Facts: The Nudibranch

Nudibranchs are a group of sea slugs that can be found from the poles to the tropics in both shallow and deep water. They are most abundant in shallow, tropical waters. There are about 3000 species and new species are constantly being identified. Nudibranchs can be thick or flattened, long or short. They are often incredibly colorful since they derive their coloring from the food they eat. With no internal or external shell, they are the only marine slugs that always lose their shells as adults. They typically have a pair of sensory organs on their head called rhinophores. The rhinophores serve to pick up smells in their environment. Many species also have sensory tentacles near their mouth that help them feel their way around. They have tiny eyes that can only detect light and shadows. The order Nudibranchia is divided into four sub-orders. The most notable sub-orders are the dorids and the aelids. Dorids usually have feathery external gills on their backs. The feathery gills maximize surface area in order to extract oxygen from the water. The name nudibranch means naked gills. Aelid nudibranchs tend to be long, slender, and frilly. They lack gills. Aelids breathe with the tentacle-like structures that cover their backs. These structures are called serrata. Most nudibranchs spend their lives on the bottom. They use their muscular foot to crawl along the seafloor. Some species, like the Spanish dancer, can swim by flapping their bodies back and forth. Many nudibranchs have unique adaptations. For example, the blue sea dragon nudibranch spends its life floating upside down at the surface of the water, partially kept afloat by a gas-filled sack in its stomach. This gives blue sea dragons access to their prey, siphonophores. Nudibranchs are carnivores. Most feed with the help of their radula, also known as the rasping tongue. The mouth, which contains the radula, is on the underside of their head. The radula shreds prey into small pieces for eating. It is composed of rows of chitinous teeth. The old teeth are continuously replaced by new rows of sharp teeth. Most nudibranchs feed on sessile organisms such as sponges, anemones, hydroids, and corals. Some hunt other sea slugs. The egg-eating nudibranch feeds on the eggs of other sea slugs. Aside from members of their own order, nudibranchs have few natural predators. Brightly colored animals are often foul-tasting, toxic, or venomous. Nudibranchs are no exception. Since they lack shells, they have alternative defense mechanisms. The bright colors of some nudibranch species serve as a warning to potential predators. This is called aposematism. However, nudibranchs rarely produce poisons or venoms of their own. They often recycle the toxins found in their prey. For example, sponges produce a wide variety of poisons that the slugs concentrate in their bodies for their own defense. Eeled nudibranchs that feed on stinging animals like hydroids may ingest their prey's stinging capsules, called nematocysts, without discharging them. They pass the nematocysts through their bodies and store them at the tips of the tentacle-like cerata in receptacles called cnidosacs, ready to fire when necessary. Other nudibranchs rely on camouflage and nocturnal habits for protection. For example, the cup coral nudibranch closely resembles the orange cup coral that it feeds on. Some nudibranchs can even incorporate the symbiotic algae called zooxanthellae from their prey, therefore increasing their food supply through photosynthesis. Nudibranchs are simultaneous hermaphrodites. This means they have both male and female reproductive organs. They can mate with any other mature member of their species. The reproductive organs are on the right side of the body, so they must position themselves so that their right sides line up when mating. They lay spirally coiled egg ribbons on hard surfaces. Larval nudibranchs typically have a shell. The shell is lost during the transition to the juvenile stage. Some species live about 1 year. Some live less than a month. Depending on the species, they may grow anywhere from only a quarter of an inch to over one foot long. For more marine facts, click the SUBSCRIBE button!!