Editing Octopus

{{Short description|Soft-bodied eight-limbed order of molluscs}}
{{About |the order of cephalopod}}
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{{Featured article}}
{{Use dmy dates|date=October 2019}}
{{Use British English|date=May 2017}}
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NOTE: This article uses the English pluralisation, "octopuses". This is correct and intentional, please maintain it. Other variants are discussed in the == Etymology and pluralisation == section.
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{{Automatic taxobox
| fossil_range = {{Fossil range|Middle Jurassic|0|[[Middle Jurassic]] – recent}}
| image = Octopus2.jpg
| image_caption = [[Common octopus]]<br/>(''Octopus vulgaris'')
| image_alt = Common octopus on seabed
| display_parents = 3
| taxon = Octopoda
| authority = [[William Elford Leach|Leach]], 1818<ref>{{cite web |url=https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=82589 |title=ITIS Report: Octopoda Leach, 1818 |publisher=Itis.gov |date=10 April 2013 |access-date=4 February 2014}}</ref>
| subdivision_ranks = Suborders
| subdivision = (traditional)
* [[Cirrina]]
* [[Incirrina]]
See {{section link||Evolution}} for families
| synonyms = * Octopoida<br/><small>Leach, 1817</small><ref name="Mikko">{{cite web |url=http://www.helsinki.fi/~mhaaramo/metazoa/protostoma/mollusca/cephalopoda/coleoidea.html |website=Mikko's Phylogeny Archive |title=Coleoidea&nbsp;– Recent cephalopods}}</ref>
}}

An '''octopus''' ({{Plural form}}: '''octopuses''' or '''octopodes'''{{efn|See {{slink||Etymology and pluralisation}} for variants.}}) is a soft-bodied, eight-limbed [[Mollusca|mollusc]] of the [[order (biology)|order]] Octopoda ({{IPAc-en|ɒ|k|ˈ|t|ɒ|p|ə|d|ə}}, {{respell|ok|TOP|ə|də}}<ref>{{cite Merriam-Webster|Octopoda|access-date=12 July 2021}}</ref>). The order consists of some 300 [[species]] and is grouped within the class [[Cephalopod]]a with [[squid]]s, [[cuttlefish]], and [[nautiloid]]s. Like other cephalopods, an octopus is [[symmetry in biology|bilaterally symmetric]] with two eyes and a [[cephalopod beak|beak]]ed mouth at the centre point of the eight limbs.{{efn|"[[Tentacle]]" is a common [[umbrella term]] for cephalopod limbs. In [[Teuthology|teuthological]] context, octopuses have "arms" with suckers along their entire length while "tentacle" is reserved for appendages with suckers only near the end of the limb, which octopuses lack.<ref>{{cite web|url=https://aquarium.ucsd.edu/blog/get-to-know-the-four-types-of-cephalopods/|title=Get to Know th Four Types of Cephalopod|publisher=U CSan Diego|date=October 11, 2018|last=Scully|first=Caitlin|work=Birch Aquarium Blog|access-date=12 September 2021|archive-date=24 September 2021|archive-url=https://web.archive.org/web/20210924222532/https://aquarium.ucsd.edu/blog/get-to-know-the-four-types-of-cephalopods/|url-status=dead}}</ref>}} An octopus can radically deform its shape, enabling it to squeeze through small gaps. They trail their appendages behind them as they swim. The [[Siphon (mollusc)|siphon]] is used for [[aquatic respiration|respiration]] and [[aquatic locomotion|locomotion]] (by [[jet propulsion#Jet-propelled animals|water jet propulsion]]). Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse [[invertebrate]]s.

Octopuses inhabit various [[ocean]] habitats, including [[coral reef]]s, [[pelagic]] waters, and the [[seabed]]; some live in the [[intertidal zone]] and others at [[abyssal zone|abyssal depths]]. Most species grow quickly, mature early, and are short-lived. In most species, the male uses a specially-adapted arm to deliver sperm directly into the female's mantle cavity, after which he becomes [[senescence|senescent]] and dies, while the female deposits fertilised eggs in a den and cares for them until they hatch, after which she also dies. Strategies to defend themselves against predators include expelling [[octopus ink|ink]], [[camouflage]], and [[deimatic display|threat displays]], the ability to jet quickly through the water and hide, and deceit. All octopuses are [[venomous]], but only the [[blue-ringed octopus]]es are known to be deadly to humans.

Octopuses appear in [[mythology]] as sea monsters such as the [[kraken]] of Norway and the [[Akkorokamui]] of the [[Ainu people|Ainu]], and possibly the [[Gorgon]] of [[ancient Greece]]. A battle with an octopus appears in [[Victor Hugo]]'s book ''[[Toilers of the Sea]]''. Octopuses appear in Japanese ''[[shunga]]'' erotic art. They are eaten and considered a delicacy by humans in many parts of the world, especially the [[Mediterranean cuisine|Mediterranean]] and Asia.

==Etymology and pluralisation==<!--Avoid changing. Used in links to subsection.-->
{{See also|Plural form of words ending in -us}}
The [[Neo-Latin|scientific Latin]] term {{wikt-lang|la|octopus}} was derived from [[Ancient Greek]] {{wikt-lang|grc|ὀκτώπους}} ({{lang|grc-Latn|oktōpous}}), a [[compound (linguistics)|compound]] form of {{wikt-lang|grc|ὀκτώ}} ({{lang|grc-Latn|oktō}}, 'eight') and {{wikt-lang|grc|πούς}} ({{lang|grc-Latn|pous}}, 'foot'), itself a variant form of {{wikt-lang|grc|ὀκτάπους}}, a word used for example by [[Alexander of Tralles]] ({{circa|525}} – {{circa|605}}).<ref name=etym>{{OEtymD|octopus}}</ref><ref>{{cite web |url=http://dictionary.reference.com/browse/octopus |title=Octopus |website=Dictionary.reference.com |access-date=4 February 2014}}</ref><ref>{{LSJ|o)kta/pous|ὀκτάπους}}, {{LSJ|o)ktw/pous|ὀκτώπους|ref}}.</ref> 

The standard [[plural]]ised form of ''octopus'' in English is ''octopuses'';<ref>{{cite journal |last1=Michel |first1=Jean-Baptiste |last2=Shen |first2=Yuan |last3=Aiden |first3=Aviva |last4=Veres |first4=Adrian |last5=Gray |first5=Matthew |last6=Pickett |first6=Joseph |last7=Hoiberg |first7=Dale |last8=Clancy |first8=Dan |last9=Norvig |first9=Peter |last10=Orwant |first10=Jon |last11=Pinker |first11=Steven |author11-link=Steven Pinker |last12=Nowak |first12=Martin |title=Quantitative Analysis of Culture Using Millions of Digitized Books |journal=Science |year=2011 |volume=331 |issue=6014 |pages=176–182 |doi=10.1126/science.1199644 |pmid=21163965 |pmc=3279742| bibcode=2011Sci...331..176M }} [https://books.google.com/ngrams/graph?content=octopuses%2Coctopi%2Coctopodes&yece.com/ngrams/graph?content=octopuses%2Coctopi%2Coctopodes&year_start=1850&year_end=2000&corpus=1orpus=15&smoothing=3&share=&direct_url=t1%3B%2Coctopuses%3B%2Cc0%3B.t1%3B%2Coctopi%3B%2Cc0%3B.t1%3B%2Coctopodes%3B%2Cc0 Relevant data at Google Ngram Viewer].</ref> the Ancient Greek plural {{lang|grc|ὀκτώποδες}}, {{lang|grc-Latn|octopodes}} ({{IPAc-en|ɒ|k|ˈ|t|ɒ|p|ə|d|iː|z}}), has also been used historically.<ref name="OD">{{cite web |url=http://oxforddictionaries.com/definition/english/octopus |archive-url=https://web.archive.org/web/20121030205455/http://oxforddictionaries.com/definition/english/octopus |url-status=dead |archive-date=30 October 2012 |title=Octopus |publisher=Oxforddictionaries.com |date=2014 |access-date=4 February 2014}}</ref> The alternative plural ''octopi'' is usually considered etymologically incorrect because it [[hypercorrection|wrongly assumes]] that ''octopus'' is a Latin [[second declension|second-declension]] {{wikt-lang|la|-us}} noun or adjective when, in either Greek or Latin, it is a [[third declension|third-declension]] noun.<ref>Peters, Pam (2004). ''The Cambridge Guide to English Usage''. Cambridge: Cambridge University Press. {{ISBN|0-521-62181-X}}, p. 388.</ref><ref>{{cite book |last1=Fowler |first1=Henry Watson |title=A Dictionary of Modern English Usage |date=1994 |isbn=978-1-85326-318-7 |page=[https://archive.org/details/dictionaryofmo00fowl/page/316 316] |publisher=Wordsworth Editions |url=https://archive.org/details/dictionaryofmo00fowl/page/316 |quote=In Latin plurals there are some traps for non-Latinists; the termination of the singular is no sure guide to that of the plural. Most Latin words in ''-us'' have plural in ''-i'', but not all, & so zeal not according to knowledge issues in such oddities as...''octopi''...; as caution the following list may be useful:...''octopus'', ''-podes'' }}</ref> Historically, the first plural to commonly appear in English language sources, in the early 19th century, is the Latinate form ''octopi'',<ref>{{cite book |last1=Tuckey |first1=James Hingston |author-link=James Hingston Tuckey |last2=Smith|first2=Christen |author2-link=Christen Smith | date=1818 |title=Narrative of an Expedition to Explore the River Zaire |work=The Library of Congress |url=https://www.loc.gov/resource/rbc0001.2020toner34663/?st=gallery |publisher=Kirk & Mercein }}</ref> followed by the English form ''octopuses'' in the latter half of the same century. The Hellenic plural is roughly contemporary in usage, although it is also the rarest.<ref>{{cite web |title=The Many Plurals of 'Octopus' |url=https://www.merriam-webster.com/words-at-play/the-many-plurals-of-octopus-octopi-octopuses-octopodes |website=www.merriam-webster.com}}</ref>

