Cuttlefish Look Like Squid—and Like Crabs, and Like Algae, and Like Rocks
Cuttlefish have been captured on film exhibiting sophisticated camouflage strategies at night, according to scientists who are using new high-resolution cameras to bring these dramatic changes into focus.
They are also using underwater spectrometers to measure color wavelength to determine how other marine creatures perceive these shifts.
The findings are helping to crack the code of cephalopods, including cuttlefish, which also employ shape-shifting strategies to conceal themselves as coral or algae.
For the last nine breeding seasons, Roger Hanlon, senior scientist at the Marine Biological Laboratory at Woods Hole, Massachusetts, and a National Geographic Society grantee, has closely studied their camouflage strategies. (National Geographic News is owned by the National Geographic Society.)
His work takes place at a cuttlefish spawning site—a five-mile (eight-kilometer) stretch of shallow, flat reef—in Spencer Gulf, Australia.
This summer Hanlon went back to Australia with collaborators from the University of Sydney and the University of Queensland and used an autonomous underwater vehicle, or AUV, with a pair of high-resolution cameras and a powerful strobe to take detailed pictures of the concealed cuttlefish at night.
The cameras were synchronized and aimed at the same spot so they captured three-dimensional images.
Researchers want to know if the cuttlefish have taken their extraordinary talent for camouflage to the next step by employing color wavelengths invisible to their predators.
One of Hanlon's co-researchers, Professor Justin Marshall of the University of Queensland, used a sophisticated underwater spectrometer. "It tells you every [color] wavelength present, and how much there is of it," Hanlon said.
He hopes the device will help reveal just how closely the cuttlefish's camouflage coloration matches their surroundings.
Humans rarely encounter frilled sharks, which prefer to remain in the oceans' depths, up to 5,000 feet (1,500 meters) below the surface. Considered living fossils, frilled sharks bear many physical characteristics of ancestors who swam the seas in the time of the dinosaurs. This 5.3-foot (1.6-meter) specimen was found in shallow water in Japan in 2007 and transferred to a marine park. It died hours after being caught.
Then, the pair will take that data this fall and superimpose them over what they know of fish color vision. This will allow them to determine how well the color of the cuttlefish matches the color vision spectrum of their predators, Hanlon explained.
All day, male cuttlefish duel for mating rights, flashing contrasting patterns to deter rivals and impress females.
Come dusk, the cuttlefish turn from colorful billboards into masters of disguise, retiring to the seafloor, where they use their extraordinary color manipulation to hide from predators such as dolphins.
Plenty of sea creatures employ camouflage at night, says Hanlon, but cuttlefish have made it an art form.
"Each animal adopts a tailor-made camouflage pattern for the particular microhabitat that it settles in. An animal that settles in sand will appear one way, and ten feet (three meters) away, where it's all algae, another will be camouflaged differently," he added.
What really thrilled Hanlon was the discovery in 2003 that the cuttlefish are performing sophisticated camouflage in a pitch-black ocean. This was the first time cuttlefish were seen matching their various surroundings at night.
It seems the cuttlefish can assess the color, contrast, even the texture, of their surroundings and emulate it—in seconds and in total darkness.
Cuttlefish in HD
Cuttlefish skin has been likened to a color television—it has a way of combining basic colors to form more complex hues and dynamic patterns. "It really is electric skin," Hanlon said, because it's all controlled by neurons in the brain that transmit impulses and information to the rest of the body.
"A cuttlefish has maybe ten million little color cells in its skin, and each one of them is controlled by a neuron. If you turn some on, but leave others switched off, you can create patterns," Hanlon explained.
Cuttlefish use pigmented organs, elastic sacs called chromatophores, to display red, yellow, brown, and black directly.
Bands of muscle radiate from each chromatophore, like the spokes of a wheel, so the creature can change the hue or opacity at will simply by contracting or relaxing those muscles to expose or conceal different color layers.
With up to 200 chromatophores per .001 square inch (square millimeter), cuttlefish skin is like high-definition TV.
Ironically enough, cuttlefish are colorblind. So how do they match their camouflage and their environment so accurately?
Leave that, Hanlon said, partly to a separate layer of cells called leucophores, which reflect white light.
"When you think about what white is, it's all colors at once. So in very shallow water, [leucophores] will look white, but as you go deeper, [the ocean] gets a little more green and blue, so those cells will reflect green and blue," he said.
"They can do some [color] matching in a passive way; it doesn't require the eye to assess anything. It's a cool trick."
Mark Norman, senior curator of mollusks at Museum Victoria in Melbourne, said the cuttlefish have a still more ingenious camouflage trick up their sleeve—or at least under their skin.
"They can also change the sculpture of their skin with bands of circular muscle," Norman explained. "As they contract, the near liquid in the center gets forced up as little nodes, or spikes, or flat blades that stick up."
By employing such "skin sculpture," he said, cuttlefish could take on the appearance of kelp or rock.
"By adding that structural component, [the cuttlefish] gets rid of outline and profile, and predators that are looking for shapes will be confused," Norman added.
He said those predators have provided the evolutionary selection pressure for the cuttlefish's camouflage strategies over millions of years, "because they are such a good, soft, rump steak-kind of meal."
While other mollusks, such as clams and nautiluses, have developed hard shells for protection, cuttlefish have instead relied on invisibility, a talent that may have applications for human technology.
Norman said the military has shown interest in cuttlefish camouflage with a view to one day incorporating similar mechanisms in soldiers' uniforms.