Cephalopod Eyes: Windows to an alien existence

Those eyes have been fascinating scientists for years. So eerily similar to our own, they are an example of convergent evolution, the term given to similar traits evolving in unrelated lineages. Convergent evolution generally occurs due to similar environmental pressures or similar needs being met. While our ancestors evolved in a radically different environment to cephalopods, the end result was a camera-type eye comprised of the same basic components. How could this happen? Well, for starters, both types of eye are derived from the same handful of opsins – light-sensitive proteins in the cells of the retina – and the same “master control” gene – Pax6. These building blocks have been preserved since at least the Cambrian Period, and are present in all bilateral animals.

However, not all bilateral animals have camera eyes – not by a long shot – and neither mollusks nor chordates began developing anything resembling a complex eye until millions of years after diverging. This makes the similarity between human and complex cephalopod eyes eerily striking. Like humans, they have an iris that controls the amount of light entering the eye, a cornea that helps refract light to focus the eye, a lens that works with the cornea to focus light by changing its shape, and a light-sensitive retina at the back of the eye. Through Pax6 and a handful of ancient opsins, humans and octopuses, two species separated by hundreds of millions of years, are able to scrutinize the world in a remarkably similar way.

There is one key difference, however, that has had scientists scratching their heads. Cephalopods can mimic their surroundings in minute detail, fine-tune their movements to the rippling light of waves, and communicate through color – but they themselves are completely colorblind. Scientists were initially baffled upon the discovery that all cephalopods have only one color receptor. Humans have three color receptors in their eyes, which absorb small, medium, and large wavelengths on the color spectrum. All three must work together to perceive the entire spectrum, and if a human is missing even one, they are rendered effectively colorblind. Because cephalopods share our camera-type eye, it was assumed that their meager one color receptor must condemn them to a life in black and white.

This year, that assumption was challenged. Father-son team Stubbs and Stubbs put forward a new hypothesis: that the single color receptor present in the eyes of octopus, cuttlefish, and squid is not as limiting as our preconceived notions had led us to believe. It fact, they argue in their study, it might be an advantage for the sneaky underwater predators. Color vision loses its usefulness in low-light environments, which characterize most of the ocean. Instead, many ocean creatures take advantage of polarized light, which humans can’t detect. Cuttlefish are particularly good at perceiving polarized light, able to respond to little over a one degree shift in its orientation. However, perception of polarized light alone doesn’t account for the dramatic use of color in cuttlefish, octopuses, and squid.

Instead, the new study has discovered a third possible method of light perception – one that would explain those weird W-shaped pupils. The Stubbs duo believe that cephalopods could be taking advantage of a natural phenomenon called chromatic aberration, which blurs images while splitting white light into its component colors. While most animals have developed small pinpoint pupils for a crisper image, this new study indicates that cephalopods might have turned a fault into an asset, emphasizing the color-dependent blurring effect by having elongated, off-axis pupils. Along with the perception of polarized light, this could be the optimal strategy for discriminating between colors in either high-light or low-light conditions.

Scientists can be forgiven for assuming that cephalopod eyes would follow the same basic mechanism of human eyes, and would be limited by the same set of rules. But it could be time to rethink this assumption, and to accept there might be more to cephalopod vision than meets the (human) eye.

Sources
Stubbs, Alexander L., and Christopher W. Stubbs. "Spectral discrimination in color blind animals via chromatic aberration and pupil shape."Proceedings of the National Academy of Sciences 113.29 (2016): 8206-8211.
Temple, S. E., et al. "High-resolution polarisation vision in a cuttlefish." Current Biology 22.4 (2012): R121-R122.
Yoshida, Masa-aki, Kei Yura, and Atsushi Ogura. "Cephalopod eye evolution was modulated by the acquisition of Pax-6 splicing variants." Scientific Reports 4 (2014).

Rachel
Fritts

Guest Writer

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