''[[Fowler's Modern English Usage]]'' states that the only acceptable plural in English is ''octopuses'', that ''octopi'' is misconceived, and ''octopodes'' [[pedantic]];<ref>{{cite book|url=https://www.oxfordreference.com/view/10.1093/acref/9780199661350.001.0001/acref-9780199661350-e-3956?rskey=siwVQG&result=3941|title=Fowler's Dictionary of Modern English Usage|last1=Butterfield|first1=Jeremy|date=2015|publisher=Oxford University Press|isbn=978-0-19-174453-2|quote=The only correct plural in English is octopuses. The Greek original is {{lang|grc|ὀκτώπους}}, {{lang|grc|-ποδ-}} (which would lead to a pedantic English pl. form ''octopodes''). The pl. form ''octopi'', which is occasionally heard (mostly in jocular use), though based on modL ''octopus'', is misconceived}}</ref><ref>{{Cite web |title=Chambers Reference Online |year=1996 |work=Chambers 21st Century Dictionary |publisher=Chambers Harrap |access-date=5 February 2024 |url=https://chambers.co.uk/search/?query=octopus&title=21st }}</ref><ref>{{cite AV media |first=Kory |last=Stamper |title=Ask the editor: octopus |url=http://www.merriam-webster.com/video/0015-octopus.htm |access-date=26 June 2013 |publisher=Merriam-Webster |archive-date=30 April 2013 |archive-url=https://web.archive.org/web/20130430140919/http://www.merriam-webster.com/video/0015-octopus.htm |url-status=dead }}</ref> the last is nonetheless used frequently enough to be acknowledged by the [[descriptivist]] ''Merriam-Webster 11th Collegiate Dictionary'' and ''Webster's New World College Dictionary''. The ''[[Oxford English Dictionary]]'' lists ''octopuses'', ''octopi'', and ''octopodes'', in that order, reflecting frequency of use, calling ''octopodes'' rare and noting that ''octopi'' is based on a misunderstanding.<ref>{{OED |octopus}}</ref> The ''[[New Oxford American Dictionary]]'' (3rd Edition, 2010) lists ''octopuses'' as the only acceptable pluralisation, and indicates that ''octopodes'' is occasionally used, but that ''octopi'' is incorrect.<ref name=NOAD>{{cite book |title=New Oxford American Dictionary|url={{google books |plainurl=y |id=sZoFRwAACAAJ}} |year=2010 |publisher=Oxford University Press |isbn=978-0-19-539288-3 |edition=3rd |editor1-first=Angus |editor1-last=Stevenson |editor2-first=Christine A. |editor2-last=Lindberg}}</ref>

==Anatomy and physiology==
===Size===
{{See also |Cephalopod size}}
[[File:North Pacific Giant Octopus.JPG|thumb|A [[giant Pacific octopus]] at Echizen Matsushima Aquarium, Japan|alt=Captured specimen of a giant octopus]]

The [[Enteroctopus dofleini|giant Pacific octopus]] (''Enteroctopus dofleini'') is often cited as the largest octopus species. Adults usually weigh {{convert|10|-|50|kg|lb|abbr=on}}, with an arm span of up to {{convert|4.8|m|ft|abbr=on}}.<ref>{{cite web|title=Giant Pacific Octopus (''Enteroctopus dofleini'')|url=https://www.adfg.alaska.gov/index.cfm?adfg=giantpacificoctopus.main|access-date=22 February 2025|website=Alaska Department of Fish and Game}}</ref> The largest specimen of this species to be scientifically documented reached a live mass of {{convert|71|kg|lb|abbr=on}}.<ref>Cosgrove, J.A. 1987. Aspects of the Natural History of ''Octopus dofleini'', the Giant Pacific Octopus. MSc Thesis. Department of Biology, University of Victoria (Canada), 101 pp.</ref> Much larger sizes have been claimed:<ref name=norman03>Norman, M. 2000. ''Cephalopods: A World Guide''. ConchBooks, Hackenheim. p. 214.</ref> one specimen was recorded as {{convert|272|kg|lb|abbr=on}} with an arm span of {{convert|9|m|ft|abbr=on}}.<ref>{{Cite journal |last=High |first=William L. |year=1976 |title=The giant Pacific octopus |url=http://spo.nmfs.noaa.gov/mfr389/mfr3893.pdf |journal=Marine Fisheries Review |volume=38 |issue=9 |pages=17–22 |access-date=4 November 2016 |archive-date=23 January 2017 |archive-url=https://web.archive.org/web/20170123114633/http://spo.nmfs.noaa.gov/mfr389/mfr3893.pdf |url-status=dead }}</ref> A carcass of the [[seven-arm octopus]], ''[[Seven-arm octopus|Haliphron atlanticus]]'', weighed {{convert|61|kg|lb|abbr=on}} and was estimated to have had a live mass of {{convert|75|kg|lb|abbr=on}}.<ref>{{cite journal |last1=O'Shea |first1=S. |year=2004 |title=The giant octopus ''Haliphron atlanticus'' (Mollusca : Octopoda) in New Zealand waters |doi=10.1080/03014223.2004.9518353 |journal=New Zealand Journal of Zoology |volume=31 |issue=1 |pages=7–13 |s2cid=84954869 |doi-access=free }}</ref><ref>{{cite journal |last1=O'Shea |first1=S. |year=2002 |title=''Haliphron atlanticus''&nbsp;– a giant gelatinous octopus |url=http://isopods.nhm.org/pdfs/27566/27566.pdf |journal=Biodiversity Update |volume=5 |page=1 }}</ref> The smallest species is ''[[Octopus wolfi]]'', which is around {{convert|2.5|cm|in|0|abbr=on}} and weighs less than {{convert|1|g|abbr=on}}.<ref>{{cite web |last=Bradford |first=Alina |date=21 July 2016 |title=Octopus Facts |publisher=Live Science |access-date=26 April 2017 |url=http://www.livescience.com/55478-octopus-facts.html}}</ref>

===External characteristics===
The octopus has an elongated body that is [[Symmetry in biology|bilaterally symmetrical]] along its dorso-ventral (back to belly) axis; the head and [[Foot (mollusc)|foot]] are on the ventral side, but act as the anterior (front). The heads contains both the mouth and the brain.<ref name=Ruppert/>{{rp|343–344}} The mouth has a sharp [[chitin]]ous [[Cephalopod beak|beak]] and is surrounded by and underneath the foot, which evolved into flexible, prehensile [[cephalopod limb]]s, known as "arms", which are attached to each other near their base by a webbed structure.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-0-03-025982-1 }}</ref>{{rp|343–344}}<ref name=Courage>{{cite book |last=Courage |first=K. H. |year=2013 |title=Octopus! The Most Mysterious Creature in the Sea |url={{google books |plainurl=y |id=eSMRlaceRIEC}} |publisher=[[Penguin Group]] |isbn=978-0-698-13767-7}}</ref>{{rp|40–41}}<ref name=Mather/>{{rp|13–15}} The arms can be described based on side and sequence position (such as L1, R1, L2, R2) and divide into four pairs.<ref name=Wells>{{cite book |last=Wells |first=M. J. |year=1978 |url={{google books |plainurl=y |id=AM_tCAAAQBAJ}} |title=Octopus, Physiology and Behaviour of an Advanced Invertebrate |publisher=[[Springer Science+Business Media]] |isbn=978-94-017-2470-8}}</ref>{{rp|12}} The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food.<ref>{{cite journal |title=Does ''Octopus vulgaris'' have preferred arms? |last1=Byrne |first1=Ruth A. |last2=Kuba |first2=Michael J. |last3=Meisel |first3=Daniela V. |last4=Griebel |first4=Ulrike |last5=Mather |first5=Jennifer A. |journal=[[Journal of Comparative Psychology]] |volume=120 |number=3 |date=August 2006 |pages=198–204 |doi=10.1037/0735-7036.120.3.198 |pmid=16893257 }}</ref> The bulbous and hollow [[Mantle (mollusc)|mantle]] is fused to the back of the head and contains most of the vital organs.<ref name=Mather>{{cite book |last1=Mather |first1=J. A. |last2=Anderson |first2=R. C. |last3=Wood |first3=J. B. |year=2010 |title=Octopus: The Ocean's Intelligent Invertebrate |url={{google books |plainurl=y |id=m-Mv7awvtIQC}} |publisher=[[Timber Press]] |isbn=978-1-60469-067-5}}
</ref>{{rp|13–15}}<ref name=Courage/>{{rp|40–41}} The mantle also has a cavity with muscular walls and a pair of gills; it is connected to the exterior by a funnel or [[Siphon (mollusc)|siphon]].<ref name=Ruppert/>{{rp|343–344}}<ref>{{Cite journal |last1=Semmens |title=Understanding octopus growth: patterns, variability and physiology |date=2004 |doi=10.1071/MF03155 |volume=55 |issue=4 |journal=Marine and Freshwater Research |page=367 |bibcode=2004MFRes..55..367S |s2cid=84208773 }}</ref>

[[File:Schematic lateral aspect of octopod features.jpg|thumb|400px|left|alt=Schematic of external anatomy|Diagram of octopus from side, with gills, funnel, eye, [[ocellus]] (eyespot), web, arms, suckers, [[hectocotylus]] and [[hectocotylus|ligula]] labelled.]]

The skin consists of a thin [[epidermis]] with mucous cells and sensory cells and a fibrous [[dermis]] made of [[collagen]] and containing various cells that allow colour change.<ref name=Ruppert/>{{rp|362}} Most of the body is made of soft tissue, allowing it to squeeze through tiny gaps; even the larger species can pass through a gap little more than {{convert|2.5|cm|in|0|abbr=on}} in diameter.<ref name=Courage/>{{rp|40–41}} Lacking skeletal support, the arms work as [[muscular hydrostat]]s and feature longitudinal, transverse, and circular muscles around a central axial nerve. They can squash and stretch, coil at any place in any direction or stiffen.<ref name=Crowfootcrawling>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoLoco.php# |title=Octopuses and Relatives: Locomotion, Crawling |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=19 April 2017 |archive-url=https://web.archive.org/web/20130522005231/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoLoco.php |archive-date=22 May 2013 |url-status=dead }}</ref><ref>{{cite journal |last1=Zelman |first1=I. |last2=Titon |first2=M. |last3=Yekutieli |first3=Y. |last4=Hanassy |first4=S. |last5=Hochner |first5=B. |last6=Flash |first6=T.|year=2013|title=Kinematic decomposition and classification of octopus arm movements |journal=Frontiers in Computational Neuroscience |volume=7 |page=60 |doi=10.3389/fncom.2013.00060 |pmid=23745113 |pmc=3662989 |doi-access=free }}</ref>

