nudibranch

File:Mystery_mollusc.jpg is the first known bathypelagic nudibranch|left]]

Nudibranchs occur in seas worldwide, ranging from the Arctic, through temperate and tropical regions, to the Southern Ocean around Antarctica.[http://www.fishermenscuba.com/DiveArticles/nudibranches.htm Nudibranchs] {{Webarchive|url=https://web.archive.org/web/20130814034543/http://www.fishermenscuba.com/DiveArticles/nudibranches.htm |date=2013-08-14 }}, Fishermen Scuba.{{Cite journal |author1=Ekimova, I. |author2=T. Korshunova |author3=D. Schepetov |author4=T. Neretina |author5=N. Sanamyan |author6=A. Martynov |date=2015 |title=Integrative systematics of northern and Arctic nudibranchs of the genus Dendronotus (Mollusca, Gastropoda), with descriptions of three new species |journal=Zoological Journal of the Linnean Society |volume=173 |issue=4 |page=e0192177 |doi=10.1111/zoj.12214 |doi-access=free }} However, they are mostly found around [Southeast Asia]. They are almost entirely restricted to salt water, although a few species are known to inhabit lower salinities in brackish water.{{Cite journal |author1=Korshunova, T. |author2=K. Lundin |author3=K. Malmberg |author4=B. Picton |author5=A. Martynov |date=2018 |title=First true brackish-water nudibranch mollusc provides new insights for phylogeny and biogeography and reveals paedomorphosis-driven evolution |journal=PLOS ONE |volume=13 |issue=1 |page=e0192177 |doi=10.1371/journal.pone.0192177 |pmid=29538398 |pmc=5851531 |bibcode=2018PLoSO..1392177K |doi-access=free }}

Nudibranchs live at virtually all depths, from the intertidal zone to depths well over {{convert|700|m|abbr=on}}. The greatest diversity of nudibranchs is seen in warm, shallow reefs, although one nudibranch species was discovered at a depth near {{convert|2500|m|abbr=on}}.{{cite web |title=Discoveries of deep-sea biomass and biodiversity using an ROV |url=http://www.mbari.org/twenty/biodiversity.htm|publisher=Monterey Bay Aquarium Research Institute |access-date=16 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131009075212/http://www.mbari.org/twenty/biodiversity.htm |archive-date=9 October 2013}} This nudibranch, described in 2024 as Bathydevius, is the only known nudibranch with a bathypelagic lifestyle and is one of the very few to be bioluminescent.{{cite web | last1=Rayne | first1=Elizabeth | title=Researchers finally identify the ocean’s “mystery mollusk” | website=Ars Technica | date=2024-12-02 | url=https://arstechnica.com/science/2024/12/glowing-sea-slug-behaves-like-a-jellyfish-lizard-and-venus-flytrap/ | access-date=2024-12-02}}

Nudibranchs are benthic animals, found crawling over the substrate. The only exceptions to this are the neustonic Glaucus nudibranchs, which float upside down just under the ocean's surface, such as the glaucus atlanticus; the pelagic nudibranchs Cephalopyge trematoides, which swim in the water column;{{Cite journal |last=Steinberg |first=J. E. |year=1956 |title=The pelagic nudibranch, Cephalopyge trematoides (Chun, 1889), in New South Wales with a note on other species in this genus |url=http://biostor.org/reference/68603 |journal=Proceedings of the Linnean Society of New South Wales |volume=81 |pages=184–192}}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 [...]" the two pelagic species of Phylliroe, and the evolutionarily distinct, bathypelagic Bathydevius.Gosliner TM, Valdes A Behrens DW 2015 Nudibranch and Sea Slug Identification Indo-Pacific New World Publications Jacksonville Florida USA

File:Aeolidiella stephanieae.png nudibranch body: Note the oral tentacles (ot), foot tentacles (ft), eye (e), rhinophores (r), and cerata (c). This species has cnidosacs (cn) at the cerata tips. Scale bar is 100 μm.]]

The body forms of nudibranchs vary greatly. Because they are opisthobranchs, unlike most other gastropods, they are apparently bilaterally symmetrical externally (but not internally) because they have undergone secondary detorsion. In all nudibranchs, the male and female sexual openings are on the right side of the body, reflecting their asymmetrical origins. They lack a mantle cavity. Some species have venomous appendages (cerata) on their sides, which deter predators. Many also have a simple gut and a mouth with a radula.

