Few things are more captivating than watching an octopus moving along the seafloor constantly changing its own appearance to match its environment, send warnings, and obscure its outline as necessary. What biological mechanisms could possibly make those brilliant colors flash so quickly? This mystery becomes even more compelling when we know that octopuses cannot see the colors themselves!
The human eye possesses two types of specialized cells in the retina: rods and cones. Rod cells are the most sensitive and are specifically adapted to operate in low-level light. The downside of this is that they cannot differentiate between different wavelengths of light—that is, they detect only “grayscale” images. This is compensated for by the cone cells, which detect only particular wavelengths, thus enabling color vision. An octopus eye, however, lacks these cone cells. But wait—then their color changing should be impossible, right? Surely, they must have some way to detect colored light! This is a question we still cannot answer definitively, though there are a few guesses. One is that the eye lens accomplishes this for them through refraction, just like a rainbow. Rainbows are formed by raindrops in the air on a sunny day, which causes the light to bend differently as it passes through, separating it into the brilliant bands we see streak across the sky. It would be as if this rainbow were cast directly onto the retina, enabling octopuses to see colors, and by extension, construct equally dazzling displays on their own skin.
Another possibility is that while their eyes are truly colorblind, perhaps the skin itself can see. This actually isn’t as crazy as it sounds; even jellyfish can detect some light (though this is not “sight” as we think of it, because they do not create an image). Some researchers have found that the chromatophores, the color changing cells, of an octopus’s skin actually do respond to some wavelengths of light due to the presence of a protein called opsin. This opsin is the same protein that detects the light in our eyes. So maybe the octopus isn’t even aware of the show it is putting on, or the beauty it possesses. Like Beethoven near the end of his life, the octopuses themselves may be the only ones unable to appreciate their own artistry. But hey, at least it keeps the eels away.