The octopus¹ has a unique ability. It can change the color, pattern and even texture² of its skin not only for purposes of camouflage³ but also as a means of communication. The most intelligent, most mobile and largest of all mollusks, these cephalopods use their almost humanlike eyes to send signals to pigmented organs in their skin called chromatophores, which expand and contract to alter their appearance. A new study by UCSB scientists has found that the skin of the California two-spot octopus (Octopus bimaculoides) can sense light even without input⁴ from the central nervous system. The animal does so by using the same family of light-sensitive proteins called opsins found in its eyes -- a process not previously⁵ described for cephalopods. The researchers' findings appear in the Journal of Experimental Biology.

 

"Octopus skin doesn't sense light in the same amount of detail as the animal does when it uses its eyes and brain," said lead author Desmond Ramirez, a doctoral student in the Department of Ecology, Evolution and Marine⁶ Biology (EEMB). "But it can sense an increase or change in light. Its skin is not detecting contrast and edge but rather brightness."

 

As part of the experiment, Ramirez shone white light on the tissue, which caused the chromatophores to expand and change color. When the light was turned off, the chromatophores relaxed and the skin returned to its original hue⁷. This process, Ramirez noted⁸, suggests that light sensors⁹ are connected to the chromatophores and that this enables a response without input from the brain or eyes. He and his co-author, Todd Oakley, an EEMB professor, dubbed¹⁰ the process Light-Activated Chromatophore Expansion (LACE).

 

In order to record the skin's sensitivity across the spectrum¹¹, Ramirez exposed octopus skin to different wavelengths¹² of light from violet to orange and found that chromatophore response time was quickest under blue light. Molecular¹³ experiments to determine which proteins were expressed in the skin followed. Ramirez found rhodopsin -- usually produced in the eye -- in the sensory¹⁴ neurons on the tissue's surface.

 

According to Oakley, this new research suggests an evolutionary¹⁵ adaptation. "We've discovered new components¹⁶ of this really complex behavior of octopus camouflage," said Oakley, who calls cephalopods the rock stars of the invertebrate¹⁷ world.

 

"It looks like the existing cellular¹⁸ mechanism¹⁹ 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," he explained. "So instead of completely inventing new things, LACE puts parts together in new ways and combinations."

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