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Sea slug is a common name which is applied to some marine invertebrates that more or less resemble terrestrial slugs. The name "sea slug" is most often applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells. The name is also applied to some taxa in other phyla, such as the sea cucumbers, which are echinoderms.
Shell-less marine gastropods
The name "sea slug" is often applied to numerous different evolutionary lineages of marine gastropod molluscs or sea snails, specifically those gastropods that are either not conchiferous (shell-bearing) or appear not to be. In evolutionary terms, losing the shell altogether, having a small internal shell, or having a shell so small that the soft parts of the animal cannot retract into it, are all features that have evolved many times independently within the class Gastropoda, on land and in the sea; these features often cause a gastropod to end up labeled with the common name "slug".
Nudibranchs (clade Nudibranchia) are a large group of marine gastropods which have no shell at all. These may be the most familiar sort of sea slug, at least to scuba divers; nudibranchs, although most are not large, are often very eye-catching because so many of species have brilliant coloration. In addition to nudibranchs, a number of other taxa of marine gastropods (some easily mistaken for nudibranchs) are also often called "sea slugs".
Within the various groups of gastropods that are called "sea slugs" numerous families are within the informal taxonomic group Opisthobranchia:
- The name "Sea slug" is perhaps most often applied to nudibranchs, many of which are very colorful and are a noticeable part of the underwater fauna. For two examples see the images of Nembrotha aurea and Glossodoris atromarginata within this article.
- The name "sea slug" is also often applied to the sacoglossans (clade Sacoglossa), the so-called sap-sucking or solar-powered sea slugs; these are frequently some shade of green.
- Another group of main gastropods that are often labeled as "sea slugs" are the various families of headshield slugs and bubble snails within the clade Cephalaspidea; some of these are very brightly colored.
- The sea hares, clade Aplysiomorpha, are often quite large animals; they have a small, flat, proteinaceous internal shell. Sea hares are sometimes described as large sea slugs.
- The clades Thecosomata and Gymnosomata are small pelagic gastropods known as "sea butterflies" and "sea angels"; these are usually simply known as "pteropods" but can also loosely be called sea slugs, especially the Gymnosomata, which have no shell as adults.
There is also one group of "sea slugs" within the informal group Pulmonata:
- One very unusual group of marine gastropods that are shell-less are the pulmonate (air-breathing) species in the family Onchidiidae, within the clade Systellommatophora.
Non-molluscan uses of the name
This name has also been attributed to other taxa of marine invertebrates with morphology similar to the sea cucumbers, and has also been applied to some Anthozoa coral cnidarians such as the order of the species Fungia.
Some sea slugs apparently emit sounds that are audible to humans, at least this observation was reported by Gosse in 1884.
"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 oftenest 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 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 behaviour in the sea slug Elysia timida". BioMed Central Ltd. Retrieved 2014-05-02.
- "How sea slugs fall in love". Retrieved 2014-05-02.
- A-Z-Animals.com. "Sea Slug (Holothuroidea) - Animals - A-Z Animals - Animal Facts, Information, Pictures, Videos, Resources and Links". A-Z Animals. Retrieved 2014-05-02.
- Rudman, W.B. "Sea cucumbers". The Sea Slug Forum. Retrieved 2014-05-02.
- P.H. Gosse. Evenings at the Microscope 1884 edition p57