The Two-Spot Octopus Can See Without Eyes

Two-spot octopus (Octopus bimaculoides). Nathan Rupert/flickr CC BY-NC-ND 2.0
Josh Davis 20 May 2015, 17:21

They are the masters of disguise. Not only can octopuses match color almost perfectly, they can also match texture, pattern and brightness of the object they try to mimic. We already know that their eyes display striking similarities to our own, but it turns out that the light-sensing abilities of these cephalopods doesn't end there. 

Researchers from the University of California, Santa Barbara, have quite astonishingly found that the skin of the two-spot octopus (Octopus bimaculoides) can also detect light. As if that wasn’t impressive enough, the scientists were then able to demonstrate that the light sensing didn't require the central nervous system (CNS). The new study will be published in the Journal of Experimental Biology.

When the team shone light on the skin of the cephalopod, their pigment containing cells, known as chromatophores, expanded, giving the tissue color. “Even though chromatophores often change quickly, I was surprised at how quickly and dramatically the chromatophores respond to light with no input from the brain,” Todd Oakley, one of the authors told IFLScience. 

They found that the skin produced the light sensing protein rhodopsin, usually found in the eye, which can sense light and dark. After exposing the skin to various light conditions, they found that different colors of light made the skin respond in different ways. For example, there was no change when exposed to red light, but a dramatic quick color change under blue.      
   
The gif. below shows the color changing skin in action. It is an isolated skin prep from the two-spot octopus viewed under a microscope. At first it is under red light, and there is no response. Then the white light is turned on and the chromatophores expand in waves, giving it color, first yellow and then getting progressively darker. They suspect that rather than the rhodopsin being linked to the CNS as in the eye, it is likely connected to the chromatophores.   

Gif showing the octopus skin changing color in response to light. Credit: Todd Oakley/UCSB

It’s been suspected that octopus skin could detect light for a while, explained Oakley. Genes that code for light sensitive proteins, known as opsins, have been found to be expressed in the skin of cuttlefish, and there has been some anecdotal evidence in older literature of octopus skin sensing light. But his study has been able to prove conclusively for the first time the skin's response to light, and that it happens independently of the eyes and the CNS.

Why they can do this though, still remains a mystery. “We do not know whether or not there is a beneficial function,” explained Oakley. They hope to continue to further explore and unravel the secrets of exactly how the octopus manages these changes, and whether other mollusks might also share this incredible skill.

Main image credit: Nathan Rupert/flickr CC BY-NC-ND 2.0

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