Toxic nudibranchs—soft, seagoing slugs—produce a brilliant defense
Nudibranchs crawl through life as slick and naked as a newborn. Snail kin whose ancestors shrugged off the shell millions of years ago, 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 billowing deep-sea vents. Members of the gastropod class, and more broadly the mollusks, the mostly finger-size morsels live fully exposed, 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’ve also traded the family shell for less burdensome weaponry: toxic secretions and 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. Other nudibranchs hoard capsules of tightly coiled stingers, called nematocysts, ingested from fire corals, anemones, and hydroids. Immune to the sting, the slugs deploy the stolen artillery along their own extremities.
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.