The eyes in nudibranchs are simple and able to discern little more than light and dark.{{cite web |url=http://ngm.nationalgeographic.com/2008/06/nudibranchs/holland-text/2 |archive-url=https://web.archive.org/web/20080617030023/http://ngm.nationalgeographic.com/2008/06/nudibranchs/holland-text/2 |url-status=dead |archive-date=June 17, 2008 |title=Nudibranchs – National Geographic Magazine |publisher=Ngm.nationalgeographic.com |date=2013-04-25 |access-date=2013-07-04}} The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.{{cite journal

| journal = Journal of Experimental Biology

| url = http://jeb.biologists.org/cgi/content/abstract/60/3/707

| pmid = 4847278

| title = The Electrophysiology of Photoreceptors in the Nudibranch Mollusc, Tritonia Diomedia

| volume = 60

| issue = 3

| pages = 707–19

| author = Chase, Ronald

| date = June 1, 1974 | doi = 10.1242/jeb.60.3.707

}}

Nudibranchs vary in adult size from {{convert|4|to|600|mm|in|abbr=on}}.{{Cite web |title=Hexabranchus sanguineus {{!}} DORIS |url=https://doris.ffessm.fr/Especes/Hexabranchus-sanguineus-Danseuse-espagnole-411 |access-date=2023-04-17 |website=doris.ffessm.fr}}

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). In most species, there is a swimming veliger larva with a coiled shell, but the shell is shed at metamorphosis when the larva transforms into the adult form. Some species have direct development, and the shell is shed before the animal emerges from the egg mass.Thompson, T. E. (1976). Biology of opisthobranch molluscs, 1, 207 pp., 21 pls. Ray Society, no. 151.

File:Blue dragon-glaucus atlanticus (8599051974).jpg is an example of a nudibranch that has its cerata positioned like wings instead of on its back.]]

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.{{cite journal |last=Dayrat |first=B. |year=2005 |title=Advantages of naming species under the PhyloCode: An example of how a new species of Discodorididae (Mollusca, Gastropoda, Euthyneura, Nudibranchia, Doridina) may be named |journal=Marine Biology Research |volume=1 |pages=216–232 |url=https://campillos.ucmerced.edu/~bdayrat/PDF%20of%20Papers/Dayrat-MBR-2005.pdf |access-date=2009-06-14 |doi=10.1080/17451000510019141 |issue=3 |bibcode=2005MBioR...1..216D |s2cid=53557429 }} By contrast, on the back of the aeolids in the clade Cladobranchia, brightly coloured sets of protruding organs called cerata are present.

Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odors.

{{Clear}}

In the course of their evolution, nudibranchs have lost their shells, while developing alternative defence mechanisms. Some species evolved an external anatomy with textures and colours that mimicked surrounding sessile invertebrate animals (often their prey sponges or soft corals) to avoid predators with camouflage. Other nudibranchs, as seen especially well on Chromodoris quadricolor, have an intensely bright and contrasting colour pattern that makes them especially conspicuous in their surroundings. Nudibranch molluscs are the most commonly cited examples of aposematism in marine ecosystems, but the evidence for this has been contested,{{Cite journal |title=Does warning colouration occur in nudibranchs? |last=Edmunds |first=M. |date=1991 |journal=Malacologia |volume=32 |pages=241–255}} mostly because few examples of mimicry are seen among species, many species are nocturnal or cryptic, and bright colours at the red end of the spectrum are rapidly attenuated as a function of water depth. For example, the Spanish dancer nudibranch (genus Hexabranchus), among the largest of tropical marine slugs, potently chemically defended, and brilliantly red and white, is nocturnal and has no known mimics.{{Cite journal|title = Defensive chemicals of the Spanish Dancer nudibranch, Hexabranchus sanguineus, and its egg ribbons: Macrolides derived from a sponge diet|last = Pawlik|first = JR|date = 1988|journal = Journal of Experimental Marine Biology and Ecology|doi = 10.1016/0022-0981(88)90225-0|volume = 119|issue = 2|pages = 99–109| bibcode=1988JEMBE.119...99P |display-authors=etal}} Other studies of nudibranch molluscs have concluded they are aposematically coloured, for example, the slugs of the family Phylidiidae from Indo-Pacific coral reefs.{{Cite journal |title=Marine benthic invertebrates use multimodal cues for defence against reef fish |journal=Marine Ecology Progress Series |date=2007 |pages=29–39 |volume=340 |doi=10.3354/meps340029 |first1=R. |last1=Ritson-Williams |first2=VJ |last2=Paul |bibcode=2007MEPS..340...29R |doi-access=free}}

