Squid can change colour in an instant by stretching pigment cells in their skin.

The longer version

Pigment sacs sit in the skin, each ringed by tiny muscles under direct nervous control. Relaxing a sac hides the colour within a speck; contracting it stretches the sac wide and floods the surface with that hue. Beneath them, reflective cells bounce back blue and green light, and some skin layers shift hue with viewing angle.

Because nerves rather than slow hormones drive everything, the whole appearance changes within a heartbeat. Such speed suits signalling to a mate, startling a predator, or erasing the body outline. Skin also senses light, helping the animal tune its colour to the background.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.