Which sea creature has eight arms and can change colour?

An octopus changes colour using millions of pigment sacs in its skin called chromatophores, each opened and closed by tiny muscles under direct nervous control. That is why the change is instant rather than gradual, and why an octopus can run a wave of colour across its body or hold two different patterns on its two sides at once.

The complication is that octopuses appear to be colourblind - they have a single visual pigment, so they should not be able to distinguish the colours they are so precisely matching. One proposal is that they see colour with their skin rather than their eyes: the light-sensitive proteins found in the retina also turn up all over the body surface. Another is that their oddly shaped pupils split light into its colours in a way that lets them judge hue by focus. Neither is settled.

The texture changes too. Papillae in the skin can raise into spikes and ridges to match rock or weed, and the mimic octopus of Indonesia goes further, apparently impersonating the shape and swimming style of sea snakes, lionfish and flatfish depending on what is threatening it.