Editing Sea slug (section)

==Shell-less marine gastropods==
The name "sea slug" is often applied to numerous different evolutionary lineages of marine [[gastropod]] [[mollusc]]s or [[sea snail]]s, specifically those gastropods that are either not conchiferous (shell-bearing) or appear not to be.<ref name="Frontiers">{{cite journal |title=Mating behaviour in the sea slug ''Elysia timida'' (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity |author1=Schmitt, V. |author2=Anthes, N. |author3=Michiels, N.K. |journal=Frontiers in Zoology |publisher=BioMed Central Ltd |volume=4 |issue=17 |date=July 2007 |page=17 | pmc=1934903|doi=10.1186/1742-9994-4-17|pmid=17610714 |doi-access=free }}</ref> 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 be labeled with the common name "slug".

[[File:Glossodoris atromarginata.jpg|thumb|right| The nudibranch ''[[Glossodoris atromarginata]]'']]

Nudibranchs (clade '''[[Nudibranch]]ia''') are a large group of marine gastropods that have no shell at all. These may be the most familiar sort of sea slug. Although most nudibranchs are not large, they are often very eye-catching because so many 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".<ref>[http://www.seaslugforum.net/find/brthomp2 Thompson, T. E., & Brown, G.H., 1984. Biology of opisthobranch Molluscs. Vol. 2. Ray Society; London. 1-229, p.49]</ref>

===Gastropod groups===
[[File:Lettuce_Sea_Slug_11-03-2006.jpg|thumb|right|The sacoglossan ''[[Elysia crispata]]'']]
[[File:Chelidonura varians.jpg|thumb|right|The cephalaspidean ''[[Chelidonura varians]]'']]
[[File:Aplysia dactylomela 2.jpg|thumb|right|The [[sea hare]] ''[[Aplysia dactylomela]]'', photographed out of water]]
Within the various groups of gastropods that are called "sea slugs", numerous families are within the informal taxonomic group '''[[Opisthobranchia]]'''. The term "sea slug" is perhaps most often applied to [[nudibranch]]s, many of which are brightly patterned and conspicuously ornate. The name "sea slug" is also often applied to the sacoglossans (clade '''[[Sacoglossa]]'''), the so-called sap-sucking or solar-powered sea slugs which are frequently a 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]]'''. The [[sea hares]], clade '''[[Aplysiomorpha]]''', have a small, flat, [[proteinaceous]] internal shell. The clades '''[[Thecosomata]]''' and '''[[Gymnosomata]]''' are small [[pelagic]] gastropods known as "sea butterflies" and "sea angels". Many species of sea butterflies retain their shells. These are commonly known as "[[pteropods]]" but are also sometimes called sea slugs; especially the Gymnosomata, which have no shell as adults.
[[File:Onchidella celtica 0377.jpg|thumb|right|The marine pulmonate ''[[Onchidella celtica]]'']]

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]]'''.<ref name="Fresh">{{cite web |date=2007 |title=How sea slugs fall in love |url=https://freshscience.org/2007/how-sea-slugs-fall-in-love |access-date=2 May 2014 |website=freshscience.org}}</ref>