An octopus can 'see' using its skin

A blindfolded octopus isn't blind.

That's because octopuses can detect changes in brightness with an organ that's not typically linked to a sense of sight — their skin, U.S. researchers have found.

"Octopus skin can sense light by itself. It doesn't need the eyes to be able to sense light," said Desmond Ramirez, lead author of a new study describing the phenomenon, in a video statement.

The study was published in the Journal of Experimental Biology.

• Summary of the paper

Ramirez, a PhD student, and co-author Todd Oakley, a professor in the Department of Ecology Evolution and Marine Biology at the University of California Santa Barbara, don't think that the skin of an octopus can detect details like contrast and the edges of objects the way eyes can.

But their ability to detect light may help explain how octopuses are so good at changing the colour of their skin to camouflage with rocks, sand or other surroundings.

No brain required

The researchers didn't test living octopuses, just samples of their skin. When they exposed the skin samples to light, pigmented spots on the skin called chromatophores expanded, making the skin look darker. The response happened without any help from the eyes or brain.

Further research suggested how the chromatophores could respond to light — they contain light-sensitive proteins called opsins. Opsins are the same light-sensitive proteins found in the octopus's eyes and used in regular vision.

"It looks like the existing cellular mechanism for light detection in octopus eyes, which has been around for quite some time, has been co-opted for light sensing in the animal's skin," Oakley said in a statement.

While the researchers think the skin's light-sensing abilities may help an octopus camouflage with its environment, they don't yet know how exactly how those abilities are used in a living animal, Oakley told CBC News in an email.

"So we don't know how it is used in camouflage."

Octopuses belong to a scientific group called mollusks that include animals such as clams, snails and chitons. Many of those other kinds of mollusks were already known to sense light with their skin and respond by doing things like moving toward or away from it, but not by changing their colour.