New research reported in the Journal of Experimental Biology demonstrates that the skin of the California two-spot octopus (Octopus bimaculoides) possesses the same cellular mechanism for detecting light as its eyes do.
The California two-spot octopus (Octopus bimaculoides). Image credit: Jeremy Selan.
The California two-spot octopus does so by using the same family of light-sensitive proteins called opsins found in its eyes – a process not previously described for cephalopods.
“Octopus skin doesn’t sense light in the same amount of detail as the animal does when it uses its eyes and brain. But it can sense an increase or change in light. Its skin is not detecting contrast and edge but rather brightness,” explained lead author Desmond Ramirez, a doctoral student at the University of California Santa Barbara.
As part of their experiments, Ramirez and his colleague, Prof Todd Oakley, shone white light on the California two-spot octopus’ 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 hue.
“This process, dubbed the process Light-Activated Chromatophore Expansion (LACE), suggests that light sensors are connected to the chromatophores and that this enables a response without input from the brain or eyes,” Ramirez said.
In order to record the skin’s sensitivity across the spectrum, the team exposed octopus skin to different wavelengths of light from violet to orange and found that chromatophore response time was quickest under blue light.
“By creating an action spectrum for the latency to LACE, we found that LACE occurred most quickly in response to blue light.”
“We fit our action spectrum data to a standard opsin curve template and estimated the λmax of LACE to be 480 nm.”
“Consistent with our hypothesis, the maximum sensitivity of the light sensors underlying LACE closely matches the known spectral sensitivity of opsin from octopus eyes.”
The scientists also conducted molecular experiments to determine which proteins were expressed in the octopus skin.
They found rhodopsin – usually produced in the eye – in the sensory neurons on the tissue’s surface.
“This study suggests an evolutionary adaptation. We’ve discovered new components of this really complex behavior of octopus camouflage,” Prof Oakley said.
“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.”
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M. Desmond Ramirez & Todd H. Oakley. 2015. Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides. J. Exp. Biol. 218, pp. 1513-1520; doi: 10.1242/jeb.110908
