Among the reefs' many marvels, stomatopods, or mantis shrimps, are the unrivaled visual masters, with the world's most complex eyes. Research by Marshall and marine biologists Tom Cronin, Roy Caldwell, and others has shown that stomatopod eyes have up to 16 separate kinds of light-sensing retinal cells, including four for UV light, plus sensitivity to patterns of polarization and exceptional spatial perception. (Humans have a paltry four retinal cell types and cannot see either UV or polarized light.) This intricate retina delivers visual information already processed to a shrimp's tiny brain, vastly reducing the work the brain has to do to interpret its world. Those compound eyes help the smashing peacock mantis shrimp locate prey. We watched one stare intently at a spot on the reef, using its powerful arms to smash at the rock again and again to reach a target we couldn't see.

The reef is a world where vision and color are clearly a matter of life and death for those wise enough to heed the message. One day I was not so wise. Bold colors can advertise danger, and most marine biologists are not so foolhardy as to reach out and grab an unfamiliar, brilliantly colored animal. But on a languid dive near Komodo, in a forest of soft corals, I spotted a gaily colored clown crab sitting on something I didn't recognize. I ignored the something and reached for the crab, who surprised me by holding his ground, unafraid. Now I know why. He could afford to stick out like a beacon because the something he was sitting on was his form of defense—a stinging hell's fire anemone. It took two weeks for the burn marks and pain to fade from my hand. Lesson learned.

Everywhere we went in the islands, anemones and corals bore bright pastel pigments that fluoresced brilliantly orange, red, or green. The molecules that create this fluorescence could serve as sunscreens, or as light absorbers to boost growth. But in some cases these colors can be co-opted by unrelated creatures. We saw one common coral with fluorescent pink splotches, which appear on damaged spots that are healing. Fish are attracted to the pink spots and bite at them. A small parasite has evolved to infest this coral, causing harm, which leads to more pink patches that attract fish. The fish nibble the spots, thus taking up the parasite and becoming its host. Even a small parasite has developed a way to use color for its own survival.

The world's coral reefs teach that color conveys information and can change over seconds or lifetimes. It can hide or reveal, warn or beckon, broadcast widely or target a select few. Science is beginning to crack these codes—vital knowledge that will help protect reef creatures and the fragile habitats they adorn so beautifully.