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| Sunday, 14 November 2004 |
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Chameleons of the reef A lot of things can be said about a shirt that sports images of coral-reef fish, but 'subtly coloured' isn't one of them. Oddly enough, that characterisation does get used when biologists talk about the reef creatures. Although the fish may dazzle the human eye with scarlet, rose, yellow, turquoise, emerald, and dozens of other shades, some theorists have proposed that, in the complexity of a reef, the riot of fish colours serve as camouflage. Popular hues Calculations based on fish vision suggest that from a distance, yellows blend in with a generic reef background, and the blues fade into vistas of water. A fish among branching corals would be hidden like a soldier wearing camouflage. Then again, some scientists have suggested the opposite notion-that brilliant colours send big, bold messages that may be come-ons or warnings or both. People can theorise till the cowfish come home about what they see on a reef, but what matters is what fish see, and that's been hard to determine. Improvements in cameras and in equipment for analysing light and colour are now inspiring new approaches to approximating a fish-eye view of the reefs. Looking at the abundant colouration from a fishy perspective, the new work demonstrates that people can be quite wrong about what's showy and what's subtle. The old questions are giving way to more-sophisticated new ones. Colours aren't just a matter of either hiding or flaunting. It may be possible to whisper and shout at the same time. "My interest is, what's all the colour about?" says N. Justin Marshall of the University of Queensland in Brisbane, Australia. A child of marine biologists, he says he was 6 years old when first dazzled by the magnificent underwater hues. He has since joined a long line of reef watchers musing over the dramatic displays. One proposal had colors developing through sexual selection, the process favouring traits that bewitch a mate. However, the fact that colours often look the same on males and females undercut that notion. Another hypothesis was that the visual drama on a reef represents conspicuous warnings that certain fish bear distasteful or toxic flesh: "Eat at your own risk." This idea dwindled in popularity as a widespread explanation when reef viewers recognised that many brightly coloured fish are tasty and quite safe. Another approach suggested that the colours represent quirks of fish metabolism. A fish might be shunting excess, colourful products of biological processes to its skin just to get them out of the way. That view hasn't found supporting evidence. When Austrian animal behaviorist Konrad Lorenz turned to reef colors during the middle of the past century, he proposed that the fish colours act as bold colour-coding that lets the abundant fish species sharing a reef keep track of who's who. "It's a very good idea," says Marshall. People may not appreciate the difficulty of identifying species mates "because we're not living among a hundred other hominids." But perhaps the colours that stand out to us actually work as camouflage for fish. Marshall argues that people analysing fish colouration need to consider what fish look like to each other, not to us. Human visual powers do well in distinguishing many kinds of yellows from greens. "We're very good at bananas," says Marshall. A reef fish's visual system, however, typically doesn't pick out fine distinctions in the yellows but is especially sensitive to shades of blue green. Also, almost half the fish analysed so far, says Marshall, seem to see ultraviolet light (UV). Species with small body size, such as damselfish, are more likely to perceive UV than big, fast-moving predators. Jill Zamzow of the University of Hawaii in Kaneohe reports that the light-screening mucus covering reef fish typically includes UV-absorbing compounds picked up from food. However, her videotapes and calculations indicate that the sunscreen should not keep fish from seeing each other in the UV range. Testing stealth To switch to a fishy perspective, consider the many yellow fish, such as the yellow form of trumpet fish (Aulostomus chinensis). "To us, they're bananas," standouts against the blue green of reefs, Marshall says. At the wavelengths in which fish see the world, the yellow of a trumpet fish swimming along 3 metres or more away becomes a "very good match" for the average reef background, says Marshall. He, George Losey of the University of Hawaii at Manoa, and their colleagues recently pieced together analyses of fish-eye pigments and measurements of background colors in various reef habitats, to perceive a bay at the island of Oahu the way its fish do. They described their work in a series of three articles on the visual biology of Hawaiian reefs, published last year in Copeia. These researchers measured the wavelengths bouncing off various parts of the reef to come up with what they call an average reef colour. They found, for example, that a common light-blue colour, familiar to fish fanciers in the bands on the blue-and-yellow angelfish Pygoplites diacanthus, matches the general bluish background a fish sees when looking into the distance through relatively deep water. "What's surprising is that some of the colorus that look bright to us are for camouflage," Marshall says. Evolutionary forces aren't pushing fish toward conspicuous colors, according to the findings of Gil Rosenthal of the Boston University Marine Program. He uses Panamanian geography to evaluate evolutionary pressure. The Isthmus of Panama formed a complete bridge between North and South America some 3 million years ago. Today, on the west of the Isthmus, in the Pacific, upwelling currents churn nutrients that feed clouds of plankton, but reefs don't flourish. To the east, in the Caribbean, waters flow clearer and multicoloured reefs thrive in the brighter environment. Many fish species on one side of the divide have close relatives on the other. Rosenthal and his colleagues measured the reflectivity of captured fish and calculated its effect under dim, blue-green Pacific light or whiter Caribbean illumination, he explained last June in Oaxaca, Mexico, at the annual meeting of the Animal Behavior Society. Science news |
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