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Coral Reef Color
MAY 2005
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Coral Reef Color
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By Les Kaufman
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Photographs by Tim Laman
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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.
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