The Skin That Sees

The fourth morning, Chiara was running late. She came in with two coffees from the corner bar. "We have not done the colour test for Polly," she said.

She put three plates on the tank floor. Bright red. Bright blue. Bright yellow. Under one of them, a small dome covered a piece of shrimp. Today, the shrimp was under the red plate.

"Here is the strange thing," Chiara said. "Octopuses are colour-blind. Their eyes have only one kind of photoreceptor. By every test we have ever done on their eyes, they cannot tell colours apart."

Polly tilted her head.

"And yet," Chiara said. "Watch."

Pasta uncurled. Two arms went out across the tank floor. They passed over the blue plate without slowing. They passed over the yellow plate without slowing. They paused over the red plate. One arm lifted the dome. Pasta took the shrimp.

"She is not blind to colour," Chiara said. "But it is not her eyes that see it."

In the last fifteen years, biologists have discovered that octopus skin is full of light-sensing cells. The same molecules that line a human retina line the surface of an octopus arm. The skin can detect different wavelengths of light. In some species, the skin can detect specific colours.

This is one answer to an old puzzle: how does a colour-blind animal camouflage itself perfectly into colourful environments? An octopus on coral matches the coral. An octopus on sand matches the sand. Its eyes cannot see colour. But its skin can. The skin sees what the skin needs to copy.

Polly walked along the rim of the tank slowly. She had never thought about her own eyes before. The idea that an animal could see with parts of itself other than its eyes was strange.

"We have published papers on this for a decade," Chiara said. "Most people still cannot believe it. The animal's skin sees."