Study proposes explanation for how cephalopods see color, despite black and white vision

July 4, 2016, Harvard University
According to a new theory, the pupil of the cuttlefish Sepia bandensis maximizes chromatic blur, allowing the animal to detect color. Credit: Roy Caldwell, UC Berkeley

For years, camera-makers have sought ways to avoid chromatic aberration—the color fringes that occur when various wavelengths of light focus at different distances behind a lens.

But where photographers see a problem, some sea creatures see possibility.

A new study, co-authored by the father-and-son team of Christopher and Alexander Stubbs, suggests that chromatic aberration may explain how cephalopods—the class of animals that includes squid, octopi and cuttlefish—can demonstrate such remarkable camouflage abilities despite only being able to see in black and white. The study is described in a July 4, 2016 paper in the Proceedings of the National Academy of Sciences.

"There's been a long-standing paradox that (cephalopods) manifest these vivid chromatic behaviors," Christopher Stubbs, the Samuel C. Moncher Professor of Physics and of Astronomy, said. "That would lead any observer, even a lay person, to conclude that they must be able to deduce things about coloration."

"I have always been fascinated by these animals, and have had the opportunity to watch them perform their camouflage act while conducting field work in Indonesia," Alexander Stubbs, a Berkley graduate student and lead author of the study, said. "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."

But what would possess a Harvard physicist to devote time and energy to one of the most persistent mysteries in biology? For Stubbs, the answer is simple—his son.

"He chased me down with an idea he'd come up with, and the more we talked about it, the more sense it made," he said. "I credit my co-author with having the a-ha moment here."

That a-ha moment, Christopher Stubbs said, was the realization that cephalopods could potentially detect color by adjusting the focal position of their eyes to detect different wavelengths of light, and then composite each into a "color" image of their world.

"You can think about it like a digital camera dithering back and forth to find the crispest image," he 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."

To understand just how cephalopods might take advantage of , Christopher Stubbs turned to code he's earlier written for astrophysics research and created a computer model of how the animals' eyes work.

"People have done a lot of physiological research on the optical properties of lenses in these animals," he explained. "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."

Though it's not definitive evidence of how cephalopods understand color, Christopher Stubbs said the mechanism described in the study does agree with earlier studies of cephalopod eyes.

"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," he said.

Ultimately, Alexander Stubbs said, the hope is that the study will offer other researchers a direction for study in the search for a conclusive answer to how squid and octopi became masters of camouflage.

The unusual pupils of cephalopods (from top, a cuttlefish Sepia bandensis; squid Sepioteuthis; and Octopus vulgaris) allow light into the eye from many directions, which spreads out the colors and allows the creatures to determine color, even though they are technically colorblind. Credit: Roy Caldwell, Klaus Stiefel, Alexander Stubbs, respectively
"This is an entirely different scheme than the multi-color visual pigments that are common in humans and many other animals. High-acuity "camera style" lens eyes in octopus, squid and cuttlefish represent a completely independent evolution of complex eyes from vertebrates so in some sense we shouldn't necessarily expect that this lineage would solve problems like color vision in the same way. These organisms seem to have the machinery for color vision, just not in a way we had previously imagined."

Alexander Stubbs said. "We also conducted an in-depth review of prior literature evaluating conflicting evidence for color vision, and found prior behavioral studies suggesting a lack of represent special cases and are consistent with our model. We hope this study will spur additional behavioral experiments by cephalopod community."

Explore further: Studying 'squid skin' to create new camouflage patterns

More information: Spectral discrimination in color blind animals via chromatic aberration and pupil shape, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1524578113

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nilbud
5 / 5 (3) Jul 04, 2016
"selective evolutionary pressure" or lunch as it's known.
Da Schneib
5 / 5 (6) Jul 04, 2016
One imagines part of the cephalopod brain scanning over focal points (which eyes can do very quickly) to build a color image, just as we scan across retina, possibly superior to our own in terms of color resolution. Living cephalopods' lenses could be measured remotely without disturbing the animal to determine this. This is a fascinating area; I have always wondered just how smart octopi and cuttlefish are, having watched them a number of times both in tanks and in documentaries.

In a tank you can watch various members of a group of cuttlefish rise above the rest and take on conspicuous shading and raise their two outermost tentacles in a gesture no cuttlefish below uses; only one does it at a time, always a large male. They change places every few minutes, and there is a brief period of display involving splaying all the tentacles and "flashing" colors involved. This is social behavior.
[contd]
Da Schneib
5 / 5 (6) Jul 04, 2016
[contd]
Meanwhile, large Eastern Pacific octopi require enrichment activities in order to remain healthy in captivity. Common activities involve brightly colored and richly textured objects that contain food. Also, to ensure an octopus does not escape its tank (which they generally try to do eventually), astroturf is placed around the rim. They apparently find its texture repellent and this prevents them from trying to crawl over it. Imagining the world their senses must reveal to them is very interesting.
Nik_2213
5 / 5 (6) Jul 05, 2016
Mechanically scanning lens *focus* to get colour vision is so neat !!

Astroturf as an anti-clamber border ? I'd wondered how you'd keep those nimble cephalopods in an open tank...
Steelwolf
3 / 5 (2) Jul 07, 2016
Yes, the entire order seems to be able to 'communicate' by color, to what degree and how fine of order they can describe and what sort of idea they can form is an interesting thought exercise. But the astroturf is a substance that actually allows them no 'solid' suction grip, they just feel it as raspy and slick with no support, so it is effective. They learned at some of the aquariums in the area in keeping their octopi in their tanks.

Many studies have shown that these are the effectively smartest top predators in the Oceans for their niche (whales have their own niche, sharks overlap with cephalapods, to an extent, but are no match for their brains.) and that they seem to be tool using to the extent of using objects to pry and to hide in, and in the case of jars, use them as a portable, see thru shell. Obviously they will have very different things on their minds, survival, mating/breeding, and if female, ensuring survival of brood, but there seems to be rich life in between
DonGateley
not rated yet Jul 11, 2016
Nature did it wrong on our side of what divided us. Show's ya evolution doesn't seek optimal. These critters must have a virtually continuous color palette. There is no color that could be indistinguishable from nearest neighbors. Their light sensing neurons can be given one and the same task, "respond as strongly and with as little noise as possible when hit by a photon." The simpler the function, the more likely a high sensitivity and high dynamic range optimum will be found.

That explains the size of their brains too. With a couple of orders of magnitude more visual information to enrich it's going to take a few more cells. Not necessarily any room left over for higher cognitive skills but, oh, wow. The possibilities.

Hey, if no one has yet built the camera that works this way, get on it! Gotta wonder what other kind of new information could be going on here and in that stream. Display, anyone?
DonGateley
not rated yet Jul 11, 2016
And, hey, since I don't know of anyone else that has ever asked for one of those, can I have one of the earliest. I have a mind blowing application.

It's not at all clear to me that with simple DSP type processing the resolution of said camera can be made to not suffer the least. De-convolution and stuff.

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