This is kind of awesome: cephalopods only have one kind of photoreceptor, so it was thought that they would be only able to see the world in shades of gray. Those amazingly clever camouflage tricks they pull? That was just matching intensities and textures, fooling our eyes. But now someone has figured out a way they could see color, and special bonus, it also explains those funky weird pupil shapes, like you see in the cuttlefish eye to the right.
They use chromatic aberration! We think of chromatic aberration as an imaging problem — it’s caused by the fact that the degree refraction of light is partly dependent on wavelength, so the blue light in an image focuses closer to the lens than the red light. When you focus optimally on the green wavelengths, for instance, that means that the red and blue colors form an out of focus, blurry image on top of the sharp greens, producing a pattern of color fringes around objects. They jump out clearly to me when I use the cheap student microscopes here, and are why I spent a lot of extra money getting planapochromat lenses for my microscope. They have lots of corrective glass to tweak the different wavelengths into the same focal plane.
But where I see an annoyance, cephalopods evolved an opportunity. Where an object comes into sharpest focus on the eye can actually tell you what wavelengths are — so by focusing backwards and forwards on something, they can extract a rough idea of its color.
And that leads into the next nifty explanation. Where I want to minimize chromatic aberration, cephalopods want to increase it…and as it turns out, having weird off-axis apertures causes more disparity in the focal plane of different wavelengths of light, which makes it easier to discriminate color using this mechanism.
It’s settled then. Cephalopods are cleverer than we are. Or maybe it’s evolution that is smarter than we are. One of those two.
Stubbs AL, Stubbs CW (2016) Spectral discrimination in color blind animals via chromatic aberration and pupil shape. Proc Natl Acad Sci U S A. 2016 Jul 5. pii: 201524578. [Epub ahead of print]