Octopuses, squids, and their leggy relatives are well known for their impressive abilities to mimic their environment not just in texture, but also in color. This might not seem too weird until you learn that nearly all cephalopods, the group of animals to which they belong, are thought to only see in black and white. How they manage this camouflage skill has been frequently debated, with suggestions of simple trial and error to being able to see polarized light.
Now a new study, published in the Proceedings of the National Academy of Sciences, proposes a new method that the animals may be using to determine the color of their environment, through an effect known as chromatic aberration. “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,” said Alexander Stubbs, who co-authored the paper.
Chromatic aberration is an effect familiar to many of us when taking photos, though we may not have known it had a name. It occurs when the lens of a camera fails to focus all wavelengths of light to the same point, often resulting in a fringe of color along the boundaries of objects in the photo. Now two physicist, a father-son team, think that cephalopods may be taking advantage of this effect to determine color in the environment without actually having any of the physiology that we normally associate with seeing it.
Cephalopods are the masters of disguise. littlesam/Shutterstock
They suggest that the creatures change the focal position of the eyes to detect the different wavelengths of light through this chromatic aberration, and then produce a composite image that reveals the full color of the environment. “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,” explained Christopher Stubbs. “So it actually looks like there's been selective evolutionary pressure for their pupil shape to maximize this phenomenon.”
To test how squids and their kind may be taking advantage of such an effect, the two physicists turned to a computer model that they had actually written for some astrophysics research. By tweaking it, they created a computer model that mimics the eyes of cephalopods. “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,” explained Christopher. It might not be conclusive proof, but the researchers hope that they may spur other researchers to explore the avenue, and the possibilities it throws up, further.
The current leading theory as to how octopuses manage to match color is by observing polarized light. As light travels as a wave, it vibrates not just on one plane, up or down, but on many in an unpolarized way. When it hits an object and is reflected, it then often becomes polarized in a direction depending on the surface it has hit. It is thought that cephalopods can detect these subtle differences in the polarized light and use them to determine the color of an object. Perhaps they may be using a mixture of these two systems.
It could be that they have simply found a new way to determine color, completely different to how we think it should be done. Narchuk/Shutterstock