Octopi may already seem alien enough with their strange combination of big brains and boneless bodies. Now a new trait is getting added to the octopus's long list of bizarre features: 'seeing' skin.

In addition to sensing light through their two highly sophisticated eyeballs, octopi can also sense light through their skin, according to a study published today in the Journal of Experimental Biology. Researchers found that the organs in that lend octopus skin its color-changing capability also contain a molecule that is key for vision, allowing the skin to change color in response to light exposure.

"We found that octopus skin is intrinsically light sensitive," study co-author Todd Oakley of the University of California, Santa Barbara, said in an interview. "That hadn't been seen before."

Oakley and his colleagues determined this by isolating the skin of a two-spot octopus (Octopus bimaculoides) from its brain, this way they could be sure that the brain was not mediating the skin's responses. They then shined light on the skin and watched what happened:

Even though the skin was not receiving any information from the brain, it responded by expanding its chromatophores, tiny organs which are attached to muscles within the skin. By expanding or contracting its chromatophores, an octopus can change its skin color.

"There are thousands of these chromatophore organs covering the skin of cephalopods," says Oakley. "You can think of them as acting like pixels on a computer monitor - their colors can be changed individually, so the animal can change the size of these thousands of 'pixels' for purposes such as blending into their surroundings or communicating with other cephalopods."

The key to octopus skin's superpower of sight is a type of protein known as opsin, according to Oakley.

"It's the same protein that's used in the eye," he says.

Opsin changes shape when light hits it, and this shape-shifting triggers a cascade of signals that cause the muscles connected to the chromatophores to either expand or contract.

"That expansion or relaxation causes changes to be made in the color of the skin," Oakley says

Further research is needed to make an explicit link between opsins and this behavior in the skin, Oakley cautions, but that skin is able to quickly react to light without help from the brain is a remarkable finding in itself.

The study can be found here.

UCSB Researchers Study Octopus Camouflage from UC Santa Barbara on Vimeo.

Photo: Brian Gratwicke | Flickr