PACI F I C OCEAN: Why Are Reef Fish So Colorful? Bright patterns on reef fish are key to astound- LIZARD ingly complex strategies to attract mates, repel ISLAND rivals and hide from predators by Justin Marshall strange event, I was stunned background—and also, it would seem, mak- to realize the black dots were ing them an obvious meal. I wondered how the eyes of an enormous the environment of the coral reef could CAIRNS school of kyphosids swim- have given rise to the virtually invisible HERON ming past on their way to drummer and frogfish as well as the highly ISLAND ) the reef edge. The bodies of conspicuous angelfish and butterfly fish. globe these fish, which are also known as drummers, are Extreme Biodiversity about 30 centimeters (nearly 12 inches) long and are a sil- t is such questions that occupy me on AUSTRALIA HAXBY ( ); WILLIAM F. very-blue color. When ver- Ifield trips to the University of Queens- map tical in water, they merged land’s research stations on Heron and Lizard perfectly with the dim, blue islands. These two islands are at either end light pervading the lagoon. of the 2,300-kilometer expanse of the Great LAURIE GRACE ( GRACE LAURIE Here was a wonderful ex- Barrier Reef (map), which is by far the ample of camouflage under- largest reef system in the world and right- trangely enough, I became water. I was humbled by my ineptitude as fully one of its seven natural wonders. The curiousS about the colors of fish not while a predator—I allowed literally tons of fish to huge expanse is a living area of 200,000 diving in the crystal-clear waters of Aus- pass within a meter or two of me and my square kilometers consisting of some 3,000 tralia’s Great Barrier Reef, surrounded by net before I even realized what they were. small reefs that include more than 400 spe- countless incredibly colorful fish. On the As a marine biologist interested in vision cies of hard and soft corals. For compari- contrary: I was in the murky, turbid wa- in the sea, however, I immediately thought son, a typical Caribbean reef might be tens ters of Heron Island’s Coral Cay Lagoon, of several questions. How is the skin of of kilometers long and have perhaps 40 near the southeastern edge of the reef, drummers so well adapted to merge with kinds of hard and soft corals. close to Shark Bay. the sea? What is it about the visual capa- Like terrestrial rain forests, coral reefs are Sitting slightly apprehensively at a depth bilities of the fish that prey on drummers isolated enclaves that are important for their of only two meters, I was trying to catch that enables them, presumably, to see the extreme biodiversity. In this respect, too, fish in a hand net. Suddenly I became dim- drummers while mine was so ineffective? the Great Barrier Reef is superlative: it is ly aware of hundreds of little black dots I noticed that many of the fish and other home to around 1,500 species of fish. This shooting past me almost at the limits of my reef creatures that the school had by now huge variety is all the more surprising in vision in the silty water. Sucking air through joined were boldly colorful, their bright light of the relatively young age of the my dive regulator and pondering this patterns making them pop out from the reef. It began to form 12 to 18 million years JUSTIN MARSHALL BARBARA BURGER JUSTIN MARSHALL 54 Scientific American Presents Why Are Reef Fish So Colorful? COPYRIGHT 1998 SCIENTIFIC AMERICAN, INC. Compared with some species of fish and FISH EYE of this beaked leatherjacket sees other creatures, humans are relatively color- the light spectrum in a slightly different way blind. People have three color receptors in than a human eye does. their eyes: the blue-, green- and red-sensi- tive cones. Some reef fish (and indeed am- 57] and the school of kyphosids I saw in the phibians, reptiles, birds and insects) possess Heron Island lagoon. But to attract a mate, four or more. The record is currently held chase rivals away or provide other warn- by the mantis shrimp (a stomatopod), a reef BY JUSTIN MARSHALL PHOTOGRAPHS ings, bright colors that are easily seen un- dweller whose eyes have 12 color recep- derwater may be the order of the day. In tors. With these additional receptors, the the blue waters of the reef, the colors yel- animals can see the region of the near ul- low and blue travel the farthest, so many traviolet, with wavelengths between about reef animals have evolved bodily patterns 350 and 400 nanometers (humans cannot of yellow and blue in striped or spotted see wavelengths shorter than about 380 combinations. Because yellow and blue nanometers). Also, they can see in greater ago and in some places is only