Cephalopods only have one kind of receptor in their eyes, so their vision should be just black and white. But their weird pupil shapes introduce chromatic aberration so they detect color via spatial shifts on their retina. So they effectively have spectrographs for eyes!
Originally shared by Kevin Clift
Octopus and other cephalopods seem to use colour, including in some cases to match their skin colour and pattern to their backgrounds, and yet they only have a single-type of monochromatic photoreceptor in their retina. Other animals, like us, perceive colour thanks to photoreceptors tuned to different frequency ranges of light. In the past researchers have proposed that mechanisms in the skin of cephalopods may detect colours, but only one type of opsin has been found in the skin, even via the recent octopus genetic sequencing effort, nor has a method of filtering that single known opsin response been found.
UC Berkeley graduate student Alexander Stubbs and others believe that they have a possible answer; the strange dumbbell, u- and w-shaped pupils of cephalopods may be used to allow light to reach multiple areas of the retina simultaneously, to create a blurred image with chromatic aberration or colour fringes which can be interpreted by the cephalopod to perceive colour at the expense of sharpness.
“We propose that these creatures might exploit a ubiquitous source of image degradation in animal eyes, turning a bug into a feature,” Stubbs said. “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.”
More here (press release): https://goo.gl/KklcYo
Video (YT ~4 mins.): https://goo.gl/7QeiyC
As a specific example, we constructed a computer model of the visual system of cephalopods (octopus, squid, and cuttlefish) that have a single unfiltered photoreceptor type. We compute a quantitative image quality budget for this visual system and show how chromatic blurring dominates the visual acuity in these animals in shallow water. We quantitatively show, through numerical simulations, how chromatic aberration can be exploited to obtain spectral information, especially through nonaxial pupils that are characteristic of coleoid cephalopods.
More here (paper open): https://goo.gl/gNV60Z