|Spanish shawl, Flabellina iodinea|
Spanish shawl, Flabellina iodinea
See text for superfamilies
A nudibranch (pronounced /nudɪbɹaŋk/) is a member of one suborder of soft-bodied, shell-less marine opisthobranch gastropod mollusks, which are noted for their often extraordinary colors and striking forms. The suborder Nudibranchia is the largest suborder of heterobranchs, with more than 3,000 described species.
Nudibranchs are often casually called "sea slugs", a non-scientific term which has led some people to assume that every sea slug is a nudibranch. However, while it is true that nudibranchs are very numerous in terms of species, and are often very attractive, there are numerous other kinds of sea slugs belonging to several taxonomic groups that are not very closely related to nudibranchs. A fair number of these other sea slugs are colorful and thus are even more easily confused with nudibranchs.
(Other marine shell-less gastropods or "sea slug" groups include additional heterobranch shell-less gastropod groups such as the Cephalaspidea sea slugs including the colorful Aglajidae, and other heterobranchs such as the Sacoglossa, the sea butterflies, the sea angels, and the often rather large sea hares. The term sea slug is also sometimes loosely applied to the only very distantly related, pelagic, caenogastropods within the superfamily Carinarioidea, and may also be casually used for the even more distantly related pulmonate sea slugs, the Onchidiidae.)
Nudibranchs occur worldwide.
Nudibranchs live at virtually all depths of salt water, but reach their greatest size and variation in warm, shallow waters.
They lack a mantle cavity.
They vary in adult size from 20 to 600 mm.
The adult form is without a shell or operculum (a bony or horny plate covering the opening of the shell, when the body is withdrawn).
The name nudibranch is appropriate, since the dorids (infraclass Anthobranchia) breathe through a branchial plume of bushy extremities on their back, rather than using gills. By contrast, on the back of the aeolids in infraclass Cladobranchia there are brightly colored sets of tentacles called cerata.
Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odors.
Nudibranchs are hermaphroditic, and thus have a set of reproductive organs for both genders, but they can rarely fertilize themselves.
Nudibranchs typically deposit their eggs within a gelatinous spiral. 
Nudibranchs are carnivorous. Some feed on sponges, others on hydroids, others on bryozoans, and some are cannibals, eating other sea slugs, or, on some occasions, members of their own species. There is also a group that feeds on tunicates and barnacles. They even sometimes feed on anemones.
Colors and camouflage
Among this group can be found the most colorful creatures on earth. In the course of evolution, sea slugs have lost their shell because they have developed other defense mechanisms. Their anatomy may resemble the texture and color of the surrounding plants, allowing them to camouflage (cryptic behavior). Others, as seen especially well on Chromodorids, have an intense and bright coloring, which warns that they are distasteful or poisonous (aposematic coloration).
Nudibranchs that feed on hydroids can store the hydroid's nematocysts (stinging cells) in the dorsal body wall, the cerata. The nematocysts wander through the alimentary tract without harming the nudibranch. Once further into the organ, the cells are brought to the specific placements on the creatures hind body via intestinal protuberances. 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 way 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. Many taxonomists in the past treated the Nudibranchia as an order, based on the authoritative work of Johannes Thiele (1931), who built on the concepts of Henri Milne-Edwards (1848). Newer insights derived from morphological data and gene-sequence research, have confirmed these ideas. On the basis of investigation of 18S rDNA sequence data, there is strong evidence for support of the monophyly of the Nudibranchia and its two major groups, the Anthobranchia/Doridoidea and Cladobranchia.
- Infraorder Anthobranchia Férussac, 1819 (dorids)
- Infraorder Cladobranchia Willan & Morton, 1984 (aeolids)
A study published in May 2001, has again revised the taxonomy of the Nudibranchia . They are thus divided into two major clades:
- Anthobranchia (= Bathydoridoidea + Doridoidea)
- Dexiarchia nom. nov. (= Doridoxoidea + Dendronotoidea + Aeolidoidea + “Arminoidea”).
The dorids (infraorder Anthobranchia) have the following characteristics: the branchial plume forms a cluster on the posterior part of the neck, around the eyes. Fringes on the mantle do not contain any intestines.
The aeolids (infraorder Cladobranchia) have the following characteristics: Instead of the branchial plume, they have cerata. They lack a mantle. Only species of the Cladobranchia are reported to house zooxanthellae.
Where to view nudibranchs
- Klussmann-Kolb A (2001). "The Reproductive Systems of the Nudibranchia (Gastropoda, Opisthobranchia): Comparative Histology and Ultrastructure of the Nidamental Glands with Aspects of Functional Morphology". Zoologischer Anzeiger. 240 (2): 119-136.
- Schrödl M.; Wägele H.2 Willan R.C. (2001). "Taxonomic Redescription of the Doridoxidae(Gastropoda: Opisthobranchia), an Enigmatic Family of Deep Water Nudibranchs, with Discussion of Basal Nudibranch Phylogeny". Zoologischer Anzeiger,. 240 (1): 83-97. doi:10.1078/0044-5231-00008. More than one of
- H. Wägele and R. C. Willan (September 2000). "Phylogeny of the Nudibranchia". Zoological Journal of the Linnean Society. 1 (1): 83–181.
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- Lots of images and information on Australian nudibranchs
- Nudibranchs in their natural environment, Scuba Diving - Narooma NSW
- Sea Slug Forum: many images and much valuable information worldwide
- Nudi Pixel: Online resource for nudibranchs and sea slugs identification using photographs
- Nudibranch gallery
- Bibliography and portal to opisthobranch, nudibranch & seaslug information
- Nudibranch photo gallery from Bali
- Scottish Nudibranchs: Online resource for identification of species found in Scottish waters
- Born to be Wild: Sea Slugs, 04/30/2008