Nudibranchs (/ˈnjuːdɪbræŋk/[2]) are a group of soft-bodiedmarinegastropodmolluscs that 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", and[4] "sea rabbit".[5] Currently, about 3,000 valid species of nudibranchs are known.[6]
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 photosyntheticSacoglossa and the colourful Aglajidae, are often confused with nudibranchs.
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 one nudibranch species was discovered at a depth near 2,500 m (8,200 ft).[10]
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.[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).[17]
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.[14]
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.[18] 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.
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 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,[19] 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.[20] Other studies of nudibranch molluscs have concluded they are aposematically coloured, for example, the slugs of the family Phylidiidae from Indo-Pacific coral reefs.[21]
Nudibranchs that feed on hydrozoids can store the hydrozoids' nematocysts (stinging cells) in the dorsal body wall, the cerata.[22] 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. Some of these species have been observed practicing autotomy, severing portions of their body in order to remove parasites, and have been observed to regrow their head if decapitated.[23]
Nudibranchs use a variety of chemical defences to aid in protection,[24] 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.[25] Some sponge-eating nudibranchs concentrate the chemical defences from their prey sponge in their bodies, rendering themselves distasteful to predators.[20][26] One method of chemical defense used by nudibranchs are secondary metabolites, which play an important role in mediating relationships among marine communities.[27] 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.[28] 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 and competition, and selective pressures.[27] 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.[29]
Another method of protection is the release of the ugdon acid from the skin.[30] 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 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.[31]
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.
Life cycle
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Nudibranchs are hermaphroditic, thus having a set of reproductive organs for both sexes, but they cannot fertilize themselves.[32] Mating usually takes a few minutes, and involves a dance-like courtship. Nudibranchs typically deposit their eggs within a gelatinous spiral,[33] 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{not a nudibranch}). The eggs contain toxins from sea sponges as a means of deterring predators.[34] 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.
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.[40]
Taxonomy
Dorids (Chromodoris willani shown) breathe with the branchial plume, which projects from around their anus.
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:[41][42]
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] 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.[43]
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).[44] They lack a mantle.[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).[41] Since 2005,[45]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).[46]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).[47]
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.[50] A study published in May 2001, again revised the taxonomy of the Nudibranchia.[51] 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:
^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 [...]"
^Gosliner TM, Valdes A Behrens DW 2015 Nudibranch and Sea Slug Identification Indo-Pacific New World Publications Jacksonville Florida USA
^ abThompson, T. E. 1976. Biology of opisthobranch molluscs, vol. 1, 207 pp., 21 pls. Ray Society, no. 151.
^Edmunds, M. (1991). "Does warning colouration occur in nudibranchs?". Malacologia. 32: 241–255.
^ abPawlik, JR; et al. (1988). "Defensive chemicals of the Spanish Dancer nudibranch, Hexabranchus sanguineus, and its egg ribbons: Macrolides derived from a sponge diet". Journal of Experimental Marine Biology and Ecology. 119 (2): 99–109. doi:10.1016/0022-0981(88)90225-0.
^Karuso, P. (1987). "Chemical Ecology of the Nudibranchs". In Scheuer, PJ (ed.). Bioorganic Marine Chemistry. Vol. 1. Springer-Verlag. pp. 31–60. doi:10.1007/978-3-642-72726-9_2. ISBN978-3-642-72728-3., a comprehensive review of the chemical ecology of the nudibranchs
^Pawlik, JR (2012). Fattorusso, E.; et al. (eds.). Antipredatory defensive roles of natural products from marine invertebrates. NY: Springer Science. pp. 677–710. ((cite book)): |work= ignored (help)
^Gosliner, T. M. (1987). Nudibranchs of Southern Africa. Sea Challengers. ISBN978-0-930118-13-6.
^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. S2CID35384332.
^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.
^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. JSTOR1986.
^Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
^ abHans 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."
^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.
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