New research confirms earlier speculation that certain species of octopus can see with their skin.
The sensory perception isn't as keen as actual eyesight, but the study–published in the Journal of Experimental Biology–presents the first evidence that octopus skin can detect light using the same light-sensitive proteins (opsins) found in eyes. The octopus can even do this without input from its central nervous system.
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The discovery proves yet again how remarkable these cephalopods are, as octopi are already known as being some of the world's most intelligent and mobile marine creatures.
"Octopus skin doesn't sense light in the same amount of detail as the animal does when it uses its eyes and brain," lead author Desmond Ramirez of the University of California at Santa Barbara, said in a press release. "But it can sense an increase or change in light. Its skin is not detecting contrast and edge, but rather brightness."
During one part of the experiment, Ramirez shined white light on a sample of skin from a California two-spot octopus. This caused chromatophores, or pigmented organs in the skin, to expand and change color. Without light, the chromatophores relaxed and the skin returned to its original hue.
Ramirez explained that this reaction suggests light sensors are connected to the chromatophores, permitting an appropriate response without input from the brain or eyes.
The researchers further discovered a compound called rhodopsin, usually produced in the eye, within the octopus skin. It likely helps the skin to detect different wavelengths of light, from violet to orange. For some reason, blue light really gets a strong response, perhaps because the marine environment of the octopus is often blue-hued.
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Co-author Todd Oakley said, "We've discovered new components of this really complex behavior of octopus camouflage. 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."
The skin of other animals, including humans, has some light-detecting abilities too. Prior research, for example, determined that human skin can "see" ultraviolet light. This obviously doesn't promote immediate camouflage, as for the octopus, but it does help to trigger tanning, which is a protective response meant to prevent skin damage.
Photo: Baby two-spot octopus at the Santa Monica Pier Aquarium. Credit: Jeremy Selan, Wikimeda Commons