The interior surfaces of the arms are covered with circular, adhesive suckers. The suckers allow the octopus to secure itself in place or to handle objects. Each sucker is typically circular and bowl-like and has two distinct parts: an outer disc-shaped [[Cephalopod limb|infundibulum]] and a inner cup-like [[acetabulum (morphology)|acetabulum]], both of which are thick muscles covered in [[connective tissue]]. A [[chitinous]] cuticle lines the outer surface. When a sucker attaches to a surface, the orifice between the two structures is sealed and the infundibulum flattens. Muscle contractions allow for attachment and detachment.<ref>{{cite journal |last1=Tramacere |first1=F. |last2=Beccai |first2=L. |last3=Kuba |first3=M. |last4=Gozzi |first4=A. |last5=Bifone |first5=A. |last6=Mazzolai |first6=B. |year=2013|title=The morphology and adhesion mechanism of ''Octopus vulgaris'' suckers |journal=PLOS ONE |volume=8 |issue=6 |page=e65074 |doi=10.1371/journal.pone.0065074 |pmid=23750233 |pmc=3672162|bibcode=2013PLoSO...865074T |doi-access=free }}</ref><ref name=kier>{{cite journal |last1=Kier |first1=W. M. |last2=Smith |first2=A. M. |title=The structure and adhesive mechanism of octopus suckers |year=2002 |journal=Integrative and Comparative Biology |volume=42 |issue=6 |pages=1146–1153 |pmid=21680399 |doi=10.1093/icb/42.6.1146|citeseerx=10.1.1.512.2605 |s2cid=15997762 }}</ref><ref name=Crowfootcrawling/> Each of the eight arms senses and responds to light, allowing the octopus to control its limbs even if its head is obscured.<ref>{{Cite journal |last1=Katz |first1=Itamar |last2=Shomrat |first2=Tal |last3=Nesher |first3=Nir |date=1 January 2021 |title=Feel the light – sight independent negative phototactic response in octopus' arms |url=https://jeb.biologists.org/content/early/2021/02/01/jeb.237529 |journal=Journal of Experimental Biology |volume=224 |issue=5 |doi=10.1242/jeb.237529 |issn=0022-0949 |pmid=33536305 |doi-access=free|bibcode=2021JExpB.224B7529K }}</ref>

[[File:Dumbo-hires (cropped).jpg| thumb |A finned ''[[Grimpoteuthis]]'' species with its atypical octopus body plan|alt=A stubby round sea-creature with short ear-like fins]]

The cranium has two [[cartilage|cartilaginous]] capsules each containing one large eye, which resembles those of fish. The [[cornea]] is formed from a [[translucent]] epidermal layer; the slit-shaped [[pupil]] forms a hole in the [[iris (anatomy)|iris]] just behind the cornea. The lens hangs behind the pupil; photoreceptive [[Retina|retinal cells]] line the back. The pupil can expand and contract; a retinal pigment screens incident light in bright conditions.<ref name=Ruppert/>{{rp|360–361}}

Some species differ in form from the typical body shape. [[Basal (phylogenetics)|Basal]] species, the [[Cirrina]], have gelatinous bodies with two [[Cephalopod fin|fins]] located above the eyes, an [[cirrate shell|internal shell]] and mostly webbed arms that are lined with fleshy papillae or [[cirrus (biology)|cirri]] underneath.<ref name="marinebio">{{cite web |title=Finned Deep-sea Octopuses, Grimpoteuthis spp |date=18 May 2017 |publisher=MarineBio |url=https://www.marinebio.org/species/finned-deep-sea-octopuses/grimpoteuthis-spp/ |access-date=14 May 2021}}</ref><ref name="Corporation2004"/>

===Circulatory system===
Octopuses have a closed [[circulatory system]], in which the blood remains inside blood vessels. They have three hearts; a systemic or main heart that circulates blood around the body and two branchial or gill hearts that pump it through the two gills. The systemic heart becomes inactive when the animal is swimming. Thus, the octopus loses energy quickly and mostly crawls.<ref name=Wells/>{{rp|19–20, 31–35}}<ref name=Courage/>{{rp|42–43}} Octopus blood contains the [[copper]]-rich protein [[haemocyanin]] to transport oxygen. This makes the blood [[viscosity|viscous]] and it requires great pressure to pump it around the body; [[blood pressure]]s can surpass {{convert|75|mmHg|kPa|-1|abbr=on}}.<ref name=Wells/>{{rp|31–35}}<ref name=Courage/>{{rp|42–43}}<ref name="Schmidt">{{cite book |last=Schmidt-Nielsen |first=Knut |author-link=Knut Schmidt-Nielsen |year=1997 |title=Animal Physiology: Adaptation and Environment |publisher=Cambridge University Press |page=117 |isbn=978-0-521-57098-5}}</ref> In cold conditions with low oxygen levels, haemocyanin transports oxygen more efficiently than [[Hemoglobin|haemoglobin]].<ref name=Oellerman/> The haemocyanin is dissolved in the [[blood plasma]] instead of carried within blood cells and gives the blood a bluish colour.<ref name=Wells/>{{rp|31–35}}<ref name=Courage/>{{rp|42–43}}<ref name=Mather/>{{rp|22}}

The systemic heart has muscular contractile walls and consists of a single [[Ventricle (heart)|ventricle]] and two [[Atrium (heart)|atria]], which attach it to each of the two gills. The blood vessels consist of arteries, capillaries and veins and are lined with a cellular [[endothelium]] unlike that of most other [[invertebrate]]s. The blood circulates through the [[aorta]] and capillary system, to the [[Venae cavae|''venae cavae'']], after which the blood is pumped through the gills by the branchial hearts and back to the main heart. Much of the venous system is contractile, which helps circulate the blood.<ref name=Ruppert/>{{rp|358}}

===Respiration===
[[File:Octopus vulgaris Cuvier, 1797 2.jpg|thumb|Octopus with open siphon. The siphon is used for respiration, waste disposal and discharging ink.|alt=An octopus on the seabed, its siphon protruding near its eye]]

[[Respiration (physiology)|Respiration]] involves drawing water into the mantle cavity through an aperture, passing it through the gills, and expelling it through the siphon. Ingress is achieved by contraction of radial muscles in the mantle wall, and flapper valves shut when strong, circular muscles expel the water through the siphon.<ref name=Crowfootjetting>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoJet.php# |title=Octopuses and Relatives: Locomotion, jet propulsion |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=26 April 2017 |archive-url=https://web.archive.org/web/20170428032706/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoJet.php |archive-date=28 April 2017 |url-status=dead }}</ref> Extensive connective tissue lattices support the respiratory muscles and allow them to inflate the respiratory chamber.<ref name=Wells/>{{rp|24–26}} The [[lamella (surface anatomy)|lamella]] structure of the gills allows for high oxygen uptake, up to 65% in water at {{convert|20|C|F}}.<ref name="Wells, M.J. 1995">{{cite journal |last1=Wells |first1=M. J. |last2=Wells |first2=J. |year=1995 |title=The control of ventilatory and cardiac responses to changes in ambient oxygen tension and oxygen demand in ''Octopus'' |journal=The Journal of Experimental Biology |volume=198 |issue=Pt 8 |pages=1717–1727 |doi=10.1242/jeb.198.8.1717 |url=http://jeb.biologists.org/content/198/8/1717 |pmid=9319626 |doi-access=free |bibcode=1995JExpB.198.1717W }}</ref> Respiration can also play a role in locomotion, as an octopus can propel its body shooting water out of the siphon.<ref name=Wells/>{{rp|18}}<ref name="Schmidt"/>

The thin skin absorbs additional oxygen. When resting, around 41% of oxygen absorption is through the skin, reduced to 33% when the octopus swims, despite the amount of oxygen absorption increasing as water flows over the body. When it is resting after a meal, skin absorption can drop to 3%.<ref name="Wells, J. 1996">{{cite journal |last=Wells |first=J. |year=1996 |title=Cutaneous respiration in ''Octopus vulgaris'' |journal=The Journal of Experimental Biology |volume=199 |issue=Pt 11 |pages=2477–2483 |doi=10.1242/jeb.199.11.2477 |url=http://jeb.biologists.org/content/199/11/2477|pmid=9320405|bibcode=1996JExpB.199.2477M }}</ref>

===Digestion and excretion===
The digestive system begins with the [[buccal mass]] which consists of the mouth with the beak, the [[pharynx]], [[radula]] and salivary glands.<ref name=Wells/>{{rp|71–74}} The radula is serrated and made of [[chitin]].<ref name=Courage/>{{rp|40–41}} Food is broken down and is forced into the [[esophagus]] by two lateral extensions of the esophageal side walls in addition to the radula. From there it is transferred to the [[gastrointestinal tract]], which is mostly suspended from the roof of the mantle cavity. The tract consists of a [[crop (anatomy)|crop]], where the food is stored; a stomach, where it is mixed with other gut material; a [[caecum]] where the food is separated into particles and liquids and which absorbs fats; the [[hepatopancreas|digestive gland]], where liver cells break down and absorb the fluid and become "brown bodies"; and the intestine, where the built-up waste is turned into faecal ropes by secretions and ejected out of the funnel via the [[rectum]].<ref name=Wells/>{{rp|75–79}}

During [[osmoregulation]], fluid is added to the [[Pericardium|pericardia]] of the branchial hearts. The octopus has two [[nephridium|nephridia]] (equivalent to vertebrate kidneys) that are associated with the branchial hearts; these and their associated ducts connect the pericardial cavities with the mantle cavity. Each branch of the [[vena cava]] has renal appendages that pass over the thin-walled nephridium before reaching the branchial heart. Urine is created in the pericardial cavity, and is altered by excretion, of mostly ammonia, and absorption from the renal appendages, as it is passed along the associated duct and through the nephridiopore into the mantle cavity.<ref name=Ruppert/>{{rp|358–359}}

[[File:Moving Octopus Vulgaris 2005-01-14.ogv|thumb|A [[common octopus]] (''Octopus vulgaris'') moving around. Its nervous system allows the arms to move with some autonomy.|alt=video of an octopus crawling about, its suckered arms moving]]