two million are also widely separated in the spectrum, detail some of the colors humans see. years old; reefs of the western Atlantic and they offer strong contrast underwater. Such impressive visual capabilities might central Pacific formed 25 million years ago. Just what does a butterfly fish look like to seem to be unnecessary on the reef, where The diversity of colored fish and inverte- another butterfly fish? How does a drum- so many creatures have evolved bold pat- brates on the Great Barrier Reef is truly mer appear to a shark? It is this goal to un- terns that emit strong visual signals. Alterna- awe-inspiring. Yet the color patterns exhib- derstand color vision and its evolution tively, it may seem incredible that these ited by these inhabitants did not evolve for from the point of view of the animals them- brightly colored fish manage to survive human eyes. The brilliant blue spots of the selves that my colleagues and I at the Uni- with markings so striking that they would semicircle angelfish, or the contrasting yel- versity of Queensland’s Vision, Touch and seem to attract the attention of even weak- low and blue fins of the yellowtail coris Hearing Research Center are striving to- eyed predators. wrasse [see illustrations at top of pages 56 and ward at present. Our research has revolved Could it be that coral reefs are colorful, 57], are a vital component of the survival around three critical questions: One, what and therefore that colorful animals fit in and strategies of these species on the reef. are the animals’ visual capabilities? To ex- may even be camouflaged? Logical though To understand this role of color and ap- plore this matter, we are carrying out ex- it may seem, the notion does not hold up pearance requires some understanding of periments in which we are quantifying to scrutiny. A reef stripped of its fish and survival on the reef and also of the optics of colors nonsubjectively, using the world’s other mobile life-forms is actually relative- the undersea realm. At its most fundamental first underwater spot-reflectance spectro- ly monochromatic. Most of the corals are level, survival for any animal species de- radiometer. Two, what are the light and brown or green, their colorful splendor mands three things: eating, not being eaten surroundings like in the habitat where these coming out only at night when the polyps and reproducing. Unfortunately for sea creatures live? Experienced divers know open or under the falsely bright illumina- creatures, the demands on appearance im- that seawater is so blue that all red light is tion of the camera strobe or video light. posed by the first two of these survival re- absorbed within 20 meters of the surface; a Another possible explanation revolves quirements conflict with those of the third. bright-red fish at this depth therefore ap- around disruptive coloration, a principle A good way to avoid being eaten, or, in- pears black. And three, under what cir- first described in detail in the 1940s and deed, to lurk undetected while waiting for cumstances, and to what other creatures, subsequently used for military camouflage. prey to swim by, is to be camouflaged to do fish show off their color patterns? The central idea is the use of large, bold pat- match the background (the scientific term Clearly, displaying bright colors to impress terns of contrasting colors that make an is “cryptic”). Masters of camouflage include a potential mate would be unwise when object blend in when viewed against an the frogfish [see illustration at bottom of page visually guided predators are lurking nearby. equally variable, contrasting background. STRIKINGLY PATTERNED REEF FISH include (from far left) the thread- whose vertical stripes make it difficult to discern head from tail. Bright fin butterfly fish, which has a “false eye” above the caudal fin to con- patterns also appear on the dorsal fin of the yellowtail coris wrasse fuse predators; the humphead Maori wrasse; and the Moorish idol, (below left) and on the anal fin of the regal angelfish (below). Why Are Reef Fish So Colorful? The Oceans 55 COPYRIGHT 1998 SCIENTIFIC AMERICAN, INC. FISH COLORS include vivid patterns, as seen above (from left to right): the midbody of a The light and dark branches, pockets and beaked leatherjacket; nose of a surf parrot fish; tail of a yellowtail coris wrasse; dorsal fin of a shafts of light on a reef provide just such a regal angelfish; midbody of a royal dottyback; fin of a regal angelfish; cheek of a harlequin background. tuskfish; dorsal fin of a harlequin tuskfish; tail of a semicircle angelfish; and another fin of a Good examples of disruptive camou- regal angelfish.