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Berghia coerulescens (Laurillard, 1830).jpg
Berghia coerulescens
Chromodoris lochi (AA3).jpg
A pair of Chromodoris lochi from Puerto Galera, the Philippines.
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
(unranked): clade Heterobranchia

clade Euthyneura
clade Nudipleura
clade Nudibranchia


See text for superfamilies

A nudibranch /ˈnjdɨbræŋk/[1] is a member of Nudibranchia, a group of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage.[2] They are noted for their often extraordinary colors and striking forms. There are more than 3,000 described species of nudibranchs.[3][better source needed]


Nudibranchs are often casually called sea slugs, but many sea slugs belong to several taxonomic groups which are not closely related to nudibranchs. A number of these other sea slugs, such as the colorful Aglajidae, are often confused with nudibranchs.

Distribution and habitat[edit]

Nudibranchs occur in seas worldwide, including both the tropics and Antarctica.[3]

Nudibranchs live at virtually all depths of salt water, from the intertidal zone to depths of well over 700 m (2,300 ft).[3] The greatest diversity of nudibranchs is seen in warm, shallow reefs, though a new nudibranch species was discovered at a depth near 2,500 m (8,200 ft).[4]

Nudibranchs are benthic animals, found crawling over the bottom substrate.[3] An exception to this is the neustonic Glaucus and Glaucilla nudibranchs, which float upside down just under the ocean's surface. Another exception is the pelagic nudibranch, Cephalopyge trematoides, which swims in the water column.[5][6]

Anatomical description[edit]

Aeolidiella stephanieae. Nudibranch body. Note the oral tentacles (ot), foot tentacles (ft), eye (e), rhinophores (r), cerata (c). This species has cnidosacs (cn) at the cerata tips.

The body forms of nudibranchs vary a great deal, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical both externally and internally because they have undergone secondary detorsion. They lack a mantle cavity. Some species have venomous appendages (cerata) on their sides, which deter predators. Many also have a simple gut and a mouth with a radula.

Their eyes are simple and able to discern little more than light and dark.[7] The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.[8]

They vary in adult size from 20 to 600 mm (0.79 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).

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.[9] By contrast, on the back of the aeolids in the clade Cladobranchia there are brightly colored sets of protruding organs called cerata.


Defense mechanisms[edit]

Nudibranchs (Nembrotha kubaryana) eating clavelina tunicate colonies.

This group includes some of the most colorful creatures on earth. In the course of their evolution, nudibranchs have lost their shell while developing alternative defense mechanisms. Some species evolved an external anatomy with textures and colors that mimicked surrounding sessile invertebrate animals (often their prey sponges or soft corals) to avoid predators (see camouflage). Other nudibranchs, as seen especially well on chromodorids, have an intensely bright and contrasting color pattern that makes them especially conspicuous in their surroundings. This is believed to be an example of aposematic coloration; the shocking coloration warns potential predators that the slugs are distasteful or poisonous.


Nudibranchs use a variety of chemical defenses to aid in protection, but it is not necessary for the strategy to be lethal in order to be effective: some successful toxins induce bradycardia or hypotension in a predator, allowing the nudibranch to escape consumption while its attacker is incapacitated.[11] Some sponge-eating nudibranchs concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators.[12] The evidence that suggests the toxins used by dorid nudibranchs do in fact come from dietary sponges lies in the similarities between the primary and secondary metabolites of prey and nudibranchs, respectively. Furthermore, nudibranchs contain a mixture of sponge chemicals when they are in the presence of multiple food sources as well as change defense chemicals with a concurrent change in diet.[13] This, however, is not the only way for nudibranchs to develop chemical defenses. Certain species are able to produce their own chemicals de novo without dietary influence. Evidence for the different methods of chemical production comes with the characteristic uniformity of chemical composition across drastically different environments and geographic locations found throughout de novo production species compared to the wide variety of dietary and environmentally dependent chemical composition in sequestering species.[14]

Another method of protection is the release of an acid from the skin.[15] Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically.

Life cycle[edit]

Nudibranch mating behavior in Nembrotha purpureolineata.
Acanthodoris lutea laying eggs


Nudibranchs typically deposit their eggs within a gelatinous spiral.[17]

Feeding and ecological role[edit]

Solar powered Pteraeolidia ianthina have adapted cerata to contain zooxanthellae which continue to photosynthesize and provide energy to the nudibranch.

