Editing Sea slug (section)

{{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>