Nudibranchs that feed on hydrozoids can store the hydrozoids' nematocysts (stinging cells) in the dorsal body wall, the cerata.{{cite journal |author=Frick, K |title=Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine |journal=In: SF Norton (Ed). Diving for Science...2003. |volume=Proceedings of the American Academy of Underwater Sciences |issue=22nd Annual Scientific Diving Symposium |year=2003 |url=http://archive.rubicon-foundation.org/4744 |archive-url=https://web.archive.org/web/20090129210100/http://archive.rubicon-foundation.org/4744 |url-status=usurped |archive-date=January 29, 2009 |access-date=2008-07-03 }} 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. The specific mechanism by which nudibranchs protect themselves from the hydrozoids and their nematocysts is yet unknown, but special cells with large vacuoles probably play an important role. Similarly, some nudibranchs can also take in plant cells (symbiotic algae from soft corals) and reuse these to make food for themselves. The related group of sacoglossan sea slugs feed on algae and retain just the chloroplasts for their own photosynthetic use, a process known as kleptoplasty. Some of these species have been observed practising autotomy, severing portions of their body to remove parasites, and have been observed to regrow their head if decapitated.{{cite web | url = https://www.cell.com/current-biology/fulltext/S0960-9822(21)00047-6?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982221000476%3Fshowall%3Dtrue | title = Extreme autotomy and whole-body regeneration in photosynthetic sea slugs: Current Biology | format = | accessdate = }}

Nudibranchs use a variety of chemical defences to aid in protection,{{cite book |title=Bioorganic Marine Chemistry |volume=1 |last=Karuso |first=P. |publisher=Springer-Verlag |year=1987 |isbn=978-3-642-72728-3 |pages=31–60 |editor-last=Scheuer |editor-first=PJ |doi=10.1007/978-3-642-72726-9_2 |chapter=Chemical Ecology of the Nudibranchs }}, a comprehensive review of the chemical ecology of the nudibranchs but the strategy need not be lethal to be effective; in fact, good arguments exist that chemical defences should evolve to be distasteful rather than toxic.{{Cite book |title=Antipredatory defensive roles of natural products from marine invertebrates. |last=Pawlik |first=JR |publisher=Springer Science |year=2012 |location=NY |pages=677–710 |work=Handbook of Marine Natural Products |editor-last=Fattorusso |editor-first=E. |display-editors=etal}} Some sponge-eating nudibranchs concentrate the chemical defences from their prey sponge in their bodies, rendering themselves distasteful to predators.{{cite book |last=Gosliner |first=T. M. |year=1987 |title=Nudibranchs of Southern Africa |publisher=Sea Challengers |isbn=978-0-930118-13-6 }} One method of chemical defense used by nudibranchs are secondary metabolites, which play an important role in mediating relationships among marine communities.{{Cite journal |last1=Avila |first1=C |last2=Iken |first2=K |last3=Fontana |first3=A |last4=Cimino |first4=G |date=2000-09-05 |title=Chemical ecology of the Antarctic nudibranch Bathydoris hodgsoni Eliot, 1907: defensive role and origin of its natural products |url=https://www.sciencedirect.com/science/article/pii/S0022098100002276 |journal=Journal of Experimental Marine Biology and Ecology |language=en |volume=252 |issue=1 |pages=27–44 |doi=10.1016/S0022-0981(00)00227-6 |pmid=10962063 |bibcode=2000JEMBE.252...27A |issn=0022-0981}} The evidence that suggests the chemical compounds used by dorid nudibranchs do in fact come from dietary sponges lies in the similarities between the 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 defence chemicals with a concurrent change in diet.{{cite journal |last1=Faulkner |first1=D. J. |first2=M. T. |last2=Ghiselin |year=1983 |title=Chemical defence and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods |journal=Marine Ecology Progress Series |volume=13 |pages=295–301 |doi=10.3354/meps013295 |bibcode=1983MEPS...13..295F |doi-access=free }} This, however, is not the only way for nudibranchs to develop chemical defences. Certain Antarctic marine species defense mechanisms are believed to be controlled by biological factors like predation, competition, and selective pressures. Certain species can produce their own chemicals de novo without dietary influence. Evidence for the different chemical production methods 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.{{cite journal |last1=Barsby |first1=T. |first2=R. G. |last2=Linington |first3=R. J. |last3=Andersen |year=2002 |title=De Novo terpenoid biosynthesis by the dendronotid nudibranch Melibe leonina |journal=Chemoecology |volume=12 |issue=4 |pages=199–202 |doi=10.1007/PL00012669 |bibcode=2002Checo..12..199B |s2cid=35384332 }}

