aN!maL aNd CoLoR

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.

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.