Many mobile nudibranchs—vulnerable as they move in daylight between feeding spots—announce their weapons with garish color designs, a palette millions of years in the making. Contrasting pigments make them highly visible against a reef's greens and browns, a visual alarm that turns predators wary—bold nibblers quickly learn to avoid the color patterns that announce unpalatable flesh. Animals able to mimic the designs, including nontoxic nudibranchs and other invertebrates like flatworms, are similarly left alone.
More reclusive nudibranchs, with nocturnal habits or small ranges, may opt for camouflage, from drab to brilliant, rather than contrast (although many of these, too, have toxic defenses). Pigments matching sponges and other edible substrates on which they linger can make even the biggest slug varieties—the length of a man's forearm—vanish where they lie.
Even the most keen-eyed diver may miss those cryptic species. But the brazen ones pop into view in bursts of Crayola colors, one munching coral, another glomming on to a rock face, a third riding a current along the seabed. A lucky sighting is a mass aggregation of dozens or even hundreds gathered at a food-rich locale to feed and mate, or a plate-size "solar powered" species that gets nutrients from photosynthetic algae farmed within its body.
Nudibranchs are blind to their own beauty, their tiny eyes discerning little more than light and dark. Instead the animals smell, taste, and feel their world using head-mounted sensory appendages called rhinophores and oral tentacles. Chemical signals help them track food—not just coral and sponges but barnacles, eggs, or small fish—and one another. Hermaphroditic, nudibranchs have both male and female organs and can fertilize one another, an ability that speeds the search for mates and doubles reproductive success. Depending on the species, pairs may lay eggs in coils, ribbons, or tangled clumps, up to two million at a time.
Not all adult encounters have such a fruitful outcome. Sometimes one nudibranch eats the other, particularly if it is of another species. A cannibal slug rears up like a cobra to engulf its kin, using jaws and teeth to finish the job. Other nudibranchs rely on enzymes, rather than teeth, to break down prey. What else can devour a nudibranch without ill consequence? Certain fish, sea spiders, turtles, sea stars, a few crabs. Some people consume them as well, after removing the toxic organs. Chileans and islanders off Russia and Alaska roast or boil sea slugs or eat them raw. (Photographer David Doubilet likened the experience to "chewing an eraser.")
Humans have also studied sea slugs' simple nervous systems for clues to learning and memory and have raided their chemical armory in search of pharmaceuticals. Fashioning remedies from marine invertebrates has a long history: Pliny the Elder, for example, wrote in the first century A.D. of using ground snails mixed with honey to treat "ulcerations of the head" and sea urchin ashes for baldness. Scientists today are isolating chemicals that may help ailing heart, bone, and brain. A sea hare (cousin to the nudibranch) recently offered up a cancer-fighting compound that made it into clinical trials.
Still, nudibranchs have hardly given up all their secrets. Scientists estimate that they've identified only half of all nudibranch species, and even the known ones are elusive. Most live no more than a year and then disappear without a trace, their boneless, shell-less bodies leaving no record of their brief, brilliant lives.