Another protection method is releasing the ugdon acid from the skin.{{cite journal |last=Edmunds |first=M. |year=1968 |title=Acid secretion in some species of Doridacea (Mollusca, Nudibranchia) |journal=Proceedings of the Malacological Society of London |volume=38 |issue=2 |pages=121–133 |url=http://mollus.oxfordjournals.org/content/38/2/121.extract |archive-url=https://archive.today/20130415132743/http://mollus.oxfordjournals.org/content/38/2/121.extract |url-status=dead |archive-date=2013-04-15 }} Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically, eating the animal from the inside out.

In 1884, Philip Henry Gosse reported observations by "Professor Grant" (possibly Robert Edmond Grant) that two species of nudibranchs emit sounds that are audible to humans.P.H. Gosse, Evenings at the Microscope, 1884 edition,[https://archive.org/stream/eveningsatmicros00goss#page/56/mode/2up/search/Professor+Grant] p57

Two very elegant species of Sea-slug, viz., Eolis punctata [i.e. Facelina annulicornis], and Tritonia arborescens [i.e. Dendronotus frondosus], certainly produce audible sounds. Professor Grant, who first observed the interesting fact in some specimens of the latter, which he was keeping in an aquarium, says of the sounds that 'they resemble very much the clink of a steel wire on the side of the jar, one stroke only been given at a time, and repeated at intervals of a minute or two; when placed in a large basin of water, the sound is much obscured and is like that of a watch, one stroke being repeated, as before, at intervals. The sound is longest and most often repeated when the Tritonia are lively and moving about and is not heard when they are cold and without any motion; in the dark, I have not observed any light emitted at the time of the stroke; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke; the sound, when in a glass vessel, is mellow and distinct.' The Professor has kept these Tritonia alive in his room for a month. During the whole period of their confinement, they have continued to produce the sounds with very little diminution of their original intensity. In a small apartment, they are audible at a distance of twelve feet. The sounds obviously proceed from the mouth of the animal, and at the instant of the stroke, we observe the lips suddenly separate as if to allow the water to rush into a small vacuum formed within. As these animals are hermaphrodites, requiring mutual impregnation, the sounds may possibly be a means of communication between them, or, if they are of an electric nature, they may be the means of defending from foreign enemies, one of the most delicate, defenceless, and beautiful Gasteropods that inhabit the deep.

{{Expand section|date=January 2018}}

File:Nembrotha rutilans Nick Hobgood.jpg]]

File:Editing Image-Acanthodoris lutea laying eggs 2.jpg laying eggs]]

Nudibranchs are hermaphroditic, thus having a set of reproductive organs for both sexes, but they cannot fertilize themselves.{{cite web |url=http://www.aquaticcommunity.com/sw/nudibranch.php |title=Nudibranch |publisher=Aquaticcommunity.com |access-date=2013-07-04}} Mating usually takes a few minutes and involves a dance-like courtship. Nudibranchs typically deposit their eggs within a gelatinous spiral,{{cite journal |author=Klussmann-Kolb A |title=The Reproductive Systems of the Nudibranchia (Gastropoda, Opisthobranchia): Comparative Histology and Ultrastructure of the Nidamental Glands with Aspects of Functional Morphology |journal=Zoologischer Anzeiger |year=2001 |volume=240 |issue=2 |pages=119–136 |doi=10.1078/0044-5231-00011}} which is often described as looking like a ribbon. The number of eggs varies; it can be as few as just 1 or 2 eggs (Vayssierea felis) or as many as an estimated 25 million (Aplysia fasciata{{Cite web |last=Rudman |first=W. B. |date=2010-06-09 |title=How many eggs do sea slugs lay? |url=http://www.seaslugforum.net/find/3086 |access-date=2024-08-14 |website=The Sea Slug Forum |language=en}}). The eggs contain toxins from sea sponges as a means of deterring predators.{{Cite web |url=http://www.dive-the-world.com/creatures-nudibranchs.php |title=Diving with Nudibranchs |website=Dive the World}} After hatching, the infants look almost identical to their adult counterparts, albeit smaller. Infants may also have fewer cerata. The lifespan of nudibranchs can range from a few weeks to a year, depending on the species.

File:Pteraeolidia ianthina Nick Hobgoood.jpg has adapted cerata to house symbiotic zooxanthellae obtained from its diet, which continue to photosynthesize and provide energy to the nudibranch.]]

