Scientists have long been fascinated by the chromatophores—tiny, circular pigment-filled structures—embedded in the skin of octopuses and other cephalopods, which allow them to change color and texture limitlessly. Previous research had shown that most other mollusks, including scallops and snails, can sense lightness or darkness through their skin (though they can't change color). Scientists wondered if the same held true for cephalopods, and whether it is tied to their camouflage ability.
Now, a new pair of studies, both published recently in the Journal of Experimental Biology, have found evidence of photosensitivity in cephalopod skin. In the first study, researchers from the University of Maryland-Baltimore found rhodopsin, a light-sensitive protein usually found in retinas, in the skin of a squid and two types of cuttlefish.
In the second, scientists from UC–Santa Barbara discovered that light causes the chromatophores in octopus skin to expand. They are most sensitive to blue light.
Scientists dubbed this behavior light-activated chromatophore expansion (or LACE). Intriguingly, the sensitivity of the LACE-related light sensors closely matches the known spectral sensitivity of opsin, a protein found in the eyes of octopuses.
Of course, octopuses and other cephalopods still have eyes through which they see in the more traditional sense, but "this is the first evidence that cephalopod dermal tissues, and specifically chromatophores, may possess the requisite combination of molecules required to respond to light," according to the Maryland scientists.
These new findings support the theory that skin-based light-sensitivity originated with an ancestral mollusk, and that this characteristic evolved over time to give cephalopods their ability to change their appearance quickly and endlessly. However, it's not clear exactly how the photosensitivity affects that ability now.