Even though sea slugs, or nudibranchs, come in brilliant colors and patterns, Cousteau photographers must look sharply to spot them. Small, sometimes less than an inch long, and often nocturnal, these beauties are not easy to capture through a lens. Unlike many birds or fishes, the nudibranchs' flashy colors are not a mating signal.
Sea slugs have such poor eyesight they can barely distinguish light and shadow, much less patterns.
Their blues and reds and yellows are for defense, to help them hide or to warn a predator that they taste bad. In many cases, the sea slug's hues come from the color of its prey, as a kind of camouflage. One species turns the blue of its favorite sponge. Another takes on the pink of rosy bryozoans on which it feeds.