Octopuses are purportedly colorblind, but they can discern one thing that we can't: polarized light. This extra visual realm might give them a leg (er, arm) up on some of the competition.
And a team of researchers has created a new way to test just how sensitive cephalopods are to this type of light. Their results were published online Monday in Current Biology.
"We now know that polarization is tuned much more finely than we thought it was," says Shelby Temple, of the Ecology of Vision Laboratory at the University of Bristol in the U.K., who led the study.
But testing polarized light is tricky, especially since we humans aren't tuned to see it. As Temple and his co-authors wrote in their paper: "For animals that can see it, the polarization of light adds another dimension to vision, analogous to adding color to a black and white image." Polarized light is different from what we see in that it comes from a single angle, and animals that can detect it seem to see it in different resolutions based on changes in its angle. (The closest we can get to using it is putting on a pair of polarized lenses to cut down on glare.)
Polarized light perception in the best-tuned animals was assumed to be limited to differences of about 10 to 20 degrees. But in the group's new experiments, the mourning cuttlefish (Sepia plangon) responded to just 1.05-degrees change of polarized light orientation.
For the experiments, the team used computer screens that had had the polarizing light filter removed (without these front filters on our liquid crystal displays—LCDs—our monitors would project polarized light images that we wouldn't be able to see).
These modified displays played digital movie versions of "looming stimuli" such as an expanding circle, which would suggest a potential predator approaching. But instead of a color or intensity-based image, the one they created was based on changing polarized light orientation only.
Octopus don't yet seem to be quite as sensitive as cuttlefish to the fine gradients in polarized light, responding only after about 10 degrees shift. But, says Temple, "it may be the way that we're testing." As he points out, cuttlefish's knee-jerk response to an approaching predator is a quick change of color, which the researchers could use as an indication that they had seen even fine shifts in the polarized light angle.
"Cuttlefish, they wear their emotions on their sleeve, quite literally," Temple says. "They're showing everything that they're doing as a neural response." In fact, the cuttlefish responded so well, that he and his colleagues thought they were doing something wrong. They were afraid that in the digital renderings they might have accidentally included a non-polarized light clue, such as brightness or intensity. But they went back and checked and found that it was, indeed, just the slight change in polarized light that was frightening the animals.
With octopus, "there's no comparison," he says. But, he concedes that it is possible that the octopuses might have seen finer resolutions of polarized light shift but just didn't have the same simple, speedy reaction as the cuttlefish.
And says Temple, "it could be that some species could do it better than others." So far, he has found that the blue ringed octopus looks to me more sensitive than the day octopus. He has plans to test different species of octopus soon.
Researchers are still working to get to the bottom of cephalopod vision, which is turning out to be highly complex. And this new work supports the idea that such sensitivity to polarized light emerged precisely because these animals don't see color well—if at all.
And if octopuses, cuttlefish and squid—and some of their predators and prey—can see polarized light so keenly, are they also using it, as they use color and luminosity, to actively create camouflage?
Other researchers are working on that very question. And Temple and his colleagues have observed that, at least in some cuttlefish, they can create a polarized light-based pattern on their skin. This play in light might "be used as part of a covert communication channel, invisible to animals lacking polarized vision," they wrote.
But the patterns remain tricky for us to pick up on. For that, Temple and his colleagues have developed a way for us to get a peak into the invisible world of polarized light and dark by modifying a digital single-reflex lens (SLR) camera and creating a computer program to feed false-color into varying degrees of polarized light. These mysterious rainbow-colored ecosystem images make it clear that, "We're not done with the story yet, for sure," Temple says.
Illustration courtesy of Ivan Phillipsen