Nudibranchs are a widespread and successful group of marine Gastropod molluscs. The name means 'naked gills'. They are shelless and uncoiled Gastropods, famous for their brilliant colours. There are more than 3000 known species.
Nudibranchs are one of the groups which are informally known as sea slugs. This is a term which includes other Gastropod groups which look similar to nudibranchs. Sea slugs is an informal term; it is not a monophyletic group.
The body forms of nudibranchs vary greatly. They are opisthobranchs, a clade which shed their shells after the larval stage.
Unlike most other gastropods they are bilaterally symmetrical. They have undergone secondary detorsion. Most species have venomous appendages on their sides. These are used to deter predators. Many also have a simple gut and a mouth with a radula.
Unusually for molluscs, they lack a mantle cavity. Nudibranchs are hermaphrodite, and thus have a set of reproductive organs for both sexes, but they rarely fertilize themselves.
Most nudibranchs are carnivorous. Some feed on sponges, others on polyps, others on bryozoans, and some eat other sea slugs, or, on some occasions, members of their own species. Other groups feed on tunicates, barnacles, or anemones.
The surface dwelling nudibranch, Glaucus atlanticus is a specialist predator of jellyfish, such as the Portuguese Man o' War. This predatory mollusc sucks air into its stomach to keep it afloat and using its muscular foot it clings to the surface film. If it finds a small victim Glaucus simply envelops it with its large mouth, but if the prey is a larger siphonophore the mollusc nibbles off its fishing tentacles, the ones carrying the most potent nematocysts. Like some others of its kind Glaucus does not digest the nematocysts; instead, it uses them to defend itself by passing them from its gut to the surface of its skin.
Colours and defenceEdit
Among this group can be found the most colourful creatures on earth. In the course of evolution, sea slugs have lost their shell, and have developed other defence mechanisms. Their anatomy may resemble the texture and color of the surrounding plants, giving them camouflage (crypsis). Many have an intense and bright colouring, which warns that they are distasteful or poisonous (warning colouration).
Nudibranchs that feed on hydroids can store the hydroids' nematocysts (stinging cells) in the dorsal body wall. The nematocysts wander through the alimentary canal without harming the nudibranch. Then, the cells are brought to specific places on the creature's hind body. Nudibranches can protect themselves from the hydroids and their nematocysts. It is not yet clear how, but special cells with large vacuoles probably play an important role. They can also take in plants' chloroplasts (plant cell organelles used for photosynthesis) and use them to make food for themselves.
Another method of protection is the release of a sour liquid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the slime automatically.
The taxonomy of the Nudibranchia is still under investigation. Meanwhile, the nudibranchs are commonly divided into two main types, dorid nudibranchs and aeolid or eolid nudibranchs: 
- Dorids are recognised by the branchial (gill) plume, which forms a cluster on the posterior part of the body, around the anus.
- Aeolids have cerata on their backs, instead of the branchial plume. Some are hosts to zooxanthellae, which are endosymbiont brown dinoflagellates of the genus Symbiodinium.
- ↑ Uncoiling of the typical gastropod body shape, see garden snail.
- ↑ Stinging cells
- ↑ Piper, Ross 2007. Extraordinary animals: an encyclopedia of curious and unusual animals. Greenwood Press.
- ↑ Frick K. 2003. Predator suites and Flabellinid Nudibranch nematocyst complements in the Gulf of Maine. In: SF Norton (ed). Diving for Science. Proceedings of the American Academy of Underwater Sciences, 22nd Annual Scientific Diving Symposium. url=http://archive.rubicon-foundation.org/4744 Archived 2011-01-06 at the Wayback Machine
- ↑ Hans Bertsch, Nudibranchs: Marine slugs with verve. Navanax inermis[..] is the bane of all nudibranchs, because it is one of the few known predators on this group of slugs. [...] Dorids mainly eat sponges, bryozoans and tunicates, whereas eolids principally eat cnidarians.
- ↑ "Astounding Nudibranch Facts". Archived from the original on 2012-11-18. Retrieved 2012-12-13.