‘Color-Blind’ Cephalopods May See Color After All

Octopuses and their kin are some of the most mysterious creatures on Earth. Now researchers say at least one mystery surrounding them has been solved: how it is that these animals can change color so effectively when they can't even see color. Their secret? They can. The report was published in the Proceedings of the National Academy of Sciences.   

Cephalopods (octopuses, squid, cuttlefish, and nautiluses) are so different from other animals that they may as well be from another planet. They’re brilliant invertebrates, which researchers used to believe was a contradiction in terms. For mostly shell-less mollusks, they’re surprisingly well protected. And while it seems they lack the optical equipment required to see colors, they nevertheless execute magnificent color changes for camouflage and communication. These contradictions, especially the last one, have stumped scientists for decades.

Study author Alexander Stubbs, a graduate student at the University of California, Berkeley, is one of those scientists. Stubbs first learned of the color-change/color-blind paradox in high school and never forgot it, although he eventually focused his studies on vertebrate animals instead. One day, he was trying to take pictures of lizard skin patterns that are only visible under ultraviolet light, and he noticed that the camera was creating a blurry, rainbow-rimmed haze around the lizard.

We’ve all seen that same haze, called chromatic aberration, after getting our pupils dilated by an eye doctor. If your eye were a camera, your pupil would be the aperture, opening or contracting to allow the entrance of more or less light, and your lens would be, well, the lens, taking in the white light and splitting it into its component colors. A smaller pupil keeps colors concentrated and close together, leading to a sharper image. A dilated pupil allows them to scatter, creating that chromatic aberration that can make it hard to read or tell one face from another.

Stubbs wondered how the phenomenon of chromatic aberration might play out in cephalopods, whose eyes are so different from our own. To find out, he teamed up with his father, Christopher Stubbs, an astrophysicist at Cambridge University.

Unlike our round apertures, the pupils of octopuses, cuttlefish, and squid are shaped like the letters U and W, or like a dumbbell. The father and son duo designed a computer model that would allow them to examine and pick apart the interplay of light and color in cephalopods’ strange-shaped eyes.

Image credit: Roy Caldwell, Klaus Stiefel, and Alexander Stubbs 

What they found there was suitably strange: The chromatic aberration that makes it so hard for us to see clearly may actually help cephalopods discern color. Their weird, narrow pupils could help focus certain colors into their retinas, thus circumventing any need for our color-vision equipment.

"We propose that these creatures might exploit a ubiquitous source of image degradation in animal eyes, turning a bug into a feature," the younger Stubbs said in a press statement. "While most organisms evolve ways to minimize this effect, the U-shaped pupils of octopus and their squid and cuttlefish relatives actually maximize this imperfection in their visual system while minimizing other sources of image error, blurring their view of the world but in a color-dependent way and opening the possibility for them to obtain color information."

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