===Nervous system and senses===
Octopuses and their relatives have a more expansive and complex [[nervous system]] than other invertebrates, containing over 500 million [[neuron]]s, around the same as a dog.<ref name="Albertin Simakov 2015"/><ref>{{cite journal | last1=Chung | first1=Wen-Sung | last2=Kurniawan | first2=Nyoman D. | last3=Marshall | first3=N. Justin | title=Comparative brain structure and visual processing in octopus from different habitats | journal=Current Biology | volume=32 | issue=1 | date=2022-01-10 | issn=1879-0445 | pmid=34798049 | doi=10.1016/j.cub.2021.10.070 | pages=97–110.e4| bibcode=2022CBio...32E..97C | doi-access=free }}</ref><ref>{{cite book |last=Budelmann |first=B. U. |year=1995 |chapter-url={{google books|plainurl=y|id=dW5e6FHOH-4C|page=PA115}} |chapter=The cephalopod nervous system: What evolution has made of the molluscan design |editor-last1=Breidbach |editor-first1=O. |editor-last2=Kutsch |editor-first2=W. |title=The nervous systems of invertebrates: An evolutionary and comparative approach |publisher=Birkhäuser |isbn=978-3-7643-5076-5 |lccn=94035125}}</ref> One part is localised in the brain, contained in a cartilaginous capsule. Two-thirds of the neurons are in the nerve cords of its arms. This allows their arms to perform actions with a degree of independence.<ref>{{cite journal |last=Hochner |first=B. |year=2012 |title=An Embodied View of Octopus Neurobiology |journal=Current Biology |volume=22 |issue=20 |pages=R887–R892 |doi=10.1016/j.cub.2012.09.001 |pmid=23098601 |doi-access=free|bibcode=2012CBio...22.R887H }}</ref> Learning mainly occurs in the brain, while arms make decisions independently when supplied with information.<ref>{{cite journal|last1=Gutnick|first1=T|last2=Zullo|first2=L|last3=Hochner|first3=B|last4=Kuba|first4=M. J.|year=2020|title=Use of peripheral sensory information for central nervous control of arm movement by Octopus|journal=Current Biology|volume=30|issue=21|pages=4322–4327|doi=10.1016/j.cub.2020.08.037|pmid=32916119}}</ref> A severed arm can still move and respond to stimuli.<ref>{{cite journal|last1=Hague|first1=T|last2=Florini|first2=M|last3=Andrews|first3=P. L. R.|year=2013|title=Preliminary in vitro functional evidence for reflex responses to noxious stimuli in the arms of ''Octopus vulgaris''|journal=Journal of Experimental Marine Biology and Ecology|volume=447|pages=100–105|doi=10.1016/j.jembe.2013.02.016|bibcode=2013JEMBE.447..100H}}</ref> Unlike in many other animals, including other mollusks, the movement of octopuses and their relatives are not organised in their brains via internal [[somatotopic arrangement|somatotopic map]]s of their bodies.<ref>{{cite journal |pmid=19765993 |doi=10.1016/j.cub.2009.07.067 |volume=19 |issue=19 |title=Nonsomatotopic organization of the higher motor centers in Octopus |first1=L. |last1=Zullo |first2=G. |last2=Sumbre |first3=C. |last3=Agnisola |first4=T. |last4=Flash |first5=B. |last5=Hochner |year=2009 |pages=1632–1636 |journal=Current Biology |s2cid=15852956 |doi-access=free |bibcode=2009CBio...19.1632Z }}</ref> Octopuses have the same [[jumping genes]] that are active in the human brain, implying an [[evolutionary convergence]] at molecular level.<ref name="Petrosino Ponte Volpe 2022">{{cite journal | last1=Petrosino | first1=Giuseppe | last2=Ponte | first2=Giovanna | last3=Volpe | first3=Massimiliano | last4=Zarrella | first4=Ilaria | last5=Ansaloni | first5=Federico | last6=Langella | first6=Concetta | last7=Di Cristina | first7=Giulia | last8=Finaurini | first8=Sara | last9=Russo | first9=Monia T. | last10=Basu | first10=Swaraj | last11=Musacchia | first11=Francesco | last12=Ristoratore | first12=Filomena | last13=Pavlinic | first13=Dinko | last14=Benes | first14=Vladimir | last15=Ferrante | first15=Maria I. | last16=Albertin | first16=Caroline | last17=Simakov | first17=Oleg | last18=Gustincich | first18=Stefano | last19=Fiorito | first19=Graziano | last20=Sanges | first20=Remo |display-authors=3 | title=Identification of LINE retrotransposons and long non-coding RNAs expressed in the octopus brain | journal=BMC Biology | volume=20 | issue=1 | date=18 May 2022 | page=116 | doi=10.1186/s12915-022-01303-5 | pmid=35581640 | pmc=9115989 | s2cid=231777147 | doi-access=free }}</ref>

[[File:Reef1072 - Flickr - NOAA Photo Library.jpg|thumb|left|Eye of [[common octopus]]|alt=Close up of an octopus showing its eye and an arm with suckers]]

Like other cephalopods, octopuses have camera-like eyes.<ref name="Albertin Simakov 2015"/> [[Colour vision]] appears to vary from species to species, for example, it is present in ''[[Amphioctopus aegina|A. aegina]]'' but absent in ''[[Common octopus|O. vulgaris]]''.<ref>{{cite journal |last1=Kawamura |first1=G. |year=2001 |title=Color Discrimination Conditioning in Two Octopus ''Octopus aegina'' and ''O. vulgaris'' |journal=Nippon Suisan Gakkaishi |volume=67 |issue=1 |pages=35–39 |doi=10.2331/suisan.67.35 |display-authors=etal |df=dmy-all |doi-access=free }}</ref> [[Opsin]]s in the skin respond to different wavelengths of light and help the animals choose a colouration that matches the surroundings and camouflages them; [[chromatophores]] in the skin can respond to light independently of the eyes.<ref name="Kingston Kuzirian 2015">{{cite journal |last1=Kingston |first1=Alexandra C. N. |last2=Kuzirian |first2=Alan M. |last3=Hanlon |first3=Roger T. |last4=Cronin |first4=Thomas W. |title=Visual phototransduction components in cephalopod chromatophores suggest dermal photoreception |journal=Journal of Experimental Biology |volume=218 |issue=10 |year=2015 |pages=1596–1602 |issn=1477-9145 |doi=10.1242/jeb.117945|pmid=25994635 |doi-access=free |bibcode=2015JExpB.218.1596K |hdl=11603/13387 |hdl-access=free }}</ref><ref name="Ramirez Oakley 2015">{{cite journal |last1=Ramirez |first1=M. Desmond |last2=Oakley |first2=Todd H. |title=Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides |journal=Journal of Experimental Biology |volume=218 |issue=10 |year=2015 |pages=1513–1520 |issn=1477-9145 |doi=10.1242/jeb.110908|pmid=25994633 |pmc=4448664 |doi-access=free |bibcode=2015JExpB.218.1513R }}</ref> An alternative hypothesis<!--Stubbs et al--> is that [[cephalopod eye]]s in species that only have a single [[photoreceptor protein]] may use [[chromatic aberration]] to turn monochromatic vision into colour vision, though this lowers image quality. This would explain pupils shaped like the letter "U", the letter "W", or a [[dumbbell]], as well as the need for colourful mating displays.<ref name="StubbsStubbs2016">{{cite journal |last1=Stubbs |first1=Alexander L. |last2=Stubbs |first2=Christopher W. |title=Spectral discrimination in color blind animals via chromatic aberration and pupil shape |journal=Proceedings of the National Academy of Sciences |volume=113 |issue=29 |year=2016 |pages=8206–8211 |issn=0027-8424 |doi=10.1073/pnas.1524578113|pmid=27382180 |pmc=4961147 |bibcode=2016PNAS..113.8206S |doi-access=free }}</ref>

Attached to the optic capsules are two organs called [[statocyst]]s (sac-like structures containing a mineralised mass and sensitive hairs), that allow the octopus to sense the orientation of its body, relative to both gravity and time ([[angular acceleration]]). An [[autonomic nervous system|autonomic]] response keeps the octopus's eyes oriented so that the pupil is always horizontal.<ref name=Ruppert/>{{rp|360–361}} Octopuses may also use the statocyst to hear. The common octopus can hear sounds between 400&nbsp;Hz and 1000&nbsp;Hz, and hears best at 600&nbsp;Hz.<ref name="HuYan2009">{{cite journal |last1=Hu |first1=Marian Y. |last2=Yan |first2=Hong Young |last3=Chung |first3=Wen-Sung |last4=Shiao |first4=Jen-Chieh |last5=Hwang |first5=Pung-Pung |title=Acoustically evoked potentials in two cephalopods inferred using the auditory brainstem response (ABR) approach |journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology |volume=153 |issue=3 |year=2009 |pages=278–283 |issn=1095-6433 |doi=10.1016/j.cbpa.2009.02.040 <!--|url=https://www.ecovis.org.au/wp-content/uploads/2015/04/squid-hearing.pdf--> |pmid=19275944 |url=http://ntur.lib.ntu.edu.tw//handle/246246/162905 |access-date=13 March 2022 |archive-date=7 April 2022 |archive-url=https://web.archive.org/web/20220407151928/http://ntur.lib.ntu.edu.tw//handle/246246/162905 |url-status=dead }}</ref>

Octopuses have an excellent [[somatosensory system]]. Their suction cups are equipped with [[chemoreceptors]] so they can [[taste]] what they touch.<ref>{{cite journal|last1=van Giesen|first1=L|last2=Kilian|first2=P. B.|last3=Allard|first3=C. A. H.|last4=Bellon|first4=N. W.|year=2020|title=Molecular basis of chemotactile sensation in Octopus|journal=Cell|volume=183|issue=3|pages=594–604|doi=10.1016/j.cell.2020.09.008|pmid=33125889|pmc=7605239}}</ref> Octopus arms move easily because the sensors recognise octopus skin and prevent self-attachment.<ref name="Nesher Levy Grasso Hochner 2014">{{cite journal |last1=Nesher |first1=Nir |last2=Levy |first2=Guy |last3=Grasso |first3=Frank W. |last4=Hochner |first4=Binyamin |title=Self-Recognition Mechanism between Skin and Suckers Prevents Octopus Arms from Interfering with Each Other |journal=Current Biology |volume=24 |issue=11 |year=2014 |issn=0960-9822 |doi=10.1016/j.cub.2014.04.024 |pages=1271–1275|pmid=24835454 |s2cid=16140159 |doi-access=free |bibcode=2014CBio...24.1271N }}</ref> Octopuses appear to have poor [[proprioception|proprioceptive]] sense and must see their arms to keep track of their position.<ref>{{cite journal|last1=Gutnick|first1=Tamar|last2=Byrne|first2=Ruth A.|last3=Hochner|first3=Binyamin|last4=Kuba|first4=Michael|year=2011|title=''Octopus vulgaris'' Uses Visual Information to Determine the Location of Its Arm|journal=Current Biology|volume=21|issue=6|pages=460–462|doi=10.1016/j.cub.2011.01.052|pmid=21396818|s2cid=10152089|doi-access=free|bibcode=2011CBio...21..460G }}</ref><ref>{{cite journal|last1=Kennedy|first1=E. B. Lane|last2=Buresch|first2=Kendra C.|last3=Boinapally|first3=Preethi|last4=Hanlon|first4=Roger T.|year=2020|title=Octopus arms exhibit exceptional flexibility|journal=Scientific Reports|volume=10|issue=1|page=20872|doi=10.1038/s41598-020-77873-7|pmid=33257824|pmc=7704652}}</ref>

===Ink sac===

The [[ink sac]] is located under the digestive gland. A gland attached to the sac produces the [[octopus ink|ink]], and the sac holds it. The sac is close enough to the funnel for the octopus to shoot out the ink with a water jet. As the animal begins to shoot, the ink passes through glands which mix it with mucus and it leaves the funnel as a thick, dark blob which helps the animal to escape from a predator.<ref name=Mather/>{{rp|107}} The main [[pigment]] in the ink is [[melanin]], which gives it its black colour.<ref>{{cite journal |last=Derby |first=C. D. |year=2014 |title=Cephalopod Ink: Production, Chemistry, Functions and Applications |journal=Marine Drugs |volume=12 |issue=5 |pages=2700–2730 |doi=10.3390/md12052700 |pmid=24824020 |pmc=4052311|doi-access=free }}</ref> Cirrate octopuses usually lack the ink sac.<ref name="marinebio"/>

