I just stumbled across this little snippet over at naturalhistorymag.com, on squid communication and their two-layered skin. The piece was pretty short so I take the liberty to quote the entire thing: "Squid Secrets As any squid knows, visual communication is a wonderful way to convey a message. It has a major downside, though; predators can tune in to the broadcast just as readily as the intended recipients (other squid) can. A recent study by Lydia M. Mäthger and Roger T. Hanlon, both biologists at the Marine Biological Laboratory in Woods Hole, Massachusetts, suggests that squid—and most likely their close relatives, cuttlefish and octopuses—have evolved a secret communication channel to which their predators are oblivious. Squid, cuttlefish, and octopuses are known for their ability to change their skin color in a spectacular way. They can blend instantly into the background or produce a startling array of patterns and hues to express their physiological or motivational state. The secret to the show is the two distinct layers of cephalopod skin: The inner layer of iridophore cells is both iridescent and reflects polarized light. The outer layer is made up of pigmented organs, or chromatophores, which expand or contract to help change the color or pattern of the skin. Cuttlefish, octopuses, and squid have a visual system to match the complexity of their skin. Unlike their vertebrate predators, they can detect differences in polarized light. Mäthger and Hanlon discovered that the two skin layers work independently, and that by taking advantage of the reflective properties of the iridophores, squid may be able to communicate with other squid via polarized light. At the same time, the squid can camouflage themselves from predators by altering the color pattern in the chromatophore layer, through which polarized light travels freely. What happens among squid stays among squid!"