Researchers from the University of California, Santa Barbara have discovered that the octopus, uniquely adept with camouflage, can "see" with its skin. The study found that the California two-spot octopus can sense light using light-sensitive proteins, similar to those found in eyes, in its skin.
The octopus changes its appearance not only to avoid predators (and to predate!) but also to communicate. These intelligent cephalopods, the largest and most mobile of the mollusks, send signals to chromatophores, pigmented organs in their skin, from their eyes. In turn their skin reacts by changing its appearance as the chromatophores expand and contract.
"Octopus skin doesn't sense light in the same amount of detail as the animal does when it uses its eyes and brain," says lead author Desmond Ramirez, a doctoral student in the Department of Ecology, Evolution and Marine Biology (EEMB). "But it can sense an increase or change in light. Its skin is not detecting contrast and edge but rather brightness."
The team exposed the octopus tissue to white light, which prompted the chromatophores to expand and change color. In the absence of the light the chromatophores relaxed, causing the skin to return to its original color. This process, which the researchers dubbed Light-Activated Chromatophore Expansion (LACE), enables a reaction without input from the eyes or brain; it also suggests that chromatophores and light sensors are connected.
The researchers also exposed the octopus skin to different wavelengths of light to record the sensitivity of the skin across the spectrum from violet to orange. The quickest chromatophore response time was to blue light. The team then used molecular experiments to determine that rhodopsin, a protein normally found in the eye, was expressed in the skin in sensory neurons at the surface.
According to co-author, Todd Oakley, an EEMB professor, this work suggests an evolutionary adaptation. "We've discovered new components of this really complex behavior of octopus camouflage," Oakley says. "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."
Octopuses are not alone among marine mollusks who can sense light with their skin, but it is not yet clear whether the skin of other animals uses the same process or tools. Octopuses can mimic color and texture of things around them, as the study describes. Cuttlefish are so adept with this kind of camouflage that they've been known to appear to have the black and white squares of a checkerboard in response to having one in their habitat. And squid can mimic the shine of the water that surrounds them with their skin.
If the process and cells are the same from mollusk to mollusk, Ramirez hopes to elucidate the relationships involved. "Do they all come from the same ancestral source or did they evolve multiple times?" he asked. "What kind of behaviors do the different groups share and what kind of behaviors does the skin sensing light underlie?"
The team hopes that their ongoing work will answer these and other questions.