Roaming around dark ocean crevices, an octopus searches for flickers of light underwater that might indicate its next meal. It looks out for small fish darting across its visual field and crabs crawling along the ocean floor. Meanwhile the octopus is wary of a shadow gliding above—perhaps it’s a sperm whale, a common predator of the octopus and its prey. Finding food and staying safe as a soft, flexible cephalopod requires a different set of visual sensitivities and skills than the ones we humans use when we’re walking through a fluorescently lit food court to search for our next meal.
Octopuses, cuttlefish and squid—the coleoid, or soft-bodied, cephalopods—are similar to humans in that they rely heavily on their visual system to guide their everyday activities. But their brain has developed an entirely different way of seeing their surroundings to help them with their aquatic endeavors. Scientists are still trying to figure out how these animals’ brain enables their unique way of seeing.
“They have eyes like ours, and they have large brains, but the brain is organized completely differently because they evolved differently,” says University of Oregon visual neuroscientist Cris Niell. “And the fact that so little is known about it—as a visual neuroscientist, I was just captivated.”
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Vision is so critical for cephalopods that they use more than two thirds of their central brain for visual processing, slightly more than the comparable measure for the human brain. Their visual system is different than that of humans, although both cephalopods and our species have somewhat unusual cameralike eyes that receive light through an aperture and focus it with a lens.
Unlike humans, cephalopods are extra sensitive to dark large things and light small things. Neill discovered this when he brought octopuses into his laboratory. There he found that they had more neural activity, measured using calcium imaging of the animals’ optic lobe, when seeing small light circles and large dark circles on a screen. This may be because the octopuses’ small prey tend to appear bright when set against a large dark background, whereas large looming objects above, such as predators, likely appear dark against a light background.
Other experiments show that octopuses also tend to detect horizontal and vertical stimuli more than diagonal visual inputs. The first findings of this date back to 1957. This “rectilinear” bias likely helps octopuses pick out prey swimming in a horizontal direction or predators moving downward.
While many other underwater creatures adapted to see a wider section of the color spectrum than humans, cephalopod adaptation took a different turn. Remarkably the animals appear to be colorblind, with the exception of a few deep-sea species. “It kind of blows our minds because almost everything sees color,” says Sönke Johnsen, a visual ecologist at Duke University.
Unlike our retinas’ photoreceptors, such as cones that help us see color, cephalopods’ photoreceptors can perceive polarized light from different directions. Their photoreceptor cells cover the retina at the back of their eyes in a pattern of alternating horizontal and vertical orientations, which allows them to detect polarized light arriving from different angles. Many other aquatic animals can sense polarized light but not with the detail that cephalopods can.
Because humans aren’t sensitive to polarized light, it’s hard to imagine what this would be like. Polarization vision allows animals to see clearly through water without having an object distorted by reflections, similar to how polarized sunglasses help us avoid seeing glare.
Underwater, polarized images are more reliable than color for gaining an accurate picture of a surrounding seascape because water can filter out certain wavelengths of light on the color spectrum but does not affect perception of polarized light at a distance. So polarized light reaching the eye of a cephalopod can provide helpful cues for seeing objects at all depths. Scientists are still attempting to understand how cephalopods use these polarized images, however.
Although polarized vision helps cephalopods see, scientists remain perplexed about how octopuses camouflage in the absence of color vision. How do the animals camouflage based on color if they can’t see it? Cephalopods, especially cuttlefish and octopuses, instantaneously change the patterns and texture of their skin to blend with their surroundings and avoid being seen. Sometimes they even pretend to be other things such as algae or rocks.
Researchers have two main theories about how this might be possible, explains Tessa Montague, a neuroscientist at Columbia University. The first is an intriguing idea that the shapes of the cephalopod pupils might help separate wavelengths of light and allow animals that would otherwise be colorblind to detect color. Of course, this would not happen with just any pupil. Cephalopods tend to have a pupil shape that allows different wavelengths to come into focus at varying distances from the lens, just behind the pupil. More experiments are needed to clarify whether the animals actually use these signals to categorize wavelengths of light into colors.
A second and less startling idea is that perhaps that seawater’s dark green-blue tint acts as a filter that cuts down on the range of colors that cephalopods need to display. The deeper the animal goes underwater, the more water filters out red and orange wavelengths. In agreement with this view, Johnsen says that colorblind camouflage is “not the puzzle everyone thinks it is.”
Some cephalopods use color not only to camouflage but for interspecies communication. “There is this whole visual vocabulary that they use for social communication,” Montague explains.
For example, many cuttlefish use a dramatic black-and-white-striped pattern to show aggression toward others of their species. They also sometimes create a spotted or wavelike pattern on their skin. “Our [lab’s] species will often look out of the tank, and when they’re looking at us and watching us, they start creating the waves,” Montague says. “So I think it’s some form of attention—like they’re attending to their surroundings and are alert.” But the meanings of many of these patterns in cuttlefish are still a mystery.
Larger questions still remain about how camouflage and colorblindness vary across species. “I think it would be good to do camouflage [studies] with octopus and at the same time do behavioral experiments on color vision with cuttlefish,” says Frederike Hanke, a zoologist at the University of Rostock in Germany. The latest genetic, neuroimaging and behavioral analysis techniques might make such studies possible.
Importantly, different cephalopods live at different depths, and they use their vision for activities such as hunting and camouflaging, so it is a dramatic simplification to refer to a single cephalopod visual system. Scientists such as Hanke and Montague are particularly interested in how these visual abilities and systems vary between octopuses and cuttlefish.
“The thing that I am always trying to convey is how mysterious this is—not just mysterious in the sense that octopuses are weird creatures ... but the fact that this is a brain that is designed completely differently than ours that does these remarkable things,” Niell says. “I find it amazing that there’s this whole unexplored territory.”