Nudibranchs are a type of invertebrate species belonging to the Gastropoda class. They are carnivorous bottom-dwellers and shell-less mollusks belonging to the same family as sea slugs. Nudibranchs are primarily a saltwater species that can be found in all oceans from the intertidal zone to the deep ocean. They do seem to prefer shallow tropical waters, and there are rare instances of nudibranchs being spotted in brackish water. There are more than 3000 known species of nudibranchs that can be separated into four subgroups. The bulk of nudibranch species fall into the dorid and eolid subgroups. All nudibranchs are generally jelly bodied and oblong in shape. A fleshy foot helps them move, and they sport an exposed gill on their back. The name nudi branch translates to “naked gill.” They have two highly sensitive tentacles on the top of their head, rhinophores, which they utilize to detect olfactory chemical cues in the water. This helps them identify prey and mates. Rhinophores are unique to both nudibranchs and sea hares. Beyond that, species vary in thickness, length and color.
Dorid nudibranchs have a smooth, thick mantle with a broad and flat silhouette. Eolid nudibranchs are slim with a thin mantle covered by appendages called cerata. These cerata may present themselves as spines, ridges or pustules. They are multifunctional, acting as both gills and extensions of the digestive system. When in danger, the cerata may also be severed to act as a decoy for predators. Cerata can actually store space for ingested toxins consumed from prey. Various nudibranch species are capable of utilizing the toxins from prey they consume as a self-made defense against predators. Some nudibranch species can create their own personal toxins as well. Vibrantly colored nudibranchs are often a reflection of what they consume and whether they are dangerous to consume themselves. Some nudibranch species are also capable of storing stinging nematocysts from jellyfish and hydroids. Finally, nudibranchs may consume and utilize corals’ photosynthetic zooxanthellae. Nudibranchs do a lot of grazing, but can actively overpower anemones, corals and even other nudibranchs. Cannibalism only seems to be the result of overcrowding and desperation.
Depending on the species, a nudibranch can grow between 0.25 and 12 inches in length and weigh up to 3.3 lbs. They live a short life span of 1 month to 1 year. All nudibranchs are hermaphrodites. They line up their bodies with another mature individual, and each nudibranch will deposit its sperm into the other’s eggs. The eggs are laid in long ribbons or strings, sheathed in protective mucus. A high diversity of nudibranchs is a good sign of a healthy ecosystem. It’s difficult to say how many species are currently threatened at the moment. There are some species becoming more rare and difficult to find in already established territories. Ocean acidification does appear to cripple nudibranch larvae. Nudibranch distribution changes are often the result of climate change, human disturbance and habitat destruction. Nudibranchs that consume tunicates also suffer from coral reef destruction.