Octopuses have long been hailed as masters of camouflage, able to change the color, pattern and texture of their skin. Now, it seems that these cephalopods not only use their skin as a means of disguise and even communication, but also as a way to "see" light.
A team of researchers at the University of California, Santa Barbara (UCSB) found that octopus skin possesses the same cellular mechanism for detecting light as its eyes do, relying on light-sensitive proteins called opsins. This way, octopuses are able to sense light without input from the central nervous system. At least, that's what researchers concluded when observing the California two-spot octopus (Octopus bimaculoides)
"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 said in a statement. "But it can sense an increase or change in light. Its skin is not detecting contrast and edge but rather brightness."
During the study, Ramirez and colleagues shone white light on octopus skin tissue, which caused pigmented organs called chromatophores to expand and change color to alter their appearance. When the light was turned off, the chromatophores relaxed and the skin returned to its original hue.
According to the UCSB team, this process suggests that light sensors are connected to the chromatophores and enable a response without input from the brain or eyes - a process referred to as Light-Activated Chromatophore Expansion (LACE).
In order to record the skin's sensitivity across the entire color spectrum, researchers exposed octopus skin to different wavelengths of light - from violet to orange - and found that chromatophore response time was quickest under blue light. They then followed with molecular experiments to determine which proteins were expressed in the skin. Amazingly, it turns out octopus skin contains the protein rhodopsin, which is usually produced in the eye.
"We've discovered new components of this really complex behavior of octopus camouflage," said co-author Todd Oakley. "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 and used for LACE."
"So instead of completely inventing new things, LACE puts parts together in new ways and combinations," he added.
Octopuses aren't the only marine mollusks whose skin can sense light; however, scientists have yet to determine whether the skin of those other animals contains the light-sensitive opsins also. Ramirez and Oakley plan to conduct further research to find out.
The latest findings are described in more detail in the Journal of Experimental Biology.
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