==Life cycle==

===Reproduction===
[[File:Tremoctopus violaceus5.jpg|thumb|upright|Adult male ''[[Tremoctopus violaceus]]'' with [[hectocotylus]]|alt=Drawing of a male octopus with one large arm ending in the sexual apparatus]]

Octopuses [[Gonochorism|have two sexes]] and have only one gonad ([[testis]] in males and [[ovary]] in females) located posteriorly. The gonad deposits [[gamete]]s into an adjacent cavity called the gonocoel. A [[gonoduct]] bridges the gonocoel with the [[mantle cavity]].<ref name=Ruppert/>{{rp|363–365}} An [[optic gland]] creates hormones that cause the octopus to mature and age and stimulate gamete production. The timing of reproduction and lifespan depends on environmental conditions such as temperature, light and nutrition, which trigger the gland.<ref name=Mather/>{{rp|147}}<ref>{{cite journal |last1=Wells |first1=Martin J. |last2=Wells |first2=J. |year=1972 |title=Optic glands and the state of the testis in ''Octopus'' |journal=Marine Behaviour and Physiology |volume=1 |issue=1–4 |pages=71–83 |doi=10.1080/10236247209386890}}</ref> The male has a specialised arm called a [[hectocotylus]] which it uses to transfer [[spermatophore]]s (packets of sperm) into the female's mantle cavity.<ref name=Ruppert/>{{rp|363–365}} The hectocotylus in ''Octopus'' is usually the R3 arm, which has a spoon-shaped depression and a suckerless tip.<ref name=Wells/>{{rp|12–14}}<ref name=Ruppert/>{{rp|363–365}} Fertilisation may occur in the mantle cavity or in the surrounding water.<ref name=Ruppert/>{{rp|363–365}}

Reproduction has been studied in some species. In the [[giant Pacific octopus]], courtship includes changes in skin texture and colour, mostly in the male. The male may cling to the top or side of the female or position himself beside her. There is some speculation that he may first use his hectocotylus to remove any spermatophore or sperm already present in the female. He picks up a spermatophore from his spermatophoric sac with the hectocotylus, inserts it into the female's mantle cavity, and deposits it in the correct location in the opening of the oviduct. Two spermatophores are transferred in this way; these are about one metre (yard) long, and the empty ends may protrude from the female's mantle.<ref name=Crowfootreproduction>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoRepr.php |title=Octopuses and Relatives: Reproduction |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=11 April 2017 |archive-url=https://web.archive.org/web/20170422215052/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoRepr.php |archive-date=22 April 2017 |url-status=dead }}</ref> A complex hydraulic mechanism releases the sperm from the spermatophore.<ref name=Ruppert/>{{rp|363–365}} 

[[File:Enteroctopus dofleini to spawn.jpg|thumb|left|Female giant Pacific octopus guarding strings of eggs|alt=A female octopus underneath hanging strings of her eggs]]

The eggs have large yolks; [[cleavage (embryo)|cleavage]] (division) is relatively shallow and a [[germinal disc]] develops at the pole. During [[gastrulation]], the disc surrounds the yolk, forming a yolk sac, which eventually forms part of the gut. The embryo forms as the dorsal side of the disc grows upward, with a shell gland, gills, mantle and eyes on its dorsal side. The arms and funnel form on the ventral side of the disc, with the former moving upward to surround the mouth. The embryo consumes the yolk during development.<ref name=Ruppert/>{{rp|363–365}} 

Over a month after mating, Giant Pacific octopuses lay eggs. The species can lay 180,000 eggs in a single clutch, while ''[[Octopus rubescens|O. rubescens]]'' clutches host up to 45,000 eggs while ''O. vulgaris'' clutches can include 500,000 eggs.<ref name="AZAmanual">{{cite web |url=https://www.aza.org/assets/2332/giant_pacific_octopus_care_manual_final_9514.pdf |title=Giant Pacific Octopus (Enteroctopus dofleini) Care Manual |publisher=AZA (Association of Zoos and Aquariums) Aquatic Invertebrate Taxonomic Advisory Group in association with AZA Animal Welfare Committee |date=9 September 2014 |access-date=31 May 2016}}</ref>{{rp|75}} Fertilised octopus eggs are laid as strings within a shelter.<ref name=Crowfootreproduction/><ref name=Mather/>{{rp|26}} Female giant Pacific octopuses nurture and protect their eggs for five months (160 days) until they hatch.<ref name=Crowfootreproduction/> In colder waters, such as those off [[Alaska]], it may take up to ten months for the eggs to completely develop.<ref name = "AZAmanual" />{{rp|74}} In the [[Argonaut (animal)|argonaut]] (paper nautilus), the female is much larger than the male. She secretes a thin shell shaped like a [[cornucopia]], in which the eggs are deposited and in which she also resides and broods the young while swimming.<ref name=Mather/>{{rp|26, 141}}

[[File:Fish3566 - Flickr - NOAA Photo Library.jpg|thumb|Octopus [[paralarva]], a [[plankton]]ic hatchling|alt=A microscopic view of a small round-bodied transparent animal with very short arms]]

Most young octopuses hatch as [[paralarva]]e,<ref name=Ruppert/>{{rp|363–365}} ''Octopus'' larvae in particular are [[plankton]]ic for weeks or months. Larvae feed on shrimp, isopods and [[amphipod]]s, eventually settling on the ocean floor to mature.<ref name=Wells/>{{rp|178}} Species that produce larger eggs instead hatch as benthic animals similar to the adults.<ref name = "AZAmanual" />{{rp|74–75}} These include the [[Southern blue-ringed octopus|southern blue-ringed]], [[Caribbean reef octopus|Caribbean reef]], [[California two-spot octopus|California two-spot]] and ''[[Eledone moschata]].''<ref>{{cite journal |last1=Forsythe |first1=J. W. |last2=Hanlon |first2=R. T. |title=A closed marine culture system for rearing ''Octopus joubini'' and other large-egged benthic octopods |journal=Laboratory Animals |year=1980 |volume=14 |issue=2 |pages=137–142 |doi=10.1258/002367780780942737 |pmid=7431823 |s2cid=19492476 |doi-access=free }}</ref>

===Lifespan===

Octopuses have short lifespans, living up to four years.<ref name=Mather/>{{rp|17}} The lifecycles of some species finish in less than half a year.<ref name=Courage/>{{rp|152}} For most octopuses, the ultimate life stage is [[senescence]]. It is the breakdown of cellular function without repair or replacement. It may last from weeks to a few months at most. Males senesce after maturity, while for females, it comes after they lay an egg clutch. During senescence, an octopus does not feed, quickly weakens, and becomes sluggish. Lesions begin to form and the octopus literally degenerates. They may die of starvation or get picked off by predators.<ref name="auto">{{cite journal |journal=Journal of Applied Animal Welfare Science |last1=Anderson |first1=Roland C. |last2=Wood |first2=James B. |last3=Byrne |first3=Ruth A. |title=Octopus Senescence: The Beginning of the End |year=2002 |volume=5 |issue=4 |pages=275–283 |url=https://www.researchgate.net/publication/7545324 |doi=10.1207/S15327604JAWS0504_02| pmid=16221078 |citeseerx=10.1.1.567.3108 |s2cid=28355735 }}</ref> Senescence is triggered by the optic glands and experimental removal of them after spawning was found to extend their lifecycle and activity.<ref name="hormones">{{cite journal |journal=[[Science (journal)|Science]] |last1=Wodinsky |first1=Jerome |title=Hormonal Inhibition of Feeding and Death in ''Octopus'': Control by Optic Gland Secretion |year=1977 |volume=198 |issue=4320 |pages=948–951 |doi=10.1126/science.198.4320.948 |pmid=17787564| bibcode=1977Sci...198..948W |s2cid=22649186 }}</ref>

==Distribution and habitat==
[[File:Octopus cyaneain Kona.jpg|thumb|''Octopus cyanea'' in Kona, Hawaii|alt=An octopus nearly hidden in a crack in some coral]]

Octopuses inhabit every ocean, with species adapted to many [[marine habitat|habitat]]s. As juveniles, common octopuses inhabit shallow [[tide pool]]s. The Hawaiian day octopus (''[[Octopus cyanea]]'') lives on coral reefs, while [[Argonaut (animal)|argonauts]] float in [[Pelagic zone|pelagic waters]]. ''[[Abdopus aculeatus]]'' is a near-shore species and can be found in [[seagrass]] beds. Some species can survive in deeper environments. The spoon-armed octopus (''[[Bathypolypus arcticus]]'') can live {{convert|1000|m|ft|abbr=on}} deep, and ''[[Vulcanoctopus hydrothermalis]]'' lives in depths of {{convert|2000|m|ft|abbr=on}} around [[hydrothermal vent]]s.<ref name=Mather/>{{rp|13–15}} Species such as ''[[Megaleledone setebos]]'' and ''[[Pareledone charcoti]]'', can survive in the waters of the Antarctic, which reach {{convert|−1.8|C|0}}.<ref name=Oellerman>{{cite journal|last1=Oellermann|first1=M|last2=Lieb|first2=B|last3=Pörtner|first3=H-O|last4=Semmens|first4=J. M.|last5=Mark|first5=F. C.|year=2015|title=Blue blood on ice: modulated blood oxygen transport facilitates cold compensation and eurythermy in an Antarctic octopod|journal=Frontiers in Zoology|volume=12|issue=6|page=6|doi=10.1186/s12983-015-0097-x|doi-access=free|pmid=25897316|pmc=4403823}}</ref> No species are known to live in fresh water.<ref>{{cite web |last=Norman |first=Mark |date=16 January 2013|title=Ask an expert: Are there any freshwater cephalopods?|publisher=ABC Science |access-date=26 April 2017 |url=http://www.abc.net.au/science/articles/2013/01/16/3670198.htm}}</ref>

The [[Cirrina|cirrate]] species are often free-swimming and live in deep-water habitats.<ref name="Corporation2004">{{cite book |author=Marshall Cavendish Corporation |title=Encyclopedia of the Aquatic World |url={{google books |plainurl=y |id=swGA8GtK4n8C|page=764}} |date=2004 |publisher=Marshall Cavendish |isbn=978-0-7614-7424-1 |page=764}}</ref> Although several species live at [[bathyal]] and [[abyssal]] depths, only a single indisputable record documents their presence in the [[hadal zone]]; a species of ''[[Grimpoteuthis]]'' (dumbo octopus) photographed at {{cvt|6957|m|ft}}.<ref>{{cite journal |last1=Jamieson |first1=A.J. |last2=Vecchione |first2=M. |title=First in situ observation of Cephalopoda at hadal depths (Octopoda: Opisthoteuthidae: Grimpoteuthis sp.) |journal=Marine Biology |year=2020 |volume=167 |issue=82 |doi=10.1007/s00227-020-03701-1 |doi-access=free |bibcode=2020MarBi.167...82J }}</ref>

