See text for superfamilies Nudibranchs (/ˈnjuːdɪbræŋk/[2]) belong to the order Nudibranchia, a group of soft-bodied marine gastropod molluscs that shed their shells after their larval stage.

[20] By contrast, on the back of the aeolids in the clade Cladobranchia, brightly coloured sets of protruding organs called cerata are present.

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.

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.

[22] Other studies of nudibranch molluscs have concluded they are aposematically coloured, for example, the slugs of the family Phylidiidae from Indo-Pacific coral reefs.

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.

[22][28] One method of chemical defense used by nudibranchs are secondary metabolites, which play an important role in mediating relationships among marine communities.

Certain Antarctic marine species defense mechanisms are believed to be controlled by biological factors like predation, competition, and selective pressures.

[33] 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 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.

[34] Some feed on sponges, others on hydroids (e.g. Cuthona),[38] others on bryozoans (phanerobranchs such as Tambja, Limacia, Plocamopherus and Triopha),[39] and some eat other sea slugs or their eggs (e.g. Favorinus)[40] or, on some occasions, are cannibals and prey on members of their own species.

Other groups feed on tunicates (e.g. Nembrotha, Goniodoris),[41] other nudibranchs (Roboastra, which are descended from tunicate-feeding species),[41] barnacles (e.g. Onchidoris bilamellata),[42] and anemones (e.g. the Aeolidiidae and other Cladobranchia).

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.

[44] Since 2005,[48] 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).

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.