Scientists have now found that octopuses (or octopi or octopodes - all 3 are correct, but octopuses is most accepted) and other cuttlefish can detect light with their skin. Octopuses and cuttlefish have opsins in their skin. Opsins are proteins normally found in the retinal receptors (including of humans) that help in vision. But instead of producing them in chromatophores, octopuses only make opsins in hairlike nerve endings in the skin.
In 2010, Roger T. Hanlon, a biologist at the Marine Biological Laboratory in Woods Hole, Mass., and his colleagues reported that cuttlefish make opsins in their skin, as well. This discovery raised the tantalizing possibility that the animals could use their skin to sense light much as their eyes do.
Dr. Hanlon teamed up with Thomas W. Cronin, a visual ecologist at the University of Maryland Baltimore County, and his colleagues to take a closer look at cephalods and their opsins. But they couldn't make much headway with that.
Later, they switched to researching octopi. When the scientists kept octopus skin in darkness or dim red light, it remained pale. But when they switched on lights, the chromatophores swiftly expanded, turning the skin dark in a matter of seconds.
Blue light turned out to trigger the fastest response. Opsins in octopus eyes are most sensitive to blue light, too.
They concluded that Octopuses can detect light with their skin.
This is of course, in addition to the fact that the retina of the OctopusEye is a better design than the Human Eye in that the light sensitive photoreceptors in an octopus receive light directly. Unlike in the human eye, where light passes through the wiring(neurons and helper cells) before reaching the light sensing photoreceptor cells. This reduces the light detected. Also, there is a hole in the vision(blind spot) at the place where the "wiring" of the human eye is taken out to connect to the brain.
|The Octopus Retina is a better design than the Human Retina|
The eye is not a single frame snapshot camera. It is more like a video stream. The eye moves rapidly in small angular amounts and continually updates the image in one's brain to "paint" the detail. We also have two eyes, and our brains combine the signals to increase the resolution further. We also typically move our eyes around the scene to gather more information. Because of these factors, the eye plus brain assembles a higher resolution image than possible with the number of photoreceptors in the retina. So the megapixel equivalent numbers below refer to the spatial detail in an image that would be required to show what the human eye could see when you view a scene.
Based on the above data for the resolution of the human eye, let's try a "small" example first.
Consider a view in front of you that is 90 degrees by 90 degrees, like looking through an open window at a scene.
The number of pixels would be
90 degrees * 60 arc-minutes/degree * 1/0.3 * 90 * 60 * 1/0.3 = 324,000,000 pixels (324 megapixels).
At any one moment, you actually do not perceive that many pixels, but your eye moves around the scene to see all the detail you want. But the human eye really sees a larger field of view, close to 180 degrees.
Let's be conservative and use 120 degrees for the field of view.
Then we would see
120 * 120 * 60 * 60 / (0.3 * 0.3) = 576 megapixels.
The full angle of human vision would require even more megapixels. This kind of image detail requires A large format camera to record.
Ramirez, M. D. & Oakley, T. H. (2015). Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides. J. Exp. Biol. doi: 10.1242/jeb.110908.