==Behaviour and ecology==

Octopuses are mostly solitary<ref name=Mather/>{{rp|17, 134}} though a few are known to live in groups and interact regularly, usually in the context of [[Dominance hierarchy|dominance]] and reproductive competition. This is likely the result of abundant food supplies combined with fewer den sites.<ref>{{cite journal |last=Scheel |first=D. |display-authors=etal |year=2017 |title=A second site occupied by ''Octopus tetricus'' at high densities, with notes on their ecology and behavior |journal=Marine and Freshwater Behaviour and Physiology |volume=50 |issue=4 |pages=285–291 |doi=10.1080/10236244.2017.1369851 |bibcode=2017MFBP...50..285S |s2cid=89738642}}</ref> The [[Larger Pacific striped octopus]] has been described as particularly social, living in groups of up to 40.<ref>{{cite journal |last=Rodaniche |first=Arcadio F. |date=1991 |title=Notes on the behavior of the Larger Pacific Striped Octopus, an undescribed species of the genus Octopus |journal=Bulletin of Marine Science |volume=49 |pages=667}}</ref><ref>{{cite journal |last1=Caldwell |first1=Roy L. |last2=Ross |first2=Richard |last3=Rodaniche |first3=Arcadio |last4=Huffard |first4=Christine L. |year=2015 |title=Behavior and Body Patterns of the Larger Pacific Striped Octopus |journal=PLOS ONE |volume=10 |issue=8 |pages=e0134152 |doi=10.1371/journal.pone.0134152 |issn=1932-6203 |pmc=4534201 |pmid=26266543 |bibcode=2015PLoSO..1034152C|doi-access=free }}</ref> Octopuses hide in dens, which are typically crevices in rocky or other hard structures, including man-made ones. Small species may use abandoned shells and bottles.<ref name=Mather/>{{rp|69, 74–75}}  They can [[Animal navigation|navigate]] to a den without having to retrace their outward route.<ref>{{cite journal |url=https://blogs.scientificamerican.com/thoughtful-animal/how-do-octopuses-navigate/ |title=How do octopuses navigate? |last=Goldman |first=Jason G. |date=24 May 2012 |journal=Scientific American |volume=168 |issue=4 |pages=491–497 |doi=10.1007/BF00199609 |s2cid=41369931 |access-date=8 June 2017}}</ref> They are not migratory.<ref name=Courage/>{{rp|45–46}}

Octopuses bring captured prey to the den to eat. Dens are often surrounded by a [[midden]] of dead and uneaten food items. These middens may attract scavengers such as fish, molluscs, and [[echinoderm]]s.<ref name=Crowfootfeeding>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoFeed.php |title=Octopuses and Relatives: Feeding, diets and growth |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=13 April 2017 |archive-url=https://web.archive.org/web/20170508060739/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoFeed.php |archive-date=8 May 2017 |url-status=dead}}</ref> On rare occasions, octopuses [[interspecific cooperation|hunt cooperatively with other species]], with fish as their partners. They regulate the [[species composition]] of the hunting {{nowrap|group{{tsp}}{{mdash}}}}{{tsp}}and the behavior of their {{nowrap|partners{{tsp}}{{mdash}}}}{{tsp}}by punching them.<ref name="Sampaio 2020">{{cite journal |last1=Sampaio |first1=Eduardo |last2=Seco |first2=Martim Costa |last3=Rosa |first3=Rui |last4=Gingins |first4=Simon |title=Octopuses punch fishes during collaborative interspecific hunting events |journal=[[Ecology (journal)|Ecology]] |publisher=[[Ecological Society of America]]/[[Wiley Publishing]] |date=18 December 2020 |volume=102 |issue=3 |pages=e03266 |issn=0012-9658 |doi=10.1002/ecy.3266 |pmid=33338268 |doi-access=free}}</ref>

===Feeding===
[[File:Veined Octopus - Amphioctopus Marginatus eating a Crab.jpg|thumb|left|[[Amphioctopus marginatus|Veined octopus]] eating a crab|alt=An octopus in an open seashell on a sandy surface, surrounding a small crab with the suckers on its arms]]

Octopuses are generally predatory and feed on prey such as [[crustacean]]s, [[Bivalvia|bivalves]], [[gastropod]]s, fish, and other cephalopods, including [[cannibalism|members of the same species]].<ref name=Hanlon>{{cite book |title=Cephalopod Behaviour |publisher=Cambridge University Press |last1=Hanlon |first1=R. T. |last2=Messenger |first2=J. B. |year=1998 |edition=1st|isbn=0-521-64583-2}}</ref>{{rp|47, 60}} Major items in the diet of the giant Pacific octopus include bivalves such as the cockle ''[[Clinocardium nuttallii]]'', clams and scallops and crustaceans such as [[crab]]s. It typically rejects [[Naticidae|moon snails]] because they are too large; [[limpet]]s, [[Crassadoma|rock scallops]], [[chiton]]s and [[abalone]], because they are too securely fixed to the rock.<ref name=Crowfootfeeding/> Small cirrate octopuses such as those of the genera ''[[Grimpoteuthis]]'' and ''[[Opisthoteuthis]]'' typically prey on polychaetes, [[copepod]]s, [[amphipod]]s and [[isopod]]s.<ref name=":9">{{Cite book |last1=Collins |first1=Martin A. |last2=Villanueva |first2=Roger |date=2006 |chapter=Taxonomy, ecology and behaviour of the cirrate octopods |title=Oceanography and Marine Biology: An Annual Review |volume=44 |pages=277–322 |doi=10.1201/9781420006391.ch6 |doi-broken-date=12 November 2024 |chapter-url=https://www.researchgate.net/publication/266220687 |access-date=5 February 2024 |isbn=978-0-8493-7044-1}}</ref>

Octopuses typically locate prey by feeling through their environment;<ref name=Mather/>{{rp|60}} some species hide and ambush their target.<ref name=Hanlon/>{{rp|54}} When prey tries to escape, the octopus jets after it.<ref name=Mather/>{{rp|61}} Octopuses may drill into the shells of crustaceans, bivalves and gastropods. It used to be thought that drilling was done by the radula, but it has now been shown that minute teeth at the tip of the salivary papilla are involved, and an enzyme in the toxic saliva is used to dissolve the calcium carbonate of the shell. This can take hours and once the shell is penetrated, the prey dies almost instantaneously. With crabs, tough-shelled species are more likely to be drilled, and soft-shelled crabs are torn apart.<ref name=Crowfoothandling>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoHand.php |title=Octopuses and Relatives: Prey handling and drilling |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=21 April 2017 |archive-url=https://web.archive.org/web/20170606005247/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoHand.php |archive-date=6 June 2017 |url-status=dead }}</ref>

Some species have other modes of feeding. ''[[Grimpoteuthis]]'' either lacks or has a small radula and swallows prey whole.<ref name="marinebio"/> In the deep-sea genus ''[[Stauroteuthis]]'',  the suckers in most species have been altered into [[photophore]]s which are believed to fool prey by directing them to the mouth, making them one of the few [[bioluminescent]] octopuses.<ref>{{cite journal |doi=10.2307/1542994 |pmid=28296499 |last1=Johnsen |first1=S. |first2=E. J. |last2=Balser |first3=E. C. |last3=Fisher |first4=E. A. |last4=Widder |year=1999 |url=http://www.biology.duke.edu/johnsenlab/pdfs/pubs/octopusbiolbull.pdf |title=Bioluminescence in the deep-sea cirrate octopod ''Stauroteuthis syrtensis'' Verrill (Mollusca: Cephalopoda) |journal=The Biological Bulletin |volume=197 |issue=1 |pages=26–39 |url-status=dead |archive-url=https://web.archive.org/web/20110305114611/http://www.biology.duke.edu/johnsenlab/pdfs/pubs/octopusbiolbull.pdf |archive-date=5 March 2011  |jstor=1542994 }}</ref>

===Locomotion===
[[File:Octopus3.jpg| thumb|right|Octopuses swim with their arms trailing behind.|alt=An octopus swimming with its round body to the front, its arms forming a streamlined tube behind]]
Octopuses mainly move about by relatively slow crawling with some swimming in a head-first position. [[Jet propulsion]] or backward swimming, is their fastest means of locomotion, while crawling is slowest.<ref name="biologists2006">{{cite journal |first=Christine L. |last=Huffard |title=Locomotion by ''Abdopus aculeatus'' (Cephalopoda: Octopodidae): walking the line between primary and secondary defenses |journal=Journal of Experimental Biology |year=2006 |volume=209 |issue=Pt 19 |pages=3697–3707 |doi=10.1242/jeb.02435 |pmid=16985187| doi-access=free |bibcode=2006JExpB.209.3697H }}</ref> While crawling, the suckers adhere and detach from the substrate as the animal hauls itself forward with its powerful arm muscles.<ref name=Crowfootcrawling/><ref name="biologists2006"/> In 2005, ''[[Abdopus|Adopus aculeatus]]'' and veined octopus (''[[Amphioctopus marginatus]]'') were found to walk on two arms, while at the same time mimicking plant matter.<ref>{{cite journal |doi=10.1126/science.1109616 |title=Underwater Bipedal Locomotion by Octopuses in Disguise |year=2005 |last1=Huffard |first1=C. L. |journal=Science |volume=307 |issue=5717 |page=1927 |pmid=15790846 |last2=Boneka |first2=F. |last3=Full |first3=R. J. |s2cid=21030132 }}</ref> This form of locomotion allows these octopuses to move quickly away from a potential predator without being recognised.<ref name="biologists2006"/> Some species of octopus can crawl out of the water briefly, which they may do between tide pools.<ref name="Wood Anderson"/><ref name=Mather/>{{rp|183}} "Stilt walking" is used by the veined octopus when carrying stacked coconut shells. The octopus carries the shells underneath it with two arms, and progresses with an awkward gait supported by its remaining arms, which are stiffened.<ref name=Finn/>
[[File:Cirroteuthis muelleri NOAA.jpg|thumb|upright=1.5<!--width for low aspect ratio image-->|Movements of the finned species ''[[Cirroteuthis muelleri]]''|alt=Three images in sequence of a two-finned sea creature swimming with an eight-cornered web]]
Most octopuses swim by expelling a jet of water from the mantle through the siphon into the sea. The direction of travel depends on the orientation of the siphon. When swimming, the head is at the front and the siphon is pointed backward but, when jetting, the visceral hump leads, the siphon points at the head and the arms trail behind, with the animal presenting a [[Spindle (textiles)|fusiform]] appearance. In an alternative method of swimming, some species flatten themselves dorso-ventrally, and swim with the arms splayed; this may provide lift and be faster than normal swimming. Jetting is used to escape from danger, but is physiologically inefficient, requiring a mantle pressure so high as to stop the heart from beating, resulting in a progressive oxygen deficit.<ref name="biologists2006"/>

