A cuttlefish's skin responds to light without the eyes

Chromatophores in cuttlefish skin expand when the skin itself is lit. The response does not require the eyes, or a signal from the brain. The skin has light-sensitive pigments of its own.

Cuttlefish change colour with organs called chromatophores, small sacs of pigment pulled open by muscle. The usual command comes from the eyes and the brain, and it is astonishingly fast. Desmond Ramirez and Todd Oakley found a second path. Shine light on the skin and the chromatophores open even when the eyes are not in the circuit. The skin expresses opsins, the same family of proteins a retina uses, and the response tracks the light falling on that patch. A shadow, a bright spot, a change the eyes might have missed or not yet answered, can be met locally.

This is not a picture. A patch of skin does not see a fish and decide to be a fish. It detects brightness, and it answers with the organ it already has. The distributed sensing is the surprise. An animal whose whole trick is appearance has built light detection into the appearance itself, so the surface being looked at is, in a limited way, looking back at the light. How much this local reflex matters in the wild, next to the eye-driven pattern, is still being weighed. The reflex is not in doubt.