All known nudibranchs are carnivorous. Some feed on sponges, others on hydroids (e.g. Cuthona),{{cite journal

| url=https://www.wheaton.edu/~/media/Files/Academics/Faculty/Rorem-Nadine/Role-prey.pdf

| journal=American Malacological Bulletin

| year=1997

| title=The role of prey mobility in the population ecology of the nudibranch Cuthona nana (Gastropoda: Opisthobranchia)

| author=NC Folino

| access-date=2013-03-07

| archive-url=https://web.archive.org/web/20120625052731/http://wheaton.edu/~/media/Files/Academics/Faculty/Rorem-Nadine/Role-prey.pdf

| archive-date=2012-06-25

| url-status=dead

}} others on bryozoans (phanerobranchs such as Tambja, Limacia, Plocamopherus and Triopha),{{Cite journal |last1=Domínguez |first1=M. |last2=Troncoso |first2=J. S. |last3=García |first3=F. J. |doi=10.1111/j.1096-3642.2008.00390.x | title = The family Aeolidiidae Gray, 1827 (Gastropoda Opisthobranchia) from Brazil, with a description of a new species belonging to the genus Berghia Trinchese, 1877 |journal=Zoological Journal of the Linnean Society |volume=153 |issue=2 |pages=349–368 |year=2008 |doi-access=free }} and some eat other sea slugs or their eggs (e.g. Favorinus){{cite web |author=Rudman, W.B. |date=1999-03-19 |title=Favorinus tsuruganus Baba & Abe, 1964. [In] Sea Slug Forum. Australian Museum |url=http://www.seaslugforum.net/find/favotsur }} or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates (e.g. Nembrotha, Goniodoris),{{Cite journal | doi = 10.1111/j.1095-8312.2004.00413.x | last1 = Valdés | first1 = Á. | title = Phylogeography and phyloecology of dorid nudibranchs (Mollusca, Gastropoda) | journal = Biological Journal of the Linnean Society | volume = 83 | issue = 4 | pages = 551–559 | year = 2004 | doi-access = free }} other nudibranchs (Roboastra, which are descended from tunicate-feeding species), barnacles (e.g. Onchidoris bilamellata),{{Cite journal |jstor=1986 |last1=Barnes |first1=H. |last2=Powell |first2=H. T. |title=Onchidoris fusca (Müller); A Predator of Barnacles |journal=Journal of Animal Ecology |volume=23 |issue=2 |pages=361–363 |year=1954 |doi=10.2307/1986|bibcode=1954JAnEc..23..361B }} 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 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 capacious 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. Many of these larvae have not been put forth as only 36 species has been studied and only 23 in the lab. The ecology of nudibranchs'{{Cite journal |last=Hadfield |first=Michael G. |date=1963 |title=The Biology of Nudibranch Larvae |url=https://www.jstor.org/stable/3564960 |journal=Oikos |volume=14 |issue=1 |pages=85–95 |doi=10.2307/3564960 |issn=0030-1299}} change with species.Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.

{{multiple image

| footer = Nudibranchs are frequently differentiated as either dorid or aeolid.

| align = right

| image1 = Chromodoris willani for nudipixel.jpg

| width1 = {{#expr: (150 * 8/6) round 0}}

| alt1 =

| caption1 = Dorids (Chromodoris willani shown) breathe with the branchial plume, which projects from around their anus.

| image2 = Nudi from tidepool.jpg

| width2 = {{#expr: (150 * 2536/2232) round 0}}

| alt2 =

| caption2 = Aeolids (Hermissenda crassicornis pictured) have many cerata over their back which are used for defense and respiration.

}}

Nudibranchs are commonly divided into two main kinds, dorid and aeolid (also spelled eolid) nudibranchs:Hans Bertsch, [http://slugsite.us/bow/nudi_han.htm Nudibranchs: Marine slugs with verve]. "Navanax inermis[..] is the bane of all nudibranchs because it is one of the few known predators of this group of slugs. [...] Dorids mainly eat sponges, bryozoans, and tunicates, whereas aeolids principally eat cnidarians."{{cite web|url=http://marinelife.about.com/od/invertebrates/tp/Facts-About-Nudibranchs.htm |title=Facts About Nudibranchs |publisher=Marinelife.about.com |date=2011-11-10 |access-date=2013-07-04}}

