Pick from the Past

Natural History, September 1999

In Living Colors

“Living jewels adorned with polished scales of gold, encrusting
lapis lazuli, rubies, sapphires, emeralds, and amethysts”

Angel fish

Photo: ©iStockphoto.com/Martin Strmko

An eighteenth-century naturalist, enraptured by the colors of coral reef fishes (but less constrained by objectivity than his modern counterparts), once described them as living jewels adorned with “polished scales of gold, encrusting lapis lazuli, rubies, sapphires, emeralds, and amethysts.” Ever since Charles Darwin’s time, scientists have been treating those colors with a more analytical eye, trying to determine how they are produced and whether they relate to specific functions.

The nineteenth-century British naturalist Abbot Thayer insisted that virtually every animal’s color pattern could be explained as camouflage. Thayer proved to be correct about many such patterns, but camouflage explains only some of those found in reef fishes. In the early 1960s, Austrian ethologist Konrad Lorenz—convinced that many coral reef fishes use their bodies as living billboards to advertise their presence, identity, or sex—coined the term plakatfarben (poster colors). Lorenz was right on target for at least some reef fishes, including several species of angelfishes and butterflyfishes.

Some fishes do their advertising in the manner of electronic neon signs rather than static billboards. Like many animals, from octopuses to chameleons, numerous reef fishes can alter their body colors when they are ready to mate, eager to fight, frightened, or otherwise in need of camouflage. Rapid color changes are sometimes brought about by the nervous system, slower color changes by hormones.

Underwater color seems to have reached its peak on the tropical reef, where the uncommonly clear and dazzlingly blue waters are illuminated by strong sunlight to a depth of at least one hundred feet. The clarity and brightness have two important consequences for visual communication, involving the kinds of eyes that reef fishes have evolved and the color patterns they display.

All eyes work under the same functional constraints that cameras do: they need lots of light to form sharp, detailed, full-color images. Most rainbow-hued reef fishes are active only during the day, when the reef is brightly lit. These diurnal species—parrotfishes, angelfishes, wrasses, butterflyfishes—have color vision at least as acute as our own. Their eyes are generally small, and behind them is a thick layer of melanin that prevents light from bouncing around inside in ways that would degrade the visual image. But not all reef fishes are diurnal. Some, such as the formerly diurnal squirrelfishes and bigeyes, were forced (by competition over time) into the shadows, where they now spend their days lurking in caves and venturing out only under cover of darkness. Like most nocturnal mammals, these fishes see only in black, white, and shades of gray. Nocturnal eyes are quite large in proportion to body size and are backed by a reflective layer that maximizes their ability to detect what little light enters their oversize pupils. A third class of fishes, mostly predators such as groupers and barracudas, are active only at dawn and dusk. The eyes of these fishes fall somewhere between diurnal and nocturnal forms.

The excellent visibility on the tropical reef makes it possible for animals there to communicate using body colors and patterns in ways that aren’t possible in turbid streams or murky swamps. And the strong hue of the reef water has profound effects on the visibility of specific colors and color combinations.

Intriguingly, fishes generate a rainbow of colors in somewhat the same way that human artists do: by combining three primary pigments—red, yellow, and blue—plus black or white. They do so by the ingenious use of guanine, melanins, and carotenoids. Guanine, one of the most common organic compounds in nature, manifests itself in pure form as tiny flattened crystals, each of which reflects light like a microscopic mirror. The visual effect produced in fish skin by these crystals depends on how they are deployed. If guanine crystals are set up in parallel arrays, they reflect the light of most wavelengths in all directions, creating a brilliant white. Set the same crystals in different arrays—oriented at a particular angle to the skin surface and spaced the right distance from one another—and they set up optical interference phenomena. The result is a range of colors that even our best painters can’t quite duplicate: the shimmering violets and iridescent blues that dance across the bodies of minnows, tarpon, and marlin.

To make black, human artists once used soot and lampblack. Fishes rely on melanins. These compounds, ranging in color from brown to black, are produced in tiny granules called melanosomes; these granules move around within large branching cells called melanophores. Using melanins and guanine alone, many fishes create striking black-on-white patterns. Furthermore, in the time it takes a human to blush, a fish can cause melanosomes to disperse or bunch together within melanophores, lightening or darkening the areas they cover.

Until the age of synthetic pigments, human artists made reds, oranges, yellows, and browns by using vegetable dyes that derived their hues from plant compounds called carotenoids. Fishes, too, collect carotenoids, which they take from their food and funnel into pigment cells in their skin. Because of the way carotenoids absorb light, different concentrations of various compounds can produce hues ranging from bright yellow and fluorescent orange to deep red.

As for shapes, coral reef fishes have evolved some that are bizarre and diverse enough to inspire characters for a Star Wars prequel—the grouper’s movable "lip" bone, for example, which can instantly reshape its mouth into a tube that works like a vacuum cleaner. Over evolutionary time, the size and design of movable mouthparts have changed in ways that enable many different reef species to exploit a wide range of specific foods. Parrotfishes use small but sturdy jaws and teeth fused into a beak to chip away at algae-encrusted coralline rock; angelfishes and butterflyfishes use projectable jaws to nibble on sponges and small crustaceans; surgeonfishes and tangs have protruding mouths with sharp teeth that allow them to graze on the reef without scraping their heads.

Delighting the eye, the variety of colors and body forms among reef fishes speaks volumes about the roles their bearers play in the life of the coral metropolis.

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