Gaze at the vivid yellows, blues, and psychedelic swirls of a single emperor angelfish and you'll sense the whimsy of evolution. Go on to explore its home in lush coral reefs and you'll soon hit sensory overload, assaulted by colors and patterns that range from sublime to garish. Coral reefs are unquestionably the world's most colorful places. But why?
Scientists have long known that color plays a role in sexual selection and warning of danger. But only in the past decade or so have we begun to understand how wavelengths of light (and therefore color) appear at different depths and how various marine creatures' eyes perceive this light and see each other—far differently than humans see them.
To document how reef animals use color, I joined photographer Tim Laman for a total-immersion course off Fiji and Indonesia. It was an eye-opener, with virtuoso dis-plays of color at every turn. Beyond the world's reefs, where waters are turbid or murky, most creatures use nonvisual means of communication such as smell, taste, touch, and sound. But in the clear, sunlit waters of coral reefs, light abounds, vision predominates, and animals—both sighted and blind—drape themselves in blazing color not only to entice mates or threaten foes but also to advertise their services, evade predators, catch prey, even hide in plain sight.
Tim and I began our work by studying the healthy reef systems of Fiji (National Geographic, November 2004). Drifting 80 feet below the sea's surface, Tim aimed his strobe lights at a patch of reef to reveal brilliant shades of red on coral branches. But when we flicked off the artificial light, we saw the reef more as fish would see it—and it was a different world. Pale blues, greens, violets, and yellows met our gaze. The red was no longer visible, its longer wavelengths absorbed by water molecules and debris. Red pigments on marine animals may simply function as gray or black at depth; why they even have a red pigment we don't know. But we are beginning to understand more about the yellows and blues that so dominate the wardrobe of reef fish—and help make them prized targets of collectors.
Justin Marshall of the University of Queensland in Australia, George Losey of the University of Hawaii, and their colleagues study fish eyes. Using a technique called microspectrophotometry, they've analyzed the visual pigments and photo-sensitivity of various reef-fish eyes to determine how and what fish see. They've also measured the wavelengths of light reflected off reef features to calculate an "average reef color." It turns out that in natural light the yellows and blues that adorn many damselfish, wrasses, and angelfish blend well with that average reef background, providing camouflage from predators.
We witnessed the phenomenon of brightly colored fish hiding in plain sight throughout Indonesia, home to the highest marine diversity on Earth. In a tiny spot just southeast of Sulawesi, clouds of colored fish swam against a collage of vivid invertebrates encrusting the reef. With such an excess of pattern and color, no one creature stood out. Up close, regal angelfish flashed eye-popping bands of yellow, violet, and white. But recent studies show that as regals swim against the reef's visually complex background, their contrasting lines merge in a predator's brain. According to Boston University marine biologist Gil Rosenthal, as a reef fish retreats, distance and motion can make it difficult for predators to perceive fine details and distinguish closely spaced outlines of contrasting colors. So at a distance, spots and stripes blur together, helping even stationary fish merge into the background of the reef and the ocean beyond.
Sulawesi is rich in cephalopods—octopuses, squids, and cuttlefish—which have the biggest brains and most mercurial colors of all the invertebrates. We got to know one octopus particularly well. It spent its days systematically moving from one outcrop to another, probing for prey with serpentine armtips thrust deep into coral crevices. Just before jumping to a new spot, it would darken (except for one bold white stripe), then crash to the ground with arms outstretched, the webbing between them blocking off routes of escape for small creatures such as hermit crabs caught under its body. The webbing would then turn a nearly transparent white. To us—and perhaps to trapped prey—these white patches looked like windows of light and escape. We speculate that this color-change act is a ruse to lure small, cowering animals up to the "windows" and thus toward the octopus's mouth.
When at rest, this octopus became camouflaged against the reef, with shifting patterns of dark and light on its skin that matched the texture and color of the backdrop. This appears to be an impressive trick, given that octopuses are color-blind: Their retinas lack the cells that receive and process color. But apparently these animals get by without color vision, simply responding to contrasts of shade and light.