Cirrate octopuses cannot produce jet propulsion and swim using their fins. Their neutrally buoyant bodies float along while the fins are spread. They can also contract their arms and surrounding web to make sudden moves known as "take-offs". Another form of locomotion is "pumping", which involves symmetrical contractions of muscles in their webs producing [[peristalsis|peristaltic waves]], moving them slowly.<ref name="marinebio"/>

===Intelligence===
{{Main |Cephalopod intelligence}}

[[File:Oktopus opening a container with screw cap 02 (cropped).jpg|thumb|upright|Octopus opening a container by unscrewing its cap|alt=A captive octopus with two arms wrapped around the cap of a plastic container]]

Octopuses are highly [[intelligence|intelligent]].<ref name="doug">{{cite journal |url=https://www.nwf.org/Magazines/National-Wildlife/1997/Armed-But-Not-Dangerous |title=Armed but not dangerous: Is the octopus really the invertebrate intellect of the sea |first=Doug |last=Stewart |journal=National Wildlife |year=1997 |volume=35 |issue=2}}</ref> [[Maze]] and [[problem solving|problem-solving]] experiments have shown evidence of a memory system that can store both [[short-term memory|short-]] and [[long-term memory]].<ref name="Zarrella Ponte Baldascino Fiorito 2015">{{cite journal |last1=Zarrella |first1=Ilaria |last2=Ponte |first2=Giovanna |last3=Baldascino |first3=Elena |last4=Fiorito |first4=Graziano |title=Learning and memory in Octopus vulgaris: a case of biological plasticity |journal=Current Opinion in Neurobiology |volume=35 |year=2015 |issn=0959-4388 |doi=10.1016/j.conb.2015.06.012 |pages=74–79|pmid=26186237 |s2cid=31682363 }}</ref> In laboratory experiments, octopuses can readily be trained to distinguish between different shapes and patterns. They have been reported to practise [[observational learning]],<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/europe/2796607.stm |title=Octopus intelligence: Jar opening |work=BBC News |date=25 February 2003 |access-date=4 February 2014}}</ref> although the validity of these findings is contested.<ref name="doug"/>  The [[veined octopus]] collects discarded [[coconut]] shells, then uses them to build a shelter, an example of [[tool use by animals|tool use]].<ref name=Finn>{{Cite journal |pmid=20064403 |year=2009 |last1=Finn |first1=J. K. |title=Defensive tool use in a coconut-carrying octopus |journal=Current Biology |volume=19 |issue=23 |pages=R1069–70 |last2=Tregenza |first2=T. |last3=Norman |first3=M. D. |doi=10.1016/j.cub.2009.10.052|s2cid=26835945 |doi-access=free |bibcode=2009CBio...19R1069F }}</ref> Octopuses have also been observed in what has been described as [[play (animal behaviour)|play]]: including moving around a bottle by jetting water at it.<ref>{{cite web |last1=Mather |first1=J. A. |last2=Anderson |first2=R. C. |year=1998 |title=What behavior can we expect of octopuses? |website=The Cephalopod Page |editor1-last=Wood |editor1-first=J. B. |url=http://www.thecephalopodpage.org/behavior.php |access-date=22 October 2006 |archive-date=5 October 2017 |archive-url=https://web.archive.org/web/20171005135515/http://www.thecephalopodpage.org/behavior.php |url-status=dead }}</ref> Octopuses often break out of aquariums and sometimes into others in search of food.<ref name="Wood Anderson">{{cite journal |url=http://www.thecephalopodpage.org/_pdf/2004Escape.pdf |title=Interspecific Evaluation of Octopus Escape Behavior |date=2004 |journal=Journal of Applied Animal Welfare Science |pages=95–106 |volume=7 |number=2 |access-date=11 September 2015 |doi=10.1207/s15327604jaws0702_2 |pmid=15234886 |last1=Wood |first1=J. B |last2=Anderson |first2=R. C |citeseerx=10.1.1.552.5888 |s2cid=16639444 }}</ref><ref>{{cite book |chapter-url={{google books |plainurl=y |id=lD8DAAAAQAAJ |page=38}} |title=Aquarium Notes – The Octopus; or, the "devil-fish" of fiction and of fact |last=Lee |first=Henry |chapter=V: The octopus out of water |date=1875 |publisher=Chapman and Hall |oclc=1544491 |location=London |access-date=11 September 2015 |pages=38–39 |quote=The marauding rascal had occasionally issued from the water in his tank, and clambered up the rocks, and over the wall into the next one; there he had helped himself to a young lump-fish, and, having devoured it, returned demurely to his own quarters by the same route, with well-filled stomach and contented mind.}}</ref><ref>{{Cite news |url= https://www.theguardian.com/world/2016/apr/13/the-great-escape-inky-the-octopus-legs-it-to-freedom-from-new-zealand-aquarium?CMP=Share_iOSApp_Other |title=The great escape: Inky the octopus legs it to freedom from aquarium |last=Ainge Roy |first=Eleanor |date=14 April 2016 |work=The Guardian (Australia)}}</ref> Evidence indicates that octopuses have [[sentience]] and can feel [[Pain in animals|pain]].<ref>{{Cite web |last=Henriques |first=Martha |date=25 July 2022 |title=The mysterious inner life of the octopus |url=https://www.bbc.com/future/article/20220720-do-octopuses-feel-pain |access-date=2024-07-29 |website=BBC |language=en-GB}}</ref>

===Camouflage and colour change===
[[File:Octopus.ogv| thumb|Video of ''Octopus cyanea'' moving and changing its colour, shape, and texture|alt=A video of an octopus changing its appearance]]

Octopuses use [[camouflage]] to hunt and to avoid predators. To do this, they use specialised skin cells that change colour. [[Chromatophore]]s contain yellow, orange, red, brown, or black pigments; most species have three of these colours, while some have two or four. Other colour-changing cells are reflective iridophores and white leucophores.<ref>{{cite web |url=http://www.docdatabase.net/more-tales-from-the-cryptic-the-common-atlantic-octopus-octopus-vulgaris-13832.html |title=Tales from the Cryptic: The Common Atlantic Octopus |access-date=27 July 2006 |last=Meyers |first=Nadia |publisher=Southeastern Regional Taxonomic Centre |archive-date=5 March 2022 |archive-url=https://web.archive.org/web/20220305152911/http://www.docdatabase.net/more-tales-from-the-cryptic-the-common-atlantic-octopus-octopus-vulgaris-13832.html |url-status=dead }}</ref> This colour-changing ability is also used to communicate with or warn other octopuses.<ref name=Mather/>{{rp|90–97}} The energy cost of the complete activation of the chromatophore system is high, nearly matching the energy used at rest.<ref>{{Cite journal |last1=Sonner |first1=Sofie C. |last2=Onthank |first2=Kirt L. |date=2024 |title=High energetic cost of color change in octopuses |journal=Proceedings of the National Academy of Sciences |language=en |volume=121 |issue=48 |pages=e2408386121 |doi=10.1073/pnas.2408386121 |pmid=39556731 |pmc=11621519 |bibcode=2024PNAS..12108386S |issn=0027-8424}}</ref>

Octopuses can create distracting patterns with waves of dark colouration across the body, a display known as the "passing cloud". Muscles in the skin change the texture of the mantle to achieve greater camouflage. In some species, the mantle can take on the bumpy appearance of algae-covered rocks. Diurnal, shallow water octopuses have more complex skin than their nocturnal and deep-sea counterparts. In the latter species, skin anatomy is limited to one colour or pattern.<ref name=Mather/>{{rp|89–97}}

Octopus' "moving rock" trick involves mimicking a rock and then inching across the open space with a speed matching that of the surrounding water.<ref>{{cite book |title=Cephalopod Behaviour |publisher=Cambridge University Press |last1=Hanlon |first1=R. T. |last2=Messenger |first2=J. B. |year=2018|pages=110–111|edition=2nd |isbn=978-0521897853}}</ref>

===Defence===
[[File:Hapalochlaena lunulata2.JPG|thumb|left|upright|[[Aposematism|Warning display]] of [[greater blue-ringed octopus]] (''Hapalochlaena lunulata'')|alt=An octopus among coral displaying conspicuous rings of turquoise outlined in black against a sandy background]]

Aside from humans, octopuses are prey for fishes, [[seabird]]s, [[sea otter]]s, [[pinniped]]s, [[cetacea]]ns, other cephalopods, and humans.<ref name=Crowfootdefense/> Octopuses typically hide or disguise themselves by camouflage and [[mimicry]]; some have conspicuous [[aposematism|warning coloration (aposematism)]] or [[deimatic behaviour<!--British English-->]] (“bluffing” a threatening appearance).<ref name=Mather/>{{rp|90–97}} An octopus may hide in their dens for as much as 40% of the day. When the octopus is approached, it may reach out an arm to investigate. 66% of ''[[Giant Pacific octopus|E. dofleini]]'' in one study had scars, with 50% missing arms.<ref name=Crowfootdefense>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoPred.php |title=Octopuses and Relatives: Predators and Defenses |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=13 April 2017 |archive-url=https://web.archive.org/web/20170421151656/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoPred.php |archive-date=21 April 2017 |url-status=dead }}</ref> The blue rings of the venomous blue-ringed octopus are hidden in muscular skin folds which contract when the animal is threatened, revealing the iridescent warning.<ref>{{cite journal |last1=Mäthger |first1=L. M. |last2=Bell |first2=G. R. |last3=Kuzirian |first3=A. M. |last4=Allen |first4=J. J. |last5=Hanlon |first5=R. T. |year=2012 |title=How does the blue-ringed octopus (''Hapalochlaena lunulata'') flash its blue rings? |journal=Journal of Experimental Biology |volume=215 |issue=21 |pages=3752–3757 |doi=10.1242/jeb.076869 |pmid=23053367 |doi-access=free |bibcode=2012JExpB.215.3752M }}</ref> The [[Atlantic white-spotted octopus]] (''Callistoctopus macropus'') becomes redder with bright white spots in a [[deimatic behaviour|deimatic display]]. Displays are often reinforced by stretching out the animal's arms, fins or web to make it look as big and threatening as possible.<ref name=Hanlon/>{{rp|80–81}}

