When at rest, this octopus became camouflaged against the reef, with shifting patterns of dark and light on its skin that matched the texture and color of the backdrop. This appears to be an impressive trick, given that octopuses are color-blind: Their retinas lack the cells that receive and process color. But apparently these animals get by without color vision, simply responding to contrasts of shade and light.
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.