Nudibranchs crawl through life as slick and naked as a newborn. They are just skin, muscle, and organs sliding on trails of slime across ocean floors and coral heads the world over.
Found from sandy shallows and reefs to the murky seabed nearly a mile down, nudibranchs thrive in waters both warm and cold and even around deep-sea vents. Members of the gastropod class, and more broadly the mollusks, their gills forming tufts on their backs. (Nudibranch means "naked gill," a feature that separates them from other sea slugs.) Although they can release their muscular foothold to tumble in a current—a few can even swim freely—they are rarely in a hurry.
So why, in habitats swirling with voracious eaters, aren't nudibranchs picked off like shrimp at a barbecue? The 3,000-plus known nudibranch species, it turns out, are well equipped to defend themselves. Not only can they be tough-skinned, bumpy, and abrasive, but they release toxic secretions and have stinging cells. A few make their own poisons, but most pilfer from the foods they eat. Species that dine on toxic sponges, for example, alter and store the irritating compounds in their bodies and secrete them from skin cells or glands when disturbed.
From :www.nationalgeographic.com