• Dorids (clade Anthobranchia, Doridacea, or Doridoidea) are recognised by having an intact digestive gland and the feather-like 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.{{Citation needed|date=October 2012}} Additionally, dorid nudibranchs commonly have distinct pockets, bumps, and/or mantle dermal formations, which are distortions on their skin, used to store bioactive defense chemicals.{{Cite journal |last1=Dean |first1=Lewis J. |last2=Prinsep |first2=Michèle R. |date=2017 |title=The chemistry and chemical ecology of nudibranchs |url=http://xlink.rsc.org/?DOI=C7NP00041C |journal=Natural Product Reports |language=en |volume=34 |issue=12 |pages=1359–1390 |doi=10.1039/C7NP00041C |pmid=29135002 |issn=0265-0568 |doi-access=free}}
• Aeolids (clade Cladobranchia) have cerata (spread across the back) instead of the branchial plume. The cerata function in place of gills and facilitate gas exchange through the epidermis. Additionally, aeolids possess a branched digestive gland, which may extend into the cerate and often has tips that contain cnidosacs (stinging cells absorbed from prey species and then used by the nudibranch).{{Cite journal |last1=Cella |first1=Kristen |last2=Carmona |first2=Leila |last3=Ekimova |first3=Irina |last4=Chichvarkhin |first4=Anton |last5=Schepetov |first5=Dimitry |last6=Gosliner |first6=Terrence M. |date=2016-12-15 |title=A Radical Solution: The Phylogeny of the Nudibranch Family Fionidae |journal=PLOS One |language=en |volume=11 |issue=12 |pages=e0167800 |doi=10.1371/journal.pone.0167800 |issn=1932-6203 |pmc=5158052 |pmid=27977703 |bibcode=2016PLoSO..1167800C |doi-access=free}} They lack a mantle.{{Citation needed|date=October 2012}} 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 subclass Opisthobranchia (the marine slugs: which consisted of nudibranchs, sidegill slugs, bubble snails, algae sap-sucking sea slugs, and sea hares). Since 2005,Taxonomy of the Gastropoda, Bouchet & Rocroi, 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).{{cite journal |url=http://www.journal-malaco.fr/bouchet&rocroi_2005_Visaya.pdf |first1=Guido T. |last1=Poppe |first2=Sheila P. |last2=Tagaro |title=The New Classification of Gastropods according to Bouchet & Rocroi, 2005 |journal=Visaya |date=February 23, 2006 |archive-url=https://web.archive.org/web/20070927101354/http://www.journal-malaco.fr/bouchet%26rocroi_2005_Visaya.pdf |archive-date=September 27, 2007 |url-status= |access-date= }} Since 2010, Opisthobranchia has been recognised as 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).{{cite journal | last1 = Jörger | first1 = K. M. | last2 = Stöger | first2 = I. | last3 = Kano | first3 = Y. | last4 = Fukuda | first4 = H. | last5 = Knebelsberger | first5 = T. | last6 = Schrödl | first6 = M. | display-authors= 3| year = 2010 | title = On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia | journal = BMC Evolutionary Biology | volume = 10 | issue = 1 | page = 323 | doi = 10.1186/1471-2148-10-323 |quote=At the basis of the Euthyneura the Nudipleura split off | pmid=20973994 | pmc=3087543 | doi-access = free | bibcode = 2010BMCEE..10..323J }}

In 2024, a brand new family of deep-sea pelagic nudibranch, Bathydeviidae, was described as containing a single genus, Bathydevius. This family does not appear to be closely related to any other extant nudibranch and is the only known bathypelagic nudibranch taxon.{{Cite journal |last=Robison |first=Bruce H. |last2=Haddock |first2=Steven H. D. |date=2024-12-01 |title=Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus, gen. et. sp. nov. |url=https://linkinghub.elsevier.com/retrieve/pii/S0967063724001845 |journal=Deep Sea Research Part I: Oceanographic Research Papers |volume=214 |pages=104414 |doi=10.1016/j.dsr.2024.104414 |issn=0967-0637|doi-access=free }}

This classification was based on the work of Johannes Thiele (1931),Thiele, J. (1931). Handbuch der systematischen Weichtierkunde, II. Verlag von Gustav Fischer, Jena, Germany. built on the concepts of Henri Milne-Edwards (1848).Milne-Edwards H (1848). Note sur la classification naturelle chez Mollusques Gasteropodes. Annales des Sciences Naturelles, series 3, 9: 102–112.