All known nudibranchs are carnivorous.[16] Some feed on sponges, others on hydroids,(e.g. Cuthona)[18] others on bryozoans (phanerobranchs such as Tambja, Limacia, Plocamopherus and Triopha),[19] and some eat other sea slugs or their eggs (e.g. Favorinus)[20] or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates (e.g. Nembrotha, Goniodoris),[21] other nudibranchs (Roboastra, which are descended from tunicate-feeding species),[21] barnacles (e.g. Onchidoris bilamellata),[22] and anemones (e.g. the Aeolidiidae and other Cladobranchia).[19]

The surface dwelling nudibranch, Glaucus atlanticus is a specialist predator of siphonophores, such as the Portuguese Man O' War. This predatory mollusk sucks air into its stomach to keep it afloat and using its muscular foot it clings to the surface film. If it finds a small victim Glaucus simply envelops it with its capacious mouth, but if the prey is a larger siphonophore the mollusk nibbles off its fishing tentacles, the ones carrying the most potent nematocysts. Like some others of its kind Glaucus does not digest the nematocysts; instead, it uses them to defend itself by passing them from its gut to the surface of its skin.[23]

For a comprehensive list of nudibranch feeding preferences, see the electronic supplement from McDonald and Nybakken.[24][25]


Aeolids have many cerata over their back which are used for defense as well as respiration. Hermissenda crassicornis.
Nudibranchs are frequently differentiated as either dorid or aeolid.

Nudibranchs are commonly divided into two main kinds, dorid nudibranchs and aeolid (also spelled eolid) nudibranchs:[26][27]

The exact systematics of nudibranchs are a topic of recent revision. Traditionally nudibranchs have been treated as the order Nudibranchia, located in the gastropod mollusc sub-class Opisthobranchia (the marine slugs: which consisted of nudibranchs, sidegill slugs, bubble snails, algae sap-sucking sea slugs, and sea hares).[26] Since 2005, pleurobranchs (which had previously been grouped among sidegill slugs) have been placed alongside nudibranchs in the clade Nudipleura (recognising them as more closely related to each other than to other opisthobranchs).[28] Since 2010, it has been recognised that Opisthobranchia was not a valid clade (it is paraphyletic) and instead Nudipleura has been placed as the first offshoot of Euthyneura (which is the dominant clade of gastropods).[29]

Traditional hierarchy[edit]


Order Nudibranchia:

Early revisions[edit]

Newer insights derived from morphological data and gene-sequence research, seemed to confirm those ideas. On the basis of investigation of 18S rDNA sequence data, there is strong evidence for support of the monophyly of the Nudibranchia and its two major groups, the Anthobranchia/Doridoidea and Cladobranchia.[30] A study published in May 2001, again revised the taxonomy of the Nudibranchia.[31] They were thus divided into two major clades:

  • Anthobranchia (= Bathydoridoidea + Doridoidea)



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


  3. ^ a b c d Nudibranchs, Fishermen Scuba.
  7. ^ USA (2013-04-25). "Nudibranchs - National Geographic Magazine". Retrieved 2013-07-04. 
  11. ^ Fuhrman, F. A.; Fuhrman, G. J.; Deriemer, K. (1979). "Toxicity and pharmacology of extracts from dorid nudibranchs". Biological Bulletin 156: 289–299. 
  13. ^ Faulkner, D. J.; Ghiselin, M. T. (1983). "Chemical defense and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods". Marine Ecology-Progress Series 13: 295–301. doi:10.3354/meps013295. 
  16. ^ a b "Nudibranch". Retrieved 2013-07-04. 
  18. ^ NC Folino (1997). "The role of prey mobility in the population ecology of the nudibranch Cuthona nana (Gastropoda: Opisthobranchia)". American Malacological Bulletin. 
  24. ^ McDonald, G. R.; Nybakken, J. W. (1997). "A worldwide review of the food of nudibranch mollusks. I. Introduction and the suborder Arminacea". Veliger 40 (2): 157–159. 
  25. ^ McDonald, G. R.; Nybakken, J. W. (1999). "A worldwide review of the food of nudibranch mollusks. II. The suborder Dendronotacea". Veliger 42 (1): 62–66. 
  26. ^ a b Hans Bertsch, Nudibranchs: Marine slugs with verve. Navanax inermis[..] is the bane of all nudibranchs, because it is one of the few known predators on this group of slugs. [...] Dorids mainly eat sponges, bryozoans and tunicates, whereas aeolids principally eat cnidarians.
  29. ^ Jörger K. M., Stöger I., Kano Y., Fukuda H., Knebelsberger T. & Schrödl M. (2010). "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". BMC Evolutionary Biology 10: 323. doi:10.1186/1471-2148-10-323. "At the basis of the Euthyneura the Nudipleura split off"

Further reading[edit]

  • Gary R. McDonald. July 29, 2006. Nudibranch Systematic Index. Institute of Marine Sciences. Paper Nudibranch_Systematic_Index. - Note: This is a good resource for getting a species list in certain genera, but it is not good for ascertaining the taxonomy of a genera, because it does not use the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005).
  • Neville Coleman (2008). Nudibranchs Encyclopedia: Catalogue of Asia/Indo-Pacific Sea Slugs. Neville Coleman's Underwater Geographic. ISBN 0-947325-41-7

External links[edit]