Octopuses, squid and cuttlefish — a group of mollusks known as cephalopods — are the ocean’schampions of camouflage.
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.
Cephalopods have large, powerful eyes to take in their surroundings. But two new studies in The Journal Experimental Biology suggest that they have another way to perceive light: their skin.
It’s possible that these animals have, in effect, evolved a body-wide eye.
When light enters the eye of a cephalopod, it strikes molecules in the retina called opsins. The collision starts a biochemical reaction that sends an electric signal from the cephalopod’s eye to its brain. (We produce a related form of opsins in our eyes as well.)
In 2010, Roger T. Hanlon, a biologist at the Marine Biological Laboratory in Woods Hole, Mass., and his colleagues reported that cuttlefish make opsins in their skin, as well. This discovery raised the tantalizing possibility that the animals could use their skin to sense light much as their eyes do.
Dr. Hanlon teamed up with Thomas W. Cronin, a visual ecologist at the University of Maryland Baltimore County, and his colleagues to take a closer look.
Cephalopods have pigment-packed cells in their skin called chromatophores, surrounded by muscles and nerve endings. When the muscles contract, they stretch out the chromatophores, allowing them to absorb more light and giving the animals new colors. Cephalopods have up to 96,000 chromatophores per square inch of skin, which they use as a kind of high-definition video display.
Dr. Hanlon, Dr. Cronin, and their colleagues developed precise molecular probes that could be used to locate the opsins in the skin. They found that in cuttlefish, opsins are produced only in chromatophores. The same turned out to be true of squid. In addition, the scientists found other enzymes in the animals’ skin that are present in the eye, where they help opsins relay signals from light to the nervous system.
Together, these studies persuaded the scientists that cephalopods may have evolved a way to perceive light, and perhaps color, directly via their skin. They next took pieces of skin from the animals and flashed light on them to see if they could get a response.
No matter how often they tried, they failed. But another pair of scientists had better luck.
Dr. Hanlon’s study inspired Todd H. Oakley, a biologist at the University of California, Santa Barbara, and M. Desmond Ramirez, a graduate student, to join the hunt for skin opsins. Instead of squid or cuttlefish, they chose to study octopuses, collecting animals from the tide pools near the campus.
The scientists discovered that octopuses, like cuttlefish, have opsins in their skin. But instead of producing them in chromatophores, octopuses only make opsins in hairlike nerve endings in the skin.
Mr. Ramirez and Dr. Oakley cut off pieces of octopus skin to see if they might respond to light. When the scientists kept the skin in darkness or dim red light, it remained pale. But when they switched on lights, the chromatophores swiftly expanded, turning the skin dark in a matter of seconds.
“We didn’t expect to see such a fast reaction,” said Dr. Oakley. He suspects that light strikes the skin opsins, which stimulate the neurons to relay the information to neighboring chromatophores.
Mr. Ramirez and Dr. Oakley exposed the skin to light in a variety of colors to test its sensitivity. Blue light turned out to trigger the fastest response. Opsins in octopus eyes are most sensitive to blue light, too.
“I’m very happy that they’ve succeeded,” said Dr. Hanlon of the studies by Dr. Oakley and Mr. Ramirez. “And a little bit envious.” Their results have prompted him to try it again.
Even if he succeeds with cuttlefish and squid, no one can yet say how exactly the animals use the light gathered by opsins to control their skin color.
“This is a really weird story,” said Dr. Hanlon, “but our animals are really weird.”