Editing Sea slug

{{Short description|Group of marine gastropods}}
{{For|the missile of the same name|Seaslug (missile)}}
{{Distinguish|sea cucumber|sea snail}}
[[File:Nembrotha aurea B.jpg|thumb|right|The [[nudibranch]] ''[[Nembrotha aurea]]'' is a gastropod.]]
[[File:Fish3923 - Flickr - NOAA Photo Library.jpg|thumb|right|A [[sea cucumber]] also looks slug-like and is sometimes loosely called a "sea slug", but it is ''not'' a gastropod.]]

'''Sea slug''' is a [[common name]] for some [[Marine biology|marine]] [[invertebrates]] with varying levels of resemblance to terrestrial [[Slug|slugs]]. Most creatures known as sea slugs are [[gastropods]], i.e. they are [[Sea snail|sea snails]] (marine gastropod [[mollusks]]) that, over evolutionary time, have either entirely lost their shells or have seemingly lost their shells due to having a significantly reduced or internal shell.<ref>{{Citation |last=Yonow |first=Nathalie |title=Sea Slugs: Unexpected Biodiversity and Distribution |date=2015 |work=The Red Sea: The Formation, Morphology, Oceanography and Environment of a Young Ocean Basin |pages=531–550 |editor-last=Rasul |editor-first=Najeeb M.A. |url=https://doi.org/10.1007/978-3-662-45201-1_30 |access-date=2024-02-09 |series=Springer Earth System Sciences |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-662-45201-1_30 |isbn=978-3-662-45201-1 |editor2-last=Stewart |editor2-first=Ian C.F.|url-access=subscription }}</ref> The name "sea slug" is often applied to [[Nudibranch|nudibranchs]] and a [[paraphyletic]] set of other marine gastropods without apparent [[Gastropod shell|shells]].<ref>Thompson, T. E. 1976. Biology of opisthobranch molluscs, vol. 1, 207 pp., 21 pls. Ray Society, no. 151.</ref>

Sea slugs have an enormous variation in body shape, color, and size. Most are partially translucent. The often bright colors of [[Coral reef|reef]]-dwelling species imply that these animals are under constant threat of predators. Still, the color can warn other animals of the sea slug's toxic stinging cells ([[nematocysts]]) or offensive taste. Like all [[Gastropod|gastropods]], they have small, razor-sharp teeth called [[Radula|radulas]]. Most sea slugs have a pair of [[Rhinophore|rhinophores]]—sensory [[tentacles]] used primarily for the sense of smell—on their head, with a small eye at the base of each rhinophore. Many have feathery structures ([[cerata]]) on the back, often in a contrasting color, which act as gills. All species of genuine sea slugs have a selected prey animal on which they depend for food, including certain [[jellyfish]], [[bryozoans]], [[sea anemones]], [[plankton]], and other species of sea slugs.<ref>{{cite book|title=The Blue Planet: A Natural History of the Oceans |last1=Byatt |first1=Andrew |last2=Fothergill |first2=Alastair |last3=Holmes |first3=Martha |others=BBC |year=2002 |publisher=DK |isbn=978-0-7894-8265-5}}</ref><ref>{{cite web|url=https://www.encyclopedia.com/plants-and-animals/animals/zoology-invertebrates/sea-slug |title=Sea Slug |date=18 May 2018 |website=encyclopedia.com |publisher=Oxford University Press}}</ref>

Sea slugs have brains. For example, ''Aplysia californica'' has a brain of about 20,000 nerve cells.<ref>{{cite web|url=https://www.scientificamerican.com/article/human-sea-slug-brains-sha |title=Human, Sea Slug Brains Share Genes for Alzheimer's and Parkinson's |last=Choi |first=Charles Q. |work=Scientific American |date=29 December 2006}}</ref>

==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>

==Diversity in sea slugs==
Like many nudibranchs, ''[[Glaucus atlanticus]]'' can store and use stinging cells, or [[nematocysts]], from its prey ([[Portuguese man o' war]]) in its finger-like [[cerata]].<ref>{{Cite journal |last1=Thompson |first1=T. E. |last2=Bennett |first2=I. |date=1969-12-19 |title=Physalia Nematocysts: Utilized by Mollusks for Defense |url=https://www.science.org/doi/10.1126/science.166.3912.1532 |journal=Science |language=en |volume=166 |issue=3912 |pages=1532–1533 |doi=10.1126/science.166.3912.1532 |pmid=17742854 |bibcode=1969Sci...166.1532T |issn=0036-8075|url-access=subscription }}</ref>
Other species, like the Pyjama slug ''[[Chromodoris quadricolor]]'', may use their striking colors to advertise their foul chemical taste.

The lettuce sea slug (''[[Elysia crispata]]'') has lettuce-like ruffles that line its body. This slug, like other [[Sacoglossa]], uses [[kleptoplasty]], a process in which the slug absorbs [[chloroplasts]] from the [[algae]] it eats, and uses "stolen" cells to [[Photosynthesis|photosynthesize]] sugars. The ruffles of the lettuce sea slug increase the slug's surface area, allowing the cells to absorb more light.

Headshield slugs, like the ''[[Chelidonura varians]]'', use their shovel-shaped heads to dig into the sand, where they spend most of their time. The shield also protects sand from entering the mantle during burrowing.

''[[Peronia indica]]'' is a [[species]] of air-breathing sea slug, a shell-less [[marine (ocean)|marine]] [[pulmonate]] [[gastropod]] [[mollusk]] in the [[family (biology)|family]] [[Onchidiidae]].<ref name="WoRMS">MolluscaBase (2019). MolluscaBase. Peronia indica (Labbé, 1934). Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=1033079 on 2019-10-05</ref>

The largest species of sea hare, the California black sea hare, ''[[Aplysia vaccaria]]'' can reach a length of 75 centimetres (30&nbsp;in) and a weight of 14 kilograms (31 lb).<ref>{{Cite web |title=WoRMS - World Register of Marine Species - Aplysia californica J. G. Cooper, 1863 |url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=240765 |access-date=2024-02-09 |website=www.marinespecies.org |language=en}}</ref> Most sea hares have several defenses; in addition to being naturally toxic, they can eject a foul ink or secrete a viscous slime to deter predators.

Some species of [[Acochlidiacea|acochlidian]] sea slugs have made evolutionary transitions to living in freshwater streams<ref>{{Cite journal|last1=Schrödl|first1=Michael|last2=Neusser|first2=Timea P.|date=2010-01-01|title=Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)|journal=Zoological Journal of the Linnean Society|language=en|volume=158|issue=1|pages=124–154|doi=10.1111/j.1096-3642.2009.00544.x|issn=0024-4082|doi-access=free}}</ref> and there is at least one evolutionary transition to land.<ref>{{Cite journal|last1=Kano|first1=Yasunori|last2=Neusser|first2=Timea P.|last3=Fukumori|first3=Hiroaki|last4=Jörger|first4=Katharina M.|last5=Schrödl|first5=Michael|date=2015-10-01|title=Sea-slug invasion of the land|journal=Biological Journal of the Linnean Society|language=en|volume=116|issue=2|pages=253–259|doi=10.1111/bij.12578|issn=0024-4066|doi-access=free}}</ref>

==References==
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[[Category:Marine gastropods|Slugs]]
[[Category:Mollusc common names]]