Science, Tech & Environment

Octopuses and other cephalopods are masters of optical illusion

Ten years ago, while doing a study in the Caribbean, marine biologist Roger Hanlon experienced something astonishing.

He was following a particular octopus along the sea floor, about five feet from the surface. Then — the octopus virtually disappeared into a clump of seaweed.

Because he was following the octopus with an underwater videocamera, Hanlon was able to “jam the camera right into its face, so to speak,” prompting the octopus to burst from camouflage mode to a startle defense — reappearing dramatically and changing its color in an instant.

When Hanlon swam to the surface, he “screamed bloody murder,” he says. His colleagues thought it was a diving accident, but Hanlon’s scream meant something different.

“It was a eureka moment,” he says.

Hanlon says cephalopods — squid, octopus and cuttlefish — are masters of optical illusion. He has been studying them for more than two decades and he is still trying to work out how they do it.

“No other animal even comes close to the speed and diversity of appearances of this animal,” he says. Cephalopods have the unique ability to change three visual aspects at once: pattern, color and texture.

Hanlon says cephalopods are the only animal group known to have fine control of its skin — the ability to change the ‘bumpiness’ we perceive as texture. They accomplish this through a complex network of nerves that connect their brain to their skin. "It's smart skin," Hanlon says. "It's all wired up."

Still, though scientists understand the physical mechanism, how the creatures know what to change is still unknown. They match their skin dimensionality by sight, not by touch, Hanlon says, which is "a vexing visual perception question."

The color aspect has a puzzling twist, too, Hanlon says: “These animals are color-blind, yet they are capable of creating color match patterns — but we don't know how.”

Hanlon is trying to solve this mystery by studying super close-up images of live, un-anesthetized squid skin. These images reveal tiny dots of pigment, called chromatophores, in three colors: yellow, red and brown. Underneath these pigments are ‘reflectors’ that produce the short color wavelengths — the blues and greens.

Based on its environment, cephalopods can rapidly change the size and shape of these chromataphores, which transforms its predominant skin color. Each chromataphore can expand up to 15 times its diameter.

Hanlon says the chromataphores “seem to be twitching all the time,” as if in a perpetual state of readiness to adapt quickly to its background. “[They] camouflage all night long,” Hanlon says. “They don't sleep as far as we know.”

Studying the third aspect of camouflage — pattern — led Hanlon to one of his major hypotheses: “We found only three to four basic pattern templates that they use to achieve all this camouflage,” Hanlon says. Given the rich visual diversity of their environments and their skill at hiding in them, Hanlon expected to find more.

Hanlon calls the first template "uniformity," in which there is little or no contrast in the skin pattern; the second he calls "mottled," which the animals use against backgrounds with small-scale light and dark splotches; Hanlon calls the third template "disruptive." This patterning “interferes with recognition of what the animal is,” he says, by blurring the outlines of its body.

Hanlon says the animals flash particular patterns based on a few visual cues they encounter in the environment. It’s not a reflex, he says, because there is so much visual analysis involved, but it is very fast: the color palette and pattern can change in less than a second.

Why these few patterns work so effectively is still a mystery, but the answer may have as much to do with visual perception as with skin color and pattern.

Hanlon analyzed his disappearing octopus video frame by frame, and he still can’t say for sure why we don't see the animal right in front of us. “We can't find any true statistical matches — whether it’s brightness or color — between the animal and the background,” he says.

This means camouflage is not looking exactly like the background; camouflage means fooling whatever is looking at you — which suggests humans are “behind the eight ball, as it were, if we think the world looks like how we see it.”

“There's much more information there, and other animals see it very differently,” Hanlon concludes.