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.
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