We’ve long known that octopuses have some of the most powerful eyes among invertebrates, but a recently published article in the Journal of Experimental Biology titled “Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides” is showing that members of cephalopoda (octopus, squid, cuttlefish) may also have light-sensitive cells in their skin that effectively transform the large outer organ — already famous for its color- and shape-shifting qualities — into a perceptive one as well. The science sections of the New York Times and The Guardian, along with National Geographic‘s Phenomena webpage, all cover the story (click on the reporter’s name to reach the original articles):

Octopuses can mimic the color and texture of a rock or a piece of  coral. Squid can give their skin a glittering sheen to match the water they are swimming in. Cuttlefish will even cloak themselves in black and white squares should a devious scientist put a checkerboard in their aquarium.

Cephalopods can perform these spectacles thanks to a dense fabric of specialized cells in their skin. But before a cephalopod can take on a new disguise, it needs to perceive the background that it is going to blend into.              — Carl Zimmer

 

Octopuses are thought to rely mainly on vision to bring about these colour changes. Despite apparently being colour blind, they use their eyes to detect the colour of their surroundings, then relax or contract their chromatophores appropriately, which assume one of three basic pattern templates to camouflage them, all within a fraction of a second. Experiments performed in the 1960s showed that chromatophores respond to light, suggesting that they can be controlled without input from the brain, but nobody had followed this up until now.      — Mo Costandi

Each chromatophore is an elastic sac of pigment, surrounded by a starburst of muscles. If the muscles relax, the sac contracts into a small dot that’s hard to see. When the muscles contract, they yank the sac into a wide disc, revealing the colour it contains. Kingston [Alexandra, one of the scientists studying cephalopod chromatophores with vision expert Tom Cronin] showed that these living pixels contain the same Rube Goldberg set-up that exists in their owners’ eyes.

Her team couldn’t, however, show that the chromatophores actually respond to light. “All the machinery is there for them to be light-sensitive but we can’t prove that. It’s been very frustrating,” says Cronin. The chromatophores might be detecting local light levels to prime them for either expansion or contraction. They could communicate with each other so that small clumps of chromatophores react to light as a unit. Or they could send signals directly to the brain to provide their owners with more information about light levels in their environment. These possibilities could all be right or wrong; no one knows.

“We don’t know if they contribute to camouflage or are just general light sensors for circadian cycling or are driving hormonal changes. They have a job to do but we don’t know what it is,” says Cronin. “That’s biology!” he adds, resignedly.     — Ed Yong