The Paper: Ramirez, M. D., & Oakley, T. H. (2015). Eye-independent , light-activated chromatophore expansion ( LACE ) and expression of phototransduction genes in the skin of Octopus bimaculoides. The Journal of Experimental Biology, 2015(218), 1513–1520. doi:10.1242/jeb.110908
Introduction: Octopuses are true masters of disguise. By changing the coloration, patterns, and even texture of their skin, octopuses are able to camouflage with their surrounding environment. They are so good at these optical illusions that within seconds, octopuses can entirely disappear as they transform into rocks, algae, or other features around them (Fig 1). To see just how incredible octopus camouflage is, watch this short video.
So how exactly are octopuses able to disguise themselves so masterfully? Structures in their skin, called chromatophores, are largely responsible for changes in pigmentation and patterning. Chromatophores are pigment containing, light reflecting cells controlled by muscles. Muscles contract or relax causing chromatophores to alter the pigment expression (Fig 2).
While we know that the chromatophores allow octopuses to change their skin color, it is still a mystery as to how the octopuses determine what background they need to blend in to, especially in such a short time frame. Experiments have shown that octopuses rely upon their well-developed, camera-like eyes to mediate their camouflage, matching their bodies to the surrounding environment based on features such as light intensity and contrast of object edges (interestingly, octopuses are color blind!). So they are likely not utilizing color information to guide their pigmentation patterns. However, when pieces of octopus skin are removed from the animal (so they no longer have contact with the eyes or the brain), chromatophores still respond when exposed to light! How is the skin independently responding to light cues when the brain and eyes are not telling it how to respond?! In an attempt to answer this question, researchers tried to unveil the mechanism behind the octopus’ disguises.
The Methods: Researchers at the University of California Santa Barbara studied how skin reacted to light in the California two-spot octopus (Octopus bimaculoides; Fig. 3). Researchers sequenced the genes present in the skin and used antibody staining to reveal what genes could be controlling the light-activated changes to the skin. Additionally, pieces of skin were dissected from octopuses, severing any nervous connection. The skin was then placed under different wavelengths to see which wavelengths elicited the greatest light-activated change to skin pigmentation.
What allows skin to respond to light? The researchers found a pretty amazing explanation for how the skin is able to respond to light without visual sensory input: the skin itself is able to act like an eye! No, the octopus is not some monster from the black lagoon covered in a million tiny creepy eyes- so no need to freak out. Turns out that the same genes present in the eyes responsible for light detection (called opsin genes, specifically r-opsin in this case) are also found expressed in the skin (Fig. 4). Furthermore, other genes necessary for vision in the eye are expressed in the skin (including some genes called G-protein α(q) and phospholipase C). The genes in the skin are, in fact, nearly identical to those in the eye. And the skin samples the scientist analyzed were most sensitive to the same wavelengths of light that the well-developed octopus eyes are specialized to see. Okay, this is pretty cool… you can freak out now.
Conclusion: Other mollusks have been shown to have skin that is reactive to light. For example, clams and other bivalves are able to sense and move towards light. Cephalopods, however, have taken skin light-sensing to a new level, using this trait to entirely shift their appearance in the blink of an eye. Although, not a fully formed eye, the skin is utilizing essentially the same mechanisms that underlie light sensing in the eyes themselves. This light-sensing ability allows the octopus to create optical illusions, disappear into its surroundings, and become invisible to predators- talk about one crazy defense mechanism!
Do you think that this finding could be applied to new technology? Tell us how in the comment section!