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Berghia coerulescens (Laurillard, 1830).jpg
Berghia coerulescens
Chromodoris lochi (AA3).jpg
Chromodoris lochi pair in Puerto Galera, the Philippines
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Infraclass: Euthyneura
Clade: Nudipleura
Order: Nudibranchia
Cuvier, 1817

See text for superfamilies

Diversity [1]
about 3000 species

Nudibranchs ( /ˈnjdɪbræŋk/ [2] ) are a group of soft-bodied, marine gastropod molluscs which shed their shells after their larval stage. [3] 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," [4] or "sea rabbit." [5] Currently, about 3,000 valid species of nudibranchs are known. [6]


The word "nudibranch" comes from the Latin nudus "naked" and the Ancient Greek βράγχια (bránkhia) "gills".

Nudibranchs are often casually called sea slugs, as they are a family of opistobranchs (sea slugs), within the phylum Mollusca (molluscs), 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 photosynthetic Sacoglossa and the colourful Aglajidae, are often confused with nudibranchs.

Distribution and habitat

Janolus sp. in Anilao, the Philippines Janolus Sp taken in Anilao Philippines.png
Janolus sp. in Anilao, the Philippines
Kalinga ornata from Anilao Pier dive site, depth 5 m Kalinga ornata.png
Kalinga ornata from Anilao Pier dive site, depth 5 m

Nudibranchs occur in seas worldwide, ranging from the Arctic, through temperate and tropical regions, to the Southern Ocean around Antarctica. [6] [7] [8] They are almost entirely restricted to salt water, although a few species are known to inhabit lower salinities in brackish water. [9]

Nudibranchs live at virtually all depths, from the intertidal zone to depths well over 700 m (2,300 ft). [7] The greatest diversity of nudibranchs is seen in warm, shallow reefs, although a new nudibranch species was discovered at a depth near 2,500 m (8,200 ft). [10]

Nudibranchs are benthic animals, found crawling over the substrate. [7] The only exceptions to this are the neustonic Glaucus nudibranchs, which float upside down just under the ocean's surface, and the pelagic nudibranchs Cephalopyge trematoides , which swim in the water column [11] [12] and Phylliroe bucephalum. [13]

Anatomical description

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 mm. 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 a great deal, but 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. [14]

The eyes in nudibranchs are simple and able to discern little more than light and dark. [15] The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors. [16]

Nudibranchs vary in adult size from 4 to 600 mm (0.16 to 23.62 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). In most species is a swimming veliger larva with a coiled shell which 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. [14]

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. [17] 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 mechanisms

Nudibranchs (Nembrotha kubaryana) eating Clavelina tunicate colonies Kubaryana eating clavelina.jpg
Nudibranchs ( Nembrotha kubaryana ) eating Clavelina tunicate colonies

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 (camouflage). Other nudibranchs, as seen especially well on chromodorids, 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, [18] 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. [19] Other studies of nudibranch molluscs have concluded they are aposematically coloured, for example, the slugs of the family Phylidiidae from Indo-Pacific coral reefs. [20]

Nudibranchs that feed on hydrozoids can store the hydrozoids' nematocysts (stinging cells) in the dorsal body wall, the cerata. [21] 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, 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.

Nudibranchs use a variety of chemical defences to aid in protection, [22] but it is not necessary for the strategy to be lethal to be effective; in fact, good arguments exist that chemical defences should evolve to be distasteful rather than toxic. [23] Some sponge-eating nudibranchs concentrate the chemical defences from their prey sponge in their bodies, rendering themselves distasteful to predators. [19] [24] 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. [25] This, however, is not the only way for nudibranchs to develop chemical defences. 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. [26]

Another method of protection is the release of an acid from the skin. [27] Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically.

Apparent production of sound

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. [28]

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, and 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 the 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.


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

Nudibranchs are hermaphroditic, thus have a set of reproductive organs for both sexes, but they cannot fertilize themselves. [29] Mating usually takes a few minutes, and involves a dance-like courtship. Nudibranchs typically deposit their eggs within a gelatinous spiral, [30] 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 ). The eggs contain toxins from sea sponges as a means of deterring predators. [31] 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 role

Pteraeolidia ianthina has adapted cerata to house symbiotic zooxanthellae obtained from its diet, which continue to photosynthesize and provide energy to the nudibranch. 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. [29] Some feed on sponges, others on hydroids (e.g. Cuthona ), [32] others on bryozoans (phanerobranchs such as Tambja , Limacia, Plocamopherus and Triopha), [33] and some eat other sea slugs or their eggs (e.g. Favorinus ) [34] or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates (e.g. Nembrotha , Goniodoris ), [35] other nudibranchs ( Roboastra , which are descended from tunicate-feeding species), [35] barnacles (e.g. Onchidoris bilamellata), [36] and anemones (e.g. the Aeolidiidae and other Cladobranchia). [33]

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. [37]


Chromodoris willani for nudipixel.jpg
Dorids ( Chromodoris willani shown) breathe with the branchial plume, which projects from around their anus.
Nudi from tidepool.jpg
Aeolids ( Hermissenda crassicornis pictured) have many cerata over their back which are used for defense and respiration.
Nudibranchs are frequently differentiated as either dorid or aeolid.