File:Nudibranch (Charleston, South Carolina, 5 March 1852).jpg

Order Nudibranchia:

• Infraorder Anthobranchia Férussac, 1819 (dorids)
• Superfamily Doridoidea Rafinesque, 1815
• Superfamily Doridoxoidea Bergh, 1900
• Superfamily Onchidoridoidea Alder & Hancock, 1845
• Superfamily Polyceroidea Alder & Hancock, 1845
• Infraorder Cladobranchia Willan & Morton, 1984 (aeolids)
• Superfamily Aeolidioidea J. E. Gray, 1827
• Superfamily Arminoidea Rafinesque, 1814
• Superfamily Dendronotoidea Allman, 1845
• Superfamily Metarminoidea Odhner in Franc, 1968

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, strong evidence supports the monophyly of the Nudibranchia and its two major groups, the Anthobranchia/Doridoidea and Cladobranchia.{{cite journal |author1=Wägele H. |author2=Willan R. C. |name-list-style=amp |title = Phylogeny of the Nudibranchia | journal = Zoological Journal of the Linnean Society |date=September 2000 | volume = 130 | issue = 1 | pages = 83–181 | doi = 10.1111/j.1096-3642.2000.tb02196.x| doi-access = free }} A study published in May 2001, again revised the taxonomy of the Nudibranchia.{{cite journal |author1=Schrödl M., Wägele H. |author2=Willan R. C. |name-list-style=amp |title=Taxonomic Redescription of the Doridoxidae (Gastropoda: Opisthobranchia), an Enigmatic Family of Deep Water Nudibranchs, with Discussion of Basal Nudibranch Phylogeny |journal=Zoologischer Anzeiger |year=2001 |volume=240 |issue=1 |pages=83–97 |doi=10.1078/0044-5231-00008}} 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, with a third described in 2024:

• Bathydeviidae
• Euctenidiacea (= Holohepatica)
• Gnathodoridacea (contains only Bathydorididae)
• Doridacea
• Doridoidea
• Phyllidioidea
• Onchidoridoidea
• Polyceroidea (= Phanerobranchiata Non Suctoria)
• Dexiarchia (= Actenidiacea)
• Pseudoeuctenidiacea ( = Doridoxida)
• Cladobranchia ( = Cladohepatica)
• Euarminida
• Dendronotida
• Aeolidida
• Unassigned Cladobranchia (previously Metarminoidea)
• Charcotiidae
• Dironidae
• Goniaeolididae
• Heroidae
• Proctonotidae
• Madrellidae
• Pinufiidae
• Embletoniidae

This gallery shows some of the great variability in the color and form of nudibranchs, and nudibranch egg ribbons.

File:Tritoniopsis elegans.jpg|Tritoniopsis elegans

File:Nembrotha cristata bunaken.jpg|Nembrotha cristata in Bunaken National Park

File:Nudibranch in tidepool.jpg|Sea clown (Triopha catalinae), Northern California

File:Chromodoris annae.JPG|Chromodoris annae from Lembeh Straits, Indonesia

File:Bailarina Española por Gustavo Gerdel.jpg|Spanish dancer (Hexabranchus sanguineus), taken at night, Red Sea

File:Nembrotha chamberlaini (AA1).jpg|Nembrotha chamberlaini from Verde Island, the Philippines

File:Glossodoris atromarginata.jpg|Glossodoris atromarginata

File:Chromodoris dianae (AA2).jpg|Chromodoris dianae from Verde Island, the Philippines

File:Nembrotha milleri mating.jpg|A pair of Nembrotha milleri mating at Verde Island, the Philippines

File:Regal Sea Goddess Nudibranch.jpg|Regal sea goddess (Felimare picta) in the Gray's Reef National Marine Sanctuary, Savannah, Georgia

File:Flabellina Affinis - La Herradura.jpg|Flabellina affinis at La Herradura (Mediterranean Sea), Spain

File:Eggs of nudibranch.JPG|Dorid nudibranch egg ribbon in Moss Beach, California

File:Nudibranch egg ribbon at Shaab Mahmoud.JPG|Nudibranch egg ribbon at Shaab Mahmoud (Red Sea), Egypt

File:Nudibranch egg ribbon at Malahi.JPG|Nudibranch egg ribbon at Malahi (Red Sea), Egypt

File:Goniobranchus Kuniei.jpg|Goniobranchus kuniei, off the coast of Papua New Guinea

File:Nudi branch.jpg|Nudibranch

File:Mushroom coral reticulidia with an orange stowaway, pantai kollo soha, wakatobi, 2018 (45088582314).jpg|Mushroom coral reticulidia (Reticulidia fungia, at Wakatobi National Park, Indonesia, 2018{{cite web |title=Family Phyllidiidae |url=https://www.ryanphotographic.com/phyllidiidae.htm|publisher=ryanphotographic.com |access-date=2023-05-07}}

