|A pair of Chromodoris lochi from Puerto Galera, the Philippines.|
See text for superfamilies
A nudibranch // is a member of Nudibranchia, a group of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms. There are more than 3,000 described species of nudibranchs.
Nudibranchs are often casually called sea slugs, but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs, such as the colorful Aglajidae, are often confused with nudibranchs.
Distribution and habitat 
Nudibranchs occur in seas worldwide, including both the tropics and Antarctica.
Nudibranchs are benthic animals, found crawling over the bottom substrate. An exception to this is the neustonic Glaucus and Glaucilla nudibranchs, which float upside down just under the ocean's surface. Another exception is the pelagic nudibranch, Cephalopyge trematoides, which swims in the water column.
Anatomical description 
The body forms of nudibranchs vary a great deal, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical both externally and internally because they have undergone secondary detorsion. They lack a mantle cavity. Some species have venomous appendages (cerata) on their sides, which are used to deter predators. Many also have a simple gut and a mouth with a radula.
Their eyes are simple and able to discern little more than light and dark. The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.
They vary in adult size from 20 to 600 millimetres (0.79 to 24 in).
The adult form is without a shell or operculum (in shelled gastropods the operculum is a bony or horny plate that can cover the opening of the shell when the body is withdrawn).
The name nudibranch is appropriate, since the dorids (infraclass Anthobranchia) breathe through a "naked gill" shaped into branchial plumes in a rosette on their backs. By contrast, on the back of the aeolids in the clade Cladobranchia there are brightly colored sets of protruding organs called cerata.
Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odors.
Defense mechanisms 
This group includes some of the most colorful creatures on earth. In the course of evolution, nudibranchs have lost their shell and developed other defense mechanisms. Some species evolved an external anatomy with textures and colors that mimicked surrounding plants to avoid predators (see camouflage). Other nudibranchs, as seen especially well on chromodorids, have an intensely bright and contrasting color pattern that makes them especially conspicuous in their surroundings. This is believed to be an example of aposematic coloration; the shocking coloration warns potential predators that the slugs are distasteful or poisonous.
Nudibranchs that feed on hydrozoids can store the hydrozoids' nematocysts (stinging cells) in the dorsal body wall, the cerata. These stolen nematocysts, called kleptocnidae, wander through the alimentary tract without harming the nudibranch. Once further into the organ, the cells are assimilated by intestinal protuberances and brought to specific placements on the creature's hind body. Nudibranchs can protect themselves from the hydrozoids and their nematocysts; the specific mechanism is yet unknown, but special cells with large vacuoles probably play an important role. Similarly, nudibranchs can also take in plant cells and reuse the chloroplasts (plant cell organelles used for photosynthesis) to make food for themselves.
Nudibranchs use a variety of chemical defenses to aid in protection, but it is not necessary for the strategy to be lethal in order to be effective: some successful toxins induce bradycardia or hypotension in a predator, allowing the nudibranch to escape consumption while its attacker is incapacitated. Some sponge-eating nudibranchs concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators. The evidence that suggests the toxins used by dorid nudibranchs do in fact come from dietary sponges lies in the similarities between the primary and secondary metabolites of prey and nudibranchs, respectively. Furthermore, nudibranchs contain a mixture of sponge chemicals when they are in the presence of multiple food sources as well as change defense chemicals with a concurrent change in diet. This, however, is not the only way for nudibranchs to develop chemical defenses. Certain species are able to produce their own chemicals de novo without dietary influence. Evidence for the different methods of chemical production comes with the characteristic uniformity of chemical composition across drastically different environments and geographic locations found throughout de novo production species compared to the wide variety of dietary and environmentally dependent chemical composition in sequestering species.
Another method of protection is the release of an acid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically.
Life cycle 
Nudibranchs typically deposit their eggs within a gelatinous spiral.
Feeding and ecological role 
All known nudibranchs are carnivorous. Some feed on sponges, others on hydroids,(e.g. Cuthona) others on bryozoans (phanerobranchs such as Tambja, Limacia, Plocamopherus and Triopha), and some eat other sea slugs or their eggs (e.g. Favorinus) or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates (e.g. Tambja, Nembrotha, Polycera, Thecacera), other nudibranchs (Roboastra, which are descended from tunicate-feeding species), barnacles (e.g. Onchidoris), and anemones (e.g. the Aeolidiidae and other Cladobranchia).