Useful in deception, color can also speak the language of love for reef creatures. But it's a quick chat. Many reef fish can blink their colors on and off in seconds, as we saw near the coast of Bali. Rising toward the shallows through a cloud of flasher wrasses, we watched the males shoot neon blue stripes across their bodies and outstretched fins, creating a miniature laser-light show. Spurred to passion by a male's display of lights, a female rose in the water column with her chosen suitor and released an explosive burst of eggs to mix with his sperm. Job done, the male instantly went drab, and the consummated pair sped to the safety of the reef. That moment of electric bliss must have exposed them to great risk from predators, so the ability to turn off color was just as important as turning it on.
The mechanism for this quick-change act is a class of skin cells called chromatophores. Controlled by both neurons and hormones, chromatophores create the appearance of color or pattern through pigments and light manipulation. Specialized chromatophores called leucophores render skin pale. To produce blue and iridescent colors like those used by the flasher wrasse, iridophores manipulate crystals of guanine, a common metabolic waste product, to scatter white light and then reflect specific wavelengths as needed. Such cells can instantly brand their bearers as terrifying, invisible, or irresistible.
With the right lighting and a bit of luck, humans can witness these vivid displays. But there's a lot that we'll never see, due to the limitations of human sight. Sailing along an island chain called Nusa Tengarra, Tim and I observed turbulence along the seam between the Pasic and Indian Oceans. This fertile mixing zone is rich with plankton, and the roiling water was jammed with plankton-feeding fish massing below the surface. We dived among great crowds of them. Clearly they were eating something—we could see their high-speed jaws flashing—but how did they spot their prey, zooplankton, which was white and all but transparent to us? Thanks to years of work by biologists George Lose, Justin Marshall, Bill McFarland, and their students, we now know that many plankton-eating fish can see ultraviolet light, which makes the zooplankton appear black and therefore more visible in the water. Humans can't see UV, and until fairly recently we thought UV light was virtually absent below the waves. We now know that UV can penetrate to depths beyond 300 feet, and that some fish not only see UV but also paint their bodies with UV reflectors to beam out messages to their kin. Damselfish, for instance, shout out to each other in UV, but their predators can't see it. Such findings make me wonder how much of the undersea world our own eyes miss.
Among the reefs' many marvels, stomatopods, or mantis shrimps, are the unrivaled visual masters, with the world's most complex eyes. Research by Marshall and marine biologists Tom Cronin, Roy Caldwell, and others has shown that stomatopod eyes have up to 16 separate kinds of light-sensing retinal cells, including four for UV light, plus sensitivity to patterns of polarization and exceptional spatial perception. (Humans have a paltry four retinal cell types and cannot see either UV or polarized light.) This intricate retina delivers visual information already processed to a shrimp's tiny brain, vastly reducing the work the brain has to do to interpret its world. Those compound eyes help the smashing peacock mantis shrimp locate prey. We watched one stare intently at a spot on the reef, using its powerful arms to smash at the rock again and again to reach a target we couldn't see.
The reef is a world where vision and color are clearly a matter of life and death for those wise enough to heed the message. One day I was not so wise. Bold colors can advertise danger, and most marine biologists are not so foolhardy as to reach out and grab an unfamiliar, brilliantly colored animal. But on a languid dive near Komodo, in a forest of soft corals, I spotted a gaily colored clown crab sitting on something I didn't recognize. I ignored the something and reached for the crab, who surprised me by holding his ground, unafraid. Now I know why. He could afford to stick out like a beacon because the something he was sitting on was his form of defense—a stinging hell's fire anemone. It took two weeks for the burn marks and pain to fade from my hand. Lesson learned.
Everywhere we went in the islands, anemones and corals bore bright pastel pigments that fluoresced brilliantly orange, red, or green. The molecules that create this fluorescence could serve as sunscreens, or as light absorbers to boost growth. But in some cases these colors can be co-opted by unrelated creatures. We saw one common coral with fluorescent pink splotches, which appear on damaged spots that are healing. Fish are attracted to the pink spots and bite at them. A small parasite has evolved to infest this coral, causing harm, which leads to more pink patches that attract fish. The fish nibble the spots, thus taking up the parasite and becoming its host. Even a small parasite has developed a way to use color for its own survival.
The world's coral reefs teach that color conveys information and can change over seconds or lifetimes. It can hide or reveal, warn or beckon, broadcast widely or target a select few. Science is beginning to crack these codes—vital knowledge that will help protect reef creatures and the fragile habitats they adorn so beautifully.