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Nudibranch

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Nudibranchs (Template:IPAc-en<ref>Longman Pronunciation Dictionary (2nd edition), Template:ISBN</ref>) are a group of soft-bodied marine gastropod molluscs, belonging to the order Nudibranchia, that shed their shells after their larval stage.<ref>Template:Cite journal</ref> They are noted for their often extraordinary colours and striking forms, and they have been given colourful nicknames to match, such as "clown", "marigold", "splendid", "dancer", "dragon", and<ref>Template:Cite journal</ref> "sea rabbit".<ref>Bronson, Wilfrid (1935). Water People.</ref> About 3,000 species of nudibranchs are known.<ref name=OceanPortal>Ocean Portal (2017). A Collage of Nudibranch Colors. Smithsonian National Museum of Natural History. Retrieved 17 April 2018.</ref>

The word nudibranch comes from the Latin Template:Wikt-lang 'naked' and the Ancient Greek Template:Wikt-lang (Template:Grc-tr) 'gills'.

Nudibranchs are often casually called sea slugs, as they are a family of opisthobranchs (sea slugs), within the phylum Mollusca (molluscs), but many sea slugs belong to several taxonomic groups that are not closely related to nudibranchs. A number of these other sea slugs, such as the photosynthetic Sacoglossa and the colourful Aglajidae, are often confused with nudibranchs.

Distribution and habitatEdit

File:Mystery mollusc.jpg
The unusual Bathydevius is the first known bathypelagic nudibranch

Nudibranchs occur in seas worldwide, ranging from the Arctic, through temperate and tropical regions, to the Southern Ocean around Antarctica.<ref name="OceanPortal" /><ref name="fishermenscuba">Nudibranchs Template:Webarchive, Fishermen Scuba.</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>

Nudibranchs live at virtually all depths, from the intertidal zone to depths well over Template:Convert.<ref name=fishermenscuba/> The greatest diversity of nudibranchs is seen in warm, shallow reefs, although one nudibranch species was discovered at a depth near Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name=":0" /><ref name="Rayne2024">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Nudibranchs are benthic animals, found crawling over the substrate.<ref name=fishermenscuba/> 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;<ref>Template:Cite journal</ref><ref>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 [...]"</ref> the two pelagic species of Phylliroe, and the evolutionarily distinct, bathypelagic Bathydevius.<ref name=":0" /><ref>Gosliner TM, Valdes A Behrens DW 2015 Nudibranch and Sea Slug Identification Indo-Pacific New World Publications Jacksonville Florida USA</ref>

Anatomical descriptionEdit

File:Aeolidiella stephanieae.png
Berghia stephanieae 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.<ref name=thompson/>

The eyes in nudibranchs are simple and able to discern little more than light and dark.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.<ref>Template:Cite journal</ref>

Nudibranchs vary in adult size from Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref name="thompson">Thompson, T. E. (1976). Biology of opisthobranch molluscs, 1, 207 pp., 21 pls. Ray Society, no. 151.</ref>

File:Blue dragon-glaucus atlanticus (8599051974).jpg
Glaucus atlanticus 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.<ref>Template:Cite journal</ref> 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.

Defence mechanismsEdit

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,<ref>Template:Cite journal</ref> 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.<ref name="Hexabranchus">Template:Cite journal</ref> Other studies of nudibranch molluscs have concluded they are aposematically coloured, for example, the slugs of the family Phylidiidae from Indo-Pacific coral reefs.<ref>Template:Cite journal</ref>