The surface dwelling nudibranch, Glaucus atlanticus is a specialist predator of siphonophores, such as the Portuguese Man O' War. This predatory mollusk 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 capacious mouth, but if the prey is a larger siphonophore the mollusk 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.
- Dorids (clade Anthobranchia, Doridacea or Doridoidea) are recognised by the branchial (gill) plume, which forms a cluster on the posterior part of the body, around the anus. Fringes on the mantle do not contain any intestines.
- Aeolids (clade Cladobranchia) have cerata (spread across the back) instead of the branchial plume. They lack a mantle. Some are hosts to zooxanthellae.
The exact systematics of nudibranchs are a topic of recent revision. Traditionally nudibranchs have been treated as the order Nudibranchia, located in the gastropod mollusc sub-class Opisthobranchia (the marine slugs: which consisted of nudibranchs, sidegill slugs, bubble snails, algae sap-sucking sea slugs, and sea hares). Since 2005, pleurobranchs (which had previously been grouped among sidegill slugs) have been placed alongside nudibranchs in the clade Nudipleura (recognising them as more closely related to each other than to other opisthobranchs). Since 2010, it has been recognised that Opisthobranchia was not a valid clade (it is paraphyletic) and instead Nudipleura has been placed as the first offshoot of Euthyneura (which is the dominant clade of gastropods).
Traditional hierarchy 
- Infraorder Anthobranchia Férussac, 1819 (dorids)
- Infraorder Cladobranchia Willan & Morton, 1984 (aeolids)
Early revisions 
Newer insights derived from morphological data and gene-sequence research, seemed to confirm those 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. A study published in May 2001, again revised the taxonomy of the Nudibranchia. They were thus divided into two major clades:
- Anthobranchia (= Bathydoridoidea + Doridoidea)
- Dexiarchia nom. nov. (= Doridoxoidea + Dendronotoidea + Aeolidoidea + “Arminoidea”).
However, according to the taxonomy by Bouchet & Rocroi (2005), currently the most up-to-date system of classifying the gastropods, the Nudibranchia are a subclade within the clade of the Nudipleura. The Nudibranchia are then divided into two clades:
- Euctenidiacea (= Holohepatica)
- Dexiarchia (= Actenidiacea)
This gallery shows some of the great variability in the color and form of nudibranchs, and also shows a nudibranch egg ribbon.
Nembrotha chamberlaini from Verde Island, the Philippines.
Chromodoris dianae from Verde Island, the Philippines.
A pair of Nembrotha milleri mating at Verde Island, the Philippines.
- Longman Pronunciation Dictionary (2nd edition), ISBN 0-582-36467-1
- Thompson, T. E. (2009). "Feeding in nudibranch larvae". Journal of the Marine Biological Association of the United Kingdom 38 (2): 239. doi:10.1017/S0025315400006044.
- Nudibranchs, Fishermen Scuba.
- J.E. Steinberg, The pelagic nudibranch, Cephalopyge trematoides (Chun, 1889), in New South Wales with a note on other species in this genus, Proceedings of The Linnean Society of New South Wales 81:184-192 (1956) 
- G.M. Mapstone & M.N. Arai, Siphonophora (Cnidaria, Hydrozoa) of Canadian Pacific Waters, p.33. "The best documented predators of pelagic cnidarians from the phylum Mollusca are the neustonic nudibranchs and snails [...and] the pelagic nudibranch [...]"
- CHASE, RONALD (June 1, 1974). "The Electrophysiology of Photoreceptors in the Nudibranch Mollusc, Tritonia Diomedia". Journal of experimental biology 60 (3): 707–19. PMID 4847278.
- Dayrat, B. (2005). "Advantages of naming species under the PhyloCode: An example of how a new species of Discodorididae (Mollusca, Gastropoda, Euthyneura, Nudibranchia, Doridina) may be named" (PDF). Marine Biology Research 1 (3): 216–232. doi:10.1080/17451000510019141. Retrieved 2009-06-14.
- Frick, K (2003). "Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine". In: SF Norton (ed). Diving for Science...2003. Proceedings of the American Academy of Underwater Sciences (22nd Annual Scientific Diving Symposium). Retrieved 2008-07-03.
- Fuhrman, F. A.; Fuhrman, G. J.; Deriemer, K. (1979). "Toxicity and pharmacology of extracts from dorid nudibranchs". Biological Bulletin 156: 289–299.
