Cephalopods often change color to confuse their prey or escape their predators. They can't see color, but their predators and prey can. How do they pull it off?
Much has been made of octopi intelligence. They can open bottles, escape cages, and occasionally disable pumps and trash aquariums. It can distinguish between objects that are black and objects that are white, ones that are different shades of grey, and even ones that are different colors — unless those colors look the same in grey-scale. When they do, the octopus can't tell the difference between them. Plenty of creatures are colorblind, but not many of them are so celebrated for their camouflage.
Scientists continue to struggle with the octopus' ability to imitate color without being able to see it. One thing they've learned is that although cephalopods lag behind us in color vision, they lap us in recognizing polarized light. Most light comes at us in waves, but the waves are all going different directions — up and down, back and forth, around in circles, and at any angle. Some sunglasses and windshields filter out much of the incoming light so that only the waves going one direction get through; this cuts down on glare. It looks like octopuses can distinguish between different polarizations of light, allowing them to sense even slight contrasts between different colors. This may help them to adjust their colors until they exactly match the shade that they see.
Scientists have recently also noticed that cephalopods have proteins that act as color receptors called opsins. The opsins can be found in their skin as well as their eyes. These proteins may be able to pick up color when the eyes do not, or they may just be able to check what the octopus is trying to blend in with and change color, cell by cell.
There's still no conclusion on why exactly cephalopods are such talented mimics when they can't see color. Some people believe they may just have evolved to use colors that work most of the time. If an octopus evolves around a lot of sand, only the ones that manage to mimic sandy colors, rather than flashing red and green, will be able to survive their predators. Are cephalopods sensing something humans can't? Or are they just playing the odds?