's texture.

Step 4 of 5

It has a brain in every arm.

Most of an octopus's nerve cells are not in its head. Each arm has its own bundle and can taste, feel and grip on its own, so the eight arms partly work things out for themselves.

Step 5 of 5

Only its beak is hard.

There is not a single bone in an octopus. The only hard part is a beak like a parrot's, so it can squeeze through any gap that the beak fits through — which can be smaller than a coin.

An octopus is almost certainly colour-blind. Nobody is quite sure how it matches a colour it cannot see — one idea is that its skin senses light directly.