- Gosliner, T. M. (1987). Nudibranchs of Southern Africa. ISBN 0-930118-13-8.
- Faulkner, D. J.; Ghiselin, M. T. (1983). "Chemical defense and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods". Marine Ecology-Progress Series 13: 295–301.
- Barsby, T.; Linington, R. G.; Andersen, R. J. (2002). "De Novo terpenoid biosynthesis by the dendronotid nudibranch Melibe leonina". Chemoecology 12 (4): 199–202. doi:10.1007/PL00012669.
- Edmunds, M. (1968). "Acid secretion in some species of Doridacea (Mollusca, Nudibranchia)". Proceedings of the Malacological Society of London 38 (2): 121–133.
- 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. doi:10.1078/0044-5231-00011.
- NC Folino (1997). "The role of prey mobility in the population ecology of the nudibranch Cuthona nana (Gastropoda: Opisthobranchia)". American Malacological Bulletin.
- Domínguez, M.; Troncoso, J. S.; García, F. J. (2008). "The family Aeolidiidae Gray, 1827 (Gastropoda Opisthobranchia) from Brazil, with a description of a new species belonging to the genus Berghia Trinchese, 1877". Zoological Journal of the Linnean Society 153 (2): 349–368. doi:10.1111/j.1096-3642.2008.00390.x.
- Rudman, W.B., (1999 (March 19)). "Favorinus tsuruganus Baba & Abe, 1964. [In] Sea Slug Forum. Australian Museum".
- Valdés, Á. (2004). "Phylogeography and phyloecology of dorid nudibranchs (Mollusca, Gastropoda)". Biological Journal of the Linnean Society 83 (4): 551–559. doi:10.1111/j.1095-8312.2004.00413.x.
- Barnes, H.; Powell, H. T. (1954). "Onchidoris fusca (Müller); A Predator of Barnacles". Journal of Animal Ecology 23 (2): 361–363. doi:10.2307/1986. JSTOR 1986.
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- McDonald, G. R.; Nybakken, J. W. (1997). "A worldwide review of the food of nudibranch mollusks. I. Introduction and the suborder Arminacea". Veliger 40 (2): 157–159.
- McDonald, G. R.; Nybakken, J. W. (1999). "A worldwide review of the food of nudibranch mollusks. II. The suborder Dendronotacea". Veliger 42 (1): 62–66.
- 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 aeolids principally eat cnidarians.
- Guido T. Poppe & Sheila P. Tagaro, The New Classification of Gastropods according to Bouchet & Rocroi, 2005; Visaya, February 23, 2006
- Jörger K. M., Stöger I., Kano Y., Fukuda H., Knebelsberger T. & Schrödl M. (2010). "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". BMC Evolutionary Biology 10: 323. doi:10.1186/1471-2148-10-323. "At the basis of the Euthyneura the Nudipleura split off"
- Wägele H. & Willan R. C. (September 2000). "Phylogeny of the Nudibranchia". Zoological Journal of the Linnean Society 1 (1): 83–181. doi:10.1111/j.1096-3642.2000.tb02196.x.
- Schrödl M., Wägele H. & 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.
Further reading 
- Gary R. McDonald. July 29, 2006. Nudibranch Systematic Index. Institute of Marine Sciences. Paper Nudibranch_Systematic_Index. - Note: This is a good resource for getting a species list in certain genera, but it is not good for ascertaining the taxonomy of a genera, because it does not use the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005).
- Neville Coleman (2008). Nudibranchs Encyclopedia: Catalogue of Asia/Indo-Pacific Sea Slugs. Neville Coleman's Underwater Geographic. ISBN 0-947325-41-7
|Wikispecies has information related to: Nudibranchia|
|Wikimedia Commons has media related to: Nudibranchia|
- Skaphandrus.com Nudibranchia
- Images, information and identification of Nudibranchs
- Sea Slug Forum by William B. Rudman
- World of Water by Neville Coleman (Encyclopedia and photos)
- Nudibranchs in their natural environment, Scuba Diving - Narooma NSW
- Nudi Pixel: Online resource for nudibranchs and sea slugs identification using photographs
- Various nudibranch species from Indonesia, Philippines and Thailand
- 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
- National Geographic Nudibranch Photo Gallery
- Sea slugs of Hawaii
- Marine Conservation Cambodia: A Photographic Database of Cambodian Nudibranch
- Slug City - Molluscs. Brain & Behavior, from the University of Illinois at Urbana-Champaign