How Do Cuttlefish Change Color? Skin Cells and Camouflage

Chromatophores: color under nervous control

The color-changing machinery of the common cuttlefish sits in its skin. The top layer is packed with chromatophores, small elastic sacs filled with pigment. In cuttlefish they come in three colors: yellow, orange-red and brown-black.

Each sac is surrounded by a starburst of tiny radial muscles. When nerves tell these muscles to contract, they pull the sac wide open, so its pigment spreads into a visible spot of color many times its resting size. When the muscles relax, the elastic sac shrinks back to a tiny, almost invisible dot.

This is very different from how most animals change color. Fish, frogs and chameleons change color by moving pigment granules inside their color cells or by rearranging tiny reflective crystals, processes that take seconds to minutes. A cuttlefish’s chromatophores are wired directly to motor nerves from the brain, so it can switch from one pattern to another almost instantly, and it can control different patches of skin independently.

Iridophores and leucophores: the reflective layers

Pigment alone produces only warm colors. Under the chromatophores lie two kinds of reflective cells that add the rest.

• Iridophores contain stacks of thin, transparent plates that reflect light in the way a soap bubble does, producing iridescent blues, greens and pinks. They can also reflect polarized light, which cuttlefish can see but people cannot.
• Leucophores contain tiny particles that scatter all colors of light equally, so they look white. Because they reflect whatever light falls on them, they take on the color of their surroundings, looking bluish in blue water and greenish over green weed.

These reflective layers work like a background screen. By opening or closing the chromatophores above them, a cuttlefish decides how much shimmer or bright white shows through, creating sharp white spots, bands and squares.

Changing skin texture with papillae

Color is only half of the disguise. A cuttlefish can also reshape its skin. Muscles in the skin raise small bumps and pointed flaps called papillae, which turn a smooth surface rough, spiky or frilly in a few seconds.

Combined with color, papillae let a cuttlefish resemble a lump of rock, a clump of seaweed or a patch of coarse gravel rather than an animal. It can also hold its arms in unusual postures to complete the disguise.

How do cuttlefish match their background?

Scientists who study the common cuttlefish group its camouflage into three main types:

• Uniform: an even color with little pattern, used over plain sand or mud.
• Mottle: small light and dark blotches, used over fine gravel or patchy ground.
• Disruptive: bold, high-contrast patches, including a white square on the back, that break up the animal’s outline among larger stones and shells.

Experiments with artificial checkerboards and gravel show that cuttlefish judge the size and contrast of light objects around them. When the light objects are roughly as large as the white square on their own back, they tend to switch to a disruptive pattern. When the objects are much smaller, they use mottle, and on a plain background they turn uniform. They also respond to edges and to objects next to them, not just to the ground beneath.

This ability starts early. Common cuttlefish can already move their chromatophores while still inside the egg, and they hatch ready to blend into the seabed.

The color blindness puzzle

The strangest part of the story is that cuttlefish are probably colorblind. Their eyes contain only one type of visual pigment, and in experiments published in 1996 and 2006, cuttlefish failed to tell colors apart when brightness was carefully matched. Yet they blend into colorful backgrounds remarkably well.

There are several possible explanations, and they may all play a part:

• Cuttlefish mainly match brightness, contrast and pattern, which matter most to many predators, and their white leucophores automatically pick up the colors around them.
• Their wide, W-shaped pupils might let them detect color indirectly, because different colors come into focus at slightly different distances. This idea, proposed in 2016, has not been confirmed.
• Light-sensitive proteins called opsins have been found in cuttlefish skin, raising the possibility that the skin itself can sense light. Whether this helps camouflage is still unknown.

Displays and signals

Cuttlefish do not use color only to hide. During the breeding season, male common cuttlefish show an intense black-and-white zebra pattern to rival males and to females. A frightened cuttlefish may suddenly turn pale and flash a pair of dark eyespots on its back, a startling display that can make a predator hesitate.

Cuttlefish can also send dark bands rippling across the body, a pattern known as a passing cloud. Scientists have suggested that it may startle small prey into moving, but its exact purpose is still debated.

How do cuttlefish compare with other color changers?

Octopuses and squid use the same basic system of chromatophores, iridophores and, in many species, leucophores, so the common octopus is just as quick to change. Among animals with backbones, chameleons and flatfish can also change color, but they take seconds to minutes. The full range of tricks is compared in animals that change color, and the cuttlefish’s place among mollusks is explained in Is a cuttlefish a fish?