Nudibranchs are commonly divided into two main kinds, dorid and aeolid (also spelled eolid) nudibranchs: [38] [39]

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). [38] Since 2005, [40] 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). [41] 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). [42]

Traditional hierarchy

This classification was based on the work of Johannes Thiele (1931), [43] who built on the concepts of Henri Milne-Edwards (1848). [44]

Order Nudibranchia:

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, strong evidence supports the monophyly of the Nudibranchia and its two major groups, the Anthobranchia/Doridoidea and Cladobranchia. [45] A study published in May 2001, again revised the taxonomy of the Nudibranchia. [46] 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:

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

See also

Related Research Articles

Sea slug group of marine invertebrates with varying levels of resemblance to terrestrial slugs

Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are actually gastropods, i.e. they are sea snails that over evolutionary time have either completely lost their shells, or have seemingly lost their shells due to having a greatly reduced or internal shell. The name "sea slug" is most often applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells.

Opisthobranchia infraclass of molluscs

Opisthobranchs is now an informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.

<i>Glaucus atlanticus</i> species of mollusc

Glaucus atlanticus is a species of small, blue sea slug, a pelagic aeolid nudibranch, a shell-less gastropod mollusk in the family Glaucidae.

Doridoidea superfamily of molluscs

Doridoidea, commonly known as dorid nudibranchs, are a taxonomic superfamily of medium to large, shell-less sea slugs, marine gastropod mollusks in the clade Doridacea, included in the clade Nudibranchia.

Metarminoidea superfamily of molluscs

Metarminoidea is a provisional taxonomic superfamily of colourful sea slugs, aeolid nudibranchs, marine opisthobranch gastropod molluscs in the clade Nudibranchia.

Phyllidiidae family of molluscs

Phyllidiidae is a family of sea slugs, dorid nudibranchs, marine gastropod mollusks in the superfamily Phyllidioidea.

<i>Phyllidia babai</i> species of mollusc

Phyllidia babai is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Phyllidiidae.

<i>Paraflabellina funeka</i> species of mollusc

The purple lady nudibranch, Paraflabellina funeka, is a species of aeolid nudibranch, and is a very colourful sea slug. It is a marine gastropod mollusc in the family Flabellinidae.

<i>Fjordia capensis</i> species of mollusc

The white-edged nudibranch, Fjordia capensis, is a species of sea slug, specifically an aeolid nudibranch, a very colourful sea slug. It is a marine gastropod mollusc in the family Flabellinidae.

<i>Trinchesia speciosa</i> species of mollusc

Trinchesia speciosa, common name the "candy nudibranch", is a species of sea slug, an aeolid nudibranch, a marine gastropod mollusc in the family Trinchesiidae.

Discodorididae family of molluscs

Discodorididae is a taxonomic family of sea slugs, specifically dorid nudibranchs, marine gastropod mollusks in the superfamily Doridoidea.

Euctenidiacea suborder of molluscs

The Euctenidiacea, common name dorid nudibranchs, are a taxonomic suborder of sea snails or slugs, marine gastropod molluscs in the order Nudibranchia. Bouchet & Rocroi (2005) rejected the name Anthobranchia on the grounds that it also included Onchidium at the time of original publication. Doridina is equivalent and used in the latest classification.

Dexiarchia suborder of molluscs

The Dexiarchia are a suborder of sea slugs, shell-less marine gastropod molluscs in the order Nudibranchia. This classification is based on the study by Schrödl et al., published in 2001, who recognized within this clade two clades Pseudoeuctenidiacea and Cladobranchia.

Aeolidida parvorder of molluscs

The Aeolidida is a taxonomic clade of sea slugs, specifically aeolid nudibranchs, marine gastropod molluscs in the clade Cladobranchia. They are distinguished from other nudibranchs by their possession of cerata containing cnidosacs.

Peltodoris is a genus of sea slugs, dorid nudibranchs, shell-less marine gastropod molluscs in the family Discodorididae.

Loy is a genus of sea slugs, dorid nudibranchs, marine gastropod mollusks in the family Corambidae within the superfamily Onchidoridoidea.

Rostanga crawfordi is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Discodorididae. Originally described as Discodoris crawfordi, it was redescribed by Rudman & Avern as Rostanga australis. The two names were synonymised by Dayrat.


A cnidosac is an anatomical feature that is found in the group of sea slugs known as aeolid nudibranchs, a clade of marine opisthobranch gastropod molluscs. A cnidosac contains cnidocytes, stinging cells that are also known as cnidoblasts or nematocysts. These stinging cells are not made by the nudibranch, but by the species that it feeds upon. However, once the nudibranch is armed with these stinging cells, they are used in its own defense.

Phyllidia multituberculata is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Phyllidiidae.

<i>Phyllodesmium poindimiei</i> species of mollusc

Phyllodesmium poindimiei is an Alcyonacea feeding, aeolid nudibranch Gastropod belonging to the family Facelinidae. Cerata are important in this clade in terms of their physical defense and efficient metabolic processes.This species is spread sporadically along tropical coastal regions such as Australia, Hawaii, and the Indo-Pacific living in diverse marine habitats such as coral reefs. Unlike other species in the Opisthobranch Mollusca clade, P. poindimiei’s lush pink cerata are used for defensive purposes other than Nematocyst (dinoflagellate) capture and toxin release. Organismal ties within these thriving, tropical ecosystems can be determinants of environment change, which affects massive coral ecosystems. Continuously changing marine ecosystems, such as coral reefs, are directly linked to the evolution of organisms that live and thrive in the tropics such as the soft nudibrach P. poindimiei.


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Further reading