One of the ocean's outstanding riddles asks why some of its most colorful residents—squids, octopuses, and cuttlefish—are restricted to a world of black and white. Coleoid cephalopods are widely believed to be colorblind, and the mechanics behind their vivid camouflaging have evaded biologists for centuries.
The question remains: How do cephalopods so masterfully disguise themselves if they can't even see the colors of their environment?
The mysterious physical properties of cephalopod sight were what led two astrophysicists to jump into the scientific fray, and propose a new theory for how these clever mollusks translate the monochrome into the colorful. According to their findings, which were published this week in the Proceedings of the National Academy of Sciences, the answer might be explained by a computer model and the basic rules of modern photography.
"I have always been fascinated by these animals, and have had the opportunity to watch them perform their camouflage act while conducting fieldwork in Indonesia," lead author Alexander Stubbs, a graduate student at the University of California, Berkeley, said in a statement. "We believe we have found an elegant mechanism that could allow these cephalopods to determine the color of their surroundings, despite having a single visual pigment in their retina."
Scientists have been studying the chimeric behavior of cephalopods since at least the fourth century BCE, when Aristotle described the octopus in Historia animalium, remarking that that it "seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also when alarmed." More recently, biologists discovered that color-changing skin cells called chromatophores are responsible for the seemingly endless array of hues that cephalopods can slip into. Pigment sacks within each cell are expanded or contracted by electrical currents which the animal controls neurally, allowing them to change color in mere milliseconds.
But where experts begin to stumble is the literal gray area of cephalopod optics. The study's authors note that only one mechanism for color discrimination is known to exist, and it's the one that primates (like us) and many other animals possess. When we see colors, what we're actually looking at are various wavelengths of reflected light that bounce off the retina at the back of our eyeballs. At the same time, millions of light-sensitive cone cells allow color-corresponding signals to reach our brain's neural cortex, which interprets them as red, blue, green, yellow, etc.
Squid or octopus eyes, however, despite sharing genetic similarities with other animals—including humans—are notably different. As opposed to vertebrate eyes, cephalopods lack a cornea, have photoreceptor cells containing only one visual pigment (thus the colorblindness), and display strange, off-axis pupils. The orthogonal alignment of their photoreceptors has led some biologists to suspect that cephalopods use polarized light to see, and don't really change color but rather modulate their light-reflecting "iridophore cells," sort of like an iridescent soap bubble.
The newly published study takes this theory one step further by suggesting that cephalopods actually adjust the focal position of their eyes to scan and detect different wavelengths of light, subsequently translating those signals into a colorful view of their surroundings.
This process is referred to as "chromatic aberration," or "color fringing," and is a well-known concept among photographers. When camera lenses accidentally disperse wavelengths on different focal planes, sometimes the edges of shapes will blur and take on a colored halo. According to the authors' findings, cephalopods use their curiously-shaped pupils to exploit this outcome and collect spectral information about the objects around them.
"You can think about it like a digital camera dithering back and forth to find the crispest image," co-author Christopher Stubbs, a professor of physics and astronomy at Harvard University, said.
"To me, what's really persuasive about this argument is… the pupils in these animals are an off-axis U shape, and that actually maximizes this chromatic signature at the expense of image sharpness. So it actually looks like there's been selective evolutionary pressure for their pupil shape to maximize this phenomenon."
In order to test their theory, the authors applied an astrophysics program they had previously written to investigate the out-of-focus properties of the Large Synoptic Survey Telescope in El Peñón, Chile. Using this code, they were able to model the visual system of a well-researched octopus that shares optical properties with all other studied cephalopods: the hammer octopus (Octopus australis). By simulating what the octopus sees, they concluded that the eye design of cephalopods almost exclusively favors color fringing over other visual benefits, such as acuity.
"People have done a lot of physiological research on the optical properties of lenses in these animals," Stubbs added. "We wrote some computer code that essentially takes test patterns and moves the retina back and forth, and superimposes that on the image and then measures the contrast. I'm not a life scientist, but I think in some ways, this is such an elegant mechanism that it would be a shame if nature didn't capitalize on it."
It's important to note that the study's results don't explain how cephalopods interpret color to camouflage themselves—only how they might absorb these spectral cues from their environments.
Maybe one day, we'll finally figure out how cephalopods came to be the ocean's illusionists. But until then, some of us are more than content to be simply amazed by them.