Nudibranchs that feed on hydrozoids can store the hydrozoids' nematocysts (stinging cells) in the dorsal body wall, the cerata.<ref>Template:Cite journal</ref> 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 whole body from their head if decapitated.<ref name="urlExtreme autotomy and whole-body regeneration in photosynthetic sea slugs: Current Biology">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Nudibranchs use a variety of chemical defences to aid in protection,<ref name="Karuso 1987">Template:Cite book, a comprehensive review of the chemical ecology of the nudibranchs</ref> 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.<ref>Template:Cite book</ref> Some sponge-eating nudibranchs concentrate the chemical defences from their prey sponge in their bodies, rendering themselves distasteful to predators.<ref name="Hexabranchus" /><ref name="Gosliner 1987">Template:Cite book</ref> One method of chemical defense used by nudibranchs are secondary metabolites, which play an important role in mediating relationships among marine communities.<ref name="auto">Template:Cite journal</ref> 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.<ref name="Faulkner and Ghiselin 1983">Template:Cite journal</ref> 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.<ref name="auto"/> 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.<ref name="Barsby et al. 2002">Template:Cite journal</ref>

Another protection method is releasing the ugdon acid from the skin.<ref name="Edmunds 1968">Template:Cite journal</ref> Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically, eating the animal from the inside out.

Apparent production of soundEdit

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.<ref>P.H. Gosse, Evenings at the Microscope, 1884 edition,[1] p57</ref>

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.

Life cycleEdit

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File:Nembrotha rutilans Nick Hobgood.jpg
Mating behavior in Nembrotha purpureolineata
File:Editing Image-Acanthodoris lutea laying eggs 2.jpg
Acanthodoris lutea laying eggs

Nudibranchs are hermaphroditic, thus having a set of reproductive organs for both sexes, but they cannot fertilize themselves.<ref name="aquaticcommunity">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Mating usually takes a few minutes and involves a dance-like courtship. Nudibranchs typically deposit their eggs within a gelatinous spiral,<ref name="Reproductive systems ">Template:Cite journal</ref> 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<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>). The eggs contain toxins from sea sponges as a means of deterring predators.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.

Feeding and ecological roleEdit

File:Pteraeolidia ianthina Nick Hobgoood.jpg
Pteraeolidia ianthina 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.<ref name="aquaticcommunity" /> Some feed on sponges, others on hydroids (e.g. Cuthona),<ref>Template:Cite journal</ref> others on bryozoans (phanerobranchs such as Tambja, Limacia, Plocamopherus and Triopha),<ref name="Domínguez2008">Template:Cite journal</ref> and some eat other sea slugs or their eggs (e.g. Favorinus)<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates (e.g. Nembrotha, Goniodoris),<ref name="Valdés2004">Template:Cite journal</ref> other nudibranchs (Roboastra, which are descended from tunicate-feeding species),<ref name="Valdés2004" /> barnacles (e.g. Onchidoris bilamellata),<ref name="Barnes1954">Template:Cite journal</ref> and anemones (e.g. the Aeolidiidae and other Cladobranchia).<ref name="Domínguez2008" />

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'<ref>Template:Cite journal</ref> change with species.<ref>Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.</ref>

TaxonomyEdit

Template:Multiple image Nudibranchs are commonly divided into two main kinds, dorid and aeolid (also spelled eolid) nudibranchs:<ref name=HansBertsch>Hans Bertsch, 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."</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

  • 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.Template:Citation needed 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.<ref>Template:Cite journal</ref>
  • 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).<ref>Template:Cite journal</ref> They lack a mantle.Template:Citation needed 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).<ref name=HansBertsch/> Since 2005,<ref>Taxonomy of the Gastropoda, Bouchet & Rocroi, 2005</ref> 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).<ref name=2005tax>Template:Cite journal</ref> 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).<ref name="Jörger 2010">Template:Cite journal</ref>

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.<ref name=":0">Template:Cite journal</ref>

Traditional hierarchyEdit

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

File:Nudibranch (Charleston, South Carolina, 5 March 1852).jpg
1852 watercolor of a Nudibranch by Jacques Burkhardt

Order Nudibranchia:

Early revisionsEdit

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.<ref>Template:Cite journal</ref> A study published in May 2001, again revised the taxonomy of the Nudibranchia.<ref name="A new taxonomic redescription of the Doridoxidae">Template:Cite journal</ref> They were thus divided into two major clades:

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:

GalleryEdit

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

See alsoEdit

ReferencesEdit

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Further readingEdit

External linksEdit

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VideosEdit

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