The Colorblind Cuttlefish That Wears Every Color

A cuttlefish has exactly one type of photoreceptor in its retina, which by every standard definition makes it colorblind, and yet it can repaint its entire skin to match almost any background within a single second.

A paradox written into the retina

The retina of a cuttlefish contains a single class of photoreceptor. That is the kind of fact that, in any other animal, would close a question rather than open one. A single photoreceptor type means no comparative wavelength channels, no trichromatic discrimination, no way to tell a yellow rock from a red one the way a human eye can. By the standard biological definition, the animal is colorblind.

And yet, as the source pack notes plainly, "although cuttlefish cannot see color, they can perceive the polarization of light." The animal changes the pattern, the texture, and the color of its skin to match almost any background, often within a single second. The mismatch between what its eyes can read and what its body can broadcast is the central wonder of this creature, and it is not a small one. It sits at the heart of every modern paper on cephalopod camouflage.

The display is also denser than most people imagine. Cuttlefish skin can carry up to 200 chromatophores per square millimeter, and each of those tiny color organs can expand its surface area by up to 500 percent when activated. A single chromatophore stretches to about 1.5 millimeters across at its widest and shrinks to roughly 0.1 millimeters when retracted. The animal is, in effect, a high-resolution adaptive screen, and the screen runs faster than the eye that drives it has any right to manage.

What it is, and how it is built

There are 116 recognized cuttlefish species. Most of them are modest animals, with typical adult body lengths between 15 and 25 centimeters. The largest, the giant cuttlefish Sepia apama, reaches 50 centimeters in mantle length and weighs more than 10.5 kilograms, or 23 pounds, which makes it more closely the size of a small dog than the palm-sized creature most readers picture.

Inside, the anatomy is unambiguously cephalopod. Cuttlefish carry oxygen with hemocyanin, a copper-based protein that turns the blood a green-blue rather than red, and they push that blood through three hearts: two branchial hearts feeding the gills, and one systemic heart feeding the rest of the body. They begin actively mating at around five months of age, hatch out at about 6 millimeters in length, and reach roughly 25 millimeters after two months.

What sits above that anatomy is the part that has fascinated researchers for decades. The cuttlefish has, in the words of the source, "one of the largest brain-to-body size ratios of all invertebrates." That brain is doing two jobs at once: navigating the reef with one of the strangest visual systems in the animal kingdom, and driving a skin that behaves, at the cellular level, more like a display panel than a hide.

Three layers of optical cell

The skin works through three classes of optical cell, stacked and coordinated. The outermost are the chromatophores, which come in three pigment types: yellow-orange, red, and brown-black. Each chromatophore is an independent organ. It is built around a single pigment cell ringed by muscle, nerve, glial, and sheath cells, all of which exist to operate the pigment-bearing core.

What that core actually holds is an elastic sac called the cytoelastic sacculus. The pigment granules sit inside it. When the surrounding muscles contract, they stretch the sac open and reveal the color. When they relax, the sac snaps shut and the color disappears. This is mechanically unlike the color change of fish, amphibians, or reptiles, where pigment vesicles physically migrate inside the cell. In cephalopods, the cell stays put and the sac changes shape. That is why the change is so fast.

Beneath the chromatophores lie the iridophores, each generally smaller than one millimeter across, which produce iridescent structural color using crystalline plates of guanine rather than pigment. Below those sit the leucophores, which use crystalline purines to scatter ambient light back up through the layers above as a broadband reflector. Pigment, structural color, and a white backlight, all stacked, all addressable.

The control system is the part that turns this from clever skin into something closer to a screen. The nerves operating the chromatophores are arranged in the brain in a pattern that maps directly onto the chromatophores they control, so neural activation and skin pattern correspond one-to-one. Roughly 20 million neurons coordinate the full display. The whole apparatus is also expensive: as researchers note, "the energy cost of the complete activation of the chromatophore system is very high," nearly as much as a resting octopus expends.

Polarization, and a strangely shaped pupil

The eye, when you finally look at it, only deepens the puzzle. The cuttlefish carries rhabdomeric photoreceptors that are sensitive to the polarization of light, a property invisible to human eyes. It uses that sensitivity when hunting silvery fish, whose reflective scales strongly polarize light, and it broadcasts rapidly changing polarization patterns on its own skin for communication. Females, in particular, display stronger polarized signals than males. This is the secret channel the animal lives on.

But polarization is not color, and a polarization channel does not, on its own, explain how a colorblind animal manages to wear every color perfectly. That is where the leading hypothesis comes in. In 2016, the researchers Stubbs and Stubbs proposed that the cuttlefish may be reading hue not from its retina but from its optics. The pupil of a cuttlefish is a smoothly curving W-shape with a wide aperture. Different wavelengths of light, as a result, focus sharply at slightly different depths inside the eye, an effect called chromatic aberration. The hypothesis is that the brain reverse-engineers the surrounding hues from how sharply each wavelength comes into focus, reading color from the physical blur rather than from any pigment-based discrimination.

It is worth being careful here. This is a hypothesis, not a settled finding. The mantis shrimp Gonodactylus smithii, for context, is believed to have the most complete polarization vision known and measures all six orthogonal components of polarization. The cuttlefish is doing something quite different and almost certainly simpler at the retina, and stranger at the lens.

A short life, and a fading screen

The end is abrupt. Most cuttlefish live only about one to two years. They mate once. The female lays her eggs within a few hours of mating and guards the clutch, generally without feeding, until a rapid senescence ends her life. During that senescence, the very eyesight that made the species famous begins to fail.

Researchers are now studying the architecture of cuttlefish skin for adaptive military fabrics and flexible display technology, which is a long way from the reef. What the animal itself leaves behind is something stranger than any of those applications. Nature built one of the most mechanically sophisticated adaptive screens on Earth, commanded it with a colorblind eye, and gave the cuttlefish a single brief lifetime to wear every color before fading to black.

Sources

• Wikipedia: Cuttlefish
• Wikipedia: Chromatophore
• Wikipedia: Polarized light