• Symposia and workshops on opisthobranchs

{{Reflist|30em}}

• Thompson, T. E. (1976). Biology of opisthobranch molluscs Vol. 1. 207 pp., 21 pls. Ray Society, no. 151.
• Thompson, T. E., & Brown, G. H. (1984). Biology of opisthobranch molluscs Vol. 2. 229 pp., 41 pls. Ray Society, no. 156.
• McDonald, Gary R. (7 July 2021). [https://escholarship.org/uc/item/5rx4j4ps Institute of Marine Sciences. Bibliographia Nudibranchia, 3rd online Edition.] A listing, by Author, of publications on nudibranchs.
• McDonald, Gary R. (7 July 2021). [https://escholarship.org/uc/item/38n512jw Institute of Marine Sciences. Nudibranch Systematic Index, 3rd online Edition.] An index of names given to nudibranchs and their subsequent use, referenced to Bibliographia Nudibranchia.
• McDonald, Gary R. & Nybakken, J. W. (November 5, 2014). [http://www.escholarship.org/uc/item/0g75h1q3 List of the Worldwide Food Habits of Nudibranchs]
• Coleman, Neville (2008). Nudibranchs Encyclopedia: Catalogue of Asia/Indo-Pacific Sea Slugs. Neville Coleman's Underwater Geographic. {{ISBN|0-947325-41-7}}

{{Wikispecies|Nudibranchia}}

{{Commons category|Nudibranchia}}

• [http://www.seaslugforum.net/ Sea Slug Forum] by William B. Rudman
• [http://www.seaslug.org.uk/nudibranchs/ Nudibranchs of the British Isles]
• [http://opistobranquis.info/en/ OPK Opistobranquis – Iberian and Mediterranean Opisthobranchs]
• [http://www.medslugs.de/E/mssmain.htm Mediterranean slug site (actually a misnomer) – Worldwide coverage]
• [http://slugsite.tierranet.com The Slug Site, Michael D. Miller 2002–2014]
• [https://web.archive.org/web/20100328164757/http://nudibranch.com.au/ Images, information and identification of Nudibranchs]
• [http://www.micktait.com/underwater-nudibranchs/ Nudibranch Photos by Mick Tait]
• [https://web.archive.org/web/20080703171829/http://scubadiving.naroomaonline.com/ Nudibranchs in their natural environment, Scuba Diving – Narooma NSW offline? 26 Nov 2014]
• [http://www.nudipixel.net/ Nudi Pixel: Online resource for nudibranchs and sea slugs identification using photographs] {{Webarchive|url=https://web.archive.org/web/20110527123406/http://www.nudipixel.net/ |date=2011-05-27 }}
• [http://www.uw-critters.de/nacktschnecken_en.html Various nudibranch species from Indonesia, Philippines and Thailand]
• [https://web.archive.org/web/20100107002612/http://www.sergeyphoto.com/underwater/nudibranchs.html Nudibranch gallery – Sergey Parinov – offline? 26 Nov 2014]
• [https://web.archive.org/web/20070531014134/http://seaslug.com/ Opisthobranch Newsletter – Bibliography and portal to opisthobranch, nudibranch & sea-slug information]
• [http://www.nudibranch.org/Scottish%20Nudibranchs/ Scottish Nudibranchs: Online resource for identification of species found in Scottish waters]
• [https://web.archive.org/web/20080517120538/http://ngm.nationalgeographic.com/2008/06/nudibranchs/doubilet-photography National Geographic Nudibranch Photo Gallery]
• [http://www.seaslugsofhawaii.com/ Sea Slugs of Hawaii]
• [http://www.life.illinois.edu/slugcity/ Slug City – Molluscs. Brain & Behavior], from the University of Illinois at Urbana-Champaign

• [https://www.youtube.com/watch?v=ef4nn45MDrs Attack of the Sea Slugs] at YouTube
• [http://www.vimeo.com/10516475 The Lynx Nudibranch]: HD clip of Phidiana lynceus carefully consuming a hydroid Myrionema amboinense.
• [http://www.life.illinois.edu/slugcity/movies.html Slug City – Molluscs. Brain & Behavior]: many videos of nudibranchs from the University of Illinois at Urbana-Champaign

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Category:Nudipleura

Category:Taxa named by Georges Cuvier