Octopus try to escape from a predator by ejecting an ink cloud, which acts as a "smoke-screen" or a [[Pseudomorph#In other fields|decoy]], as well as to interfere with the attacker's sense of smell.<ref>{{Cite journal |last1=Caldwell |first1=R. L. |year=2005 |title=An Observation of Inking Behavior Protecting Adult ''Octopus bocki'' from Predation by Green Turtle (''Chelonia mydas'') Hatchlings |journal=Pacific Science |volume=59 |issue=1 |pages=69–72 |doi=10.1353/psc.2005.0004 |url=http://scholarspace.manoa.hawaii.edu/bitstream/10125/24161/1/PacSci_069_072.pdf |hdl=10125/24161 |s2cid=54223984 |hdl-access=free }}</ref> When severed by a predator, some octopuses can [[autotomy|detach their arm]],<ref name=Hanlon/>{{rp|86–87}} which can [[limb regeneration|grow back]].<ref name=Mather/>{{rp|85}} Some octopuses, such as the [[mimic octopus]], can combine their flexible bodies with their colour-changing ability to mimic other, more dangerous animals, such as [[Pterois|lionfish]], sea snakes, and [[eel]]s.<ref>{{cite journal |last1=Norman |first1=M. D. |last2=Finn |first2=J. |last3=Tregenza |first3=T. |title=Dynamic mimicry in an Indo-Malayan octopus |journal=Proceedings of the Royal Society |volume=268 |issue=1478 |pages=1755–8 |date=2001 |pmid=11522192 |pmc=1088805 |doi=10.1098/rspb.2001.1708 |url=http://marinebio.org/upload/files/mimic.pdf |access-date=1 October 2008 |archive-url=https://web.archive.org/web/20120210191131/http://marinebio.org/upload/files/mimic.pdf |archive-date=10 February 2012 |url-status=dead |df=dmy-all }}</ref><ref>{{cite journal |last=Norman |first=M. D. |year=2005 |title=The 'Mimic Octopus' (''Thaumoctopus mimicus'' n. gen. et sp.), a new octopus from the tropical Indo-West Pacific (Cephalopoda: Octopodidae) |url=http://www.mapress.com/mr/content/v25/2005f/n2p070.htm |journal=Molluscan Research |volume=25 |issue=2 |pages=57–70 |doi=10.11646/mr.25.2.1 |s2cid=260016769 }}</ref>

===Pathogens and parasites===
Cephalopods are known to be the intermediate or final [[host (biology)|hosts]] of various parasitic [[Cestoda|cestodes]], [[nematode]]s and copepods; 150 species of [[protist]]an and [[metazoa]]n parasites are recognised.<ref>{{cite journal |last1=Pascal |first1=Santiago |last2=Gestal |first2=Camino |last3=Estevez |first3=J. |last4=Arias |first4=Christian Andrés |year=1996 |title=Parasites in commercially-exploited cephalopods (Mollusca, Cephalopoda) in Spain: An updated perspective |journal=Aquaculture |volume=142 |issue=1–2 |pages=1–10 |doi=10.1016/0044-8486(96)01254-9 |bibcode=1996Aquac.142....1P }}</ref> The [[Dicyemidae]] are a family of tiny worms found in the renal appendages of many species;<ref>{{cite journal |last1=Furuya |first1=Hidetaka |last2=Tsuneki |first2=Kazuhiko |date=2003 |title=Biology of Dicyemid Mesozoans |journal=Zoological Science |volume=20 |issue=5 |pages=519–532 |doi=10.2108/zsj.20.519 |pmid=12777824|s2cid=29839345 |doi-access=free }}</ref> it is unclear whether they are parasitic or [[endosymbiont]]s. [[Coccidia]]ns in the genus ''[[Aggregata]]'' living in the gut cause serious illness in the host. Octopuses have an [[innate immune system]]; their [[hemocyte (invertebrate immune system cell)|haemocytes]] locate the foreign invader and attack it via [[phagocytosis]], encapsulation, infiltration, or [[cytotoxicity]]. The haemocytes also contribute to healing injures.<ref>{{cite journal |last1=Castellanos-Martínez |first1=Sheila |last2=Gestal |first2=Camino |year=2013 |title=Pathogens and immune response of cephalopods |journal=Journal of Experimental Marine Biology and Ecology |volume=447 |pages=14–22 |url=https://www.academia.edu/6443538 |doi=10.1016/j.jembe.2013.02.007 |bibcode=2013JEMBE.447...14C }}</ref> A gram-negative bacterium, ''[[Vibrio lentus]]'', can cause skin lesions, exposure of muscle and sometimes death.<ref>{{cite journal |last1=Farto |first1=R. |last2=Armada |first2=S. P. |last3=Montes |first3=M. |last4=Guisande |first4=J. A. |last5=Pérez |first5=M. J. |last6=Nieto |first6=T. P. |year=2003 |title=''Vibrio lentus'' associated with diseased wild octopus (''Octopus vulgaris'') |journal=Journal of Invertebrate Pathology |volume=83 |issue=2 |pages=149–156 |doi=10.1016/S0022-2011(03)00067-3|pmid=12788284 |bibcode=2003JInvP..83..149F }}</ref>

==Evolution==
{{Further|Evolution of cephalopods}}

The scientific name Octopoda was first given as the order of octopuses in 1818 by English biologist [[William Elford Leach]],<ref>{{cite WoRMS |last=Gofas |first=S.|year=2009 |title=Octopoda |id=11718 |access-date=5 May 2017 }}</ref> who classified them as Octopoida the previous year.<ref name="Mikko"/> Octopoda consists of around 300 known species<ref name=Mather/>{{rp|145}} and were historically divided into two suborders, the [[Incirrina]] and the Cirrina.<ref>{{cite book|last1=Boyle|first1=P|last2=Rodhouse|first2=P|year=2008|title=Cephalopods: Ecology and Fisheries|publisher=Wiley|page=72|isbn=9781405145435}}</ref> More recent evidence suggests cirrates are the most basal species, not a unique [[clade]].<ref name="Sanchez et al, 2018"/> The incirrate octopuses (the majority of species) lack the cirri and paired swimming fins of the cirrates.<ref name="Corporation2004"/> In addition, the internal shell of incirrates is either present as a pair of [[Stylet (anatomy)|stylets]] or absent altogether.<ref>{{cite journal |last1=Fuchs |first1=D. |first2=C. |last2=Ifrim |first3=W. |last3=Stinnesbeck |year=2008 |title=A new ''Palaeoctopus'' (Cephalopoda: Coleoidea) from the Late Cretaceous of Vallecillo, north-eastern Mexico, and implications for the evolution of Octopoda |journal=Palaeontology |volume=51 |issue=5 |pages=1129–1139 |doi=10.1111/j.1475-4983.2008.00797.x |bibcode=2008Palgy..51.1129F |doi-access=free}}</ref>

===Fossil history and phylogeny===
[[File:Muensterella_scutellaris_348.jpg|thumb|The octopuses evolved from the [[Muensterelloidea]] (fossil pictured) in the [[Jurassic]] period.<ref name="Fuchs Iba 2019"/>|alt=Fossil of crown group coleoid on a slab of Jurassic rock from Germany]]

The Cephalopoda descended from a mollusc resembling the [[Monoplacophora]] in the [[Cambrian]] some 530 million years ago. The Coleoidea, which brought their shells inside the body, diverged from the [[Nautiloid|nautiloids]] in the [[Devonian]] some 416 million years ago. Around 276 million years ago, during the [[Permian]], the coleoids split into two groups, the [[Octopodiformes|Vampyropoda]] and the [[Squid|Decabrachia]].<ref name="Kröger 2011">{{cite journal |last1=Kröger |first1=Björn |last2=Vinther |first2=Jakob |last3=Fuchs |first3=Dirk |title=Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules |journal=BioEssays |volume=33 |issue=8 |year=2011 |pages=602–613 |issn=0265-9247 |doi=10.1002/bies.201100001|pmid=21681989 |s2cid=2767810 }}</ref> The octopuses arose from the [[Muensterelloidea]] within the Vampyropoda in the [[Jurassic]]. The earliest octopus likely lived near the sea floor ([[Benthic zone|benthic]] to [[Demersal zone|demersal]]) in shallow marine environments.<ref name="Kröger 2011"/><ref name="Fuchs Schweigert 2018">{{Cite journal |last1=Fuchs |first1=Dirk |last2=Schweigert |first2=Günter |year=2018 |title=First Middle–Late Jurassic gladius vestiges provide new evidence on the detailed origin of incirrate and cirrate octopuses (Coleoidea) |journal=PalZ |volume=92 |issue=2 |pages=203–217 |doi=10.1007/s12542-017-0399-8 |bibcode=2018PalZ...92..203F |s2cid=135245479 |issn=0031-0220}}</ref><ref name="Fuchs Iba 2019">{{cite journal |last1=Fuchs |first1=Dirk |last2=Iba |first2=Yasuhiro |last3=Heyng |first3=Alexander |last4=Iijima |first4=Masaya |last5=Klug |first5=Christian |last6=Larson |first6=Neal L. |last7=Schweigert |first7=Günter |last8=Brayard |first8=Arnaud |title=The Muensterelloidea: phylogeny and character evolution of Mesozoic stem octopods |journal=Papers in Palaeontology |volume=6 |issue=1 |year=2019 |pages=31–92 |issn=2056-2802 |doi=10.1002/spp2.1254|s2cid=198256507 }}</ref> Octopuses consist mostly of soft tissue, and so fossils are relatively rare. As soft-bodied cephalopods, they lack the external shell of most molluscs, including other cephalopods like the nautiloids and the (extinct) [[Ammonoidea]].<ref name="CephGroup">{{cite web |title=A Broad Brush History of the Cephalopoda |url=http://www.thecephalopodpage.org/evolution.php |publisher=The Cephalopod Group |access-date=27 March 2017 |archive-date=16 July 2018 |archive-url=https://web.archive.org/web/20180716165558/http://www.thecephalopodpage.org/evolution.php |url-status=dead }}</ref> They have eight limbs like other [[Coleoidea]], but lack the extra specialised feeding appendages known as [[tentacle]]s which are longer and thinner with suckers only at their club-like ends.<ref>{{cite web |last1=Young |first1=R. E. |first2=M. |last2=Vecchione |first3=K. M. |last3=Mangold |year=1999 |url=http://tolweb.org/accessory/Cephalopoda_Glossary?acc_id=587 |title=Cephalopoda Glossary |publisher=Tree of Life web project |access-date=30 May 2017}}</ref><ref>{{cite journal|last1=Nödl|first1=M-T|last2=Fossati|first2=S. M.|last3=Domingues|first3=P|last4=Sánchez|first4=F. J.|last5=Zullo|first5=L|year=2015|title=The making of an octopus arm|journal=EvoDevo|volume=6|page=19|doi=10.1186/s13227-015-0012-8|doi-access=free |pmid=26052417 |pmc=4458049 }}</ref> The vampire squid (''[[Vampyroteuthis]]'') also lacks tentacles but has sensory filaments.<ref>{{cite web |last=Seibel |first=B. |title=''Vampyroteuthis infernalis'', Deep-sea Vampire squid |publisher=The Cephalopod Page |url=http://www.thecephalopodpage.org/vampy.php |access-date=31 May 2017 |archive-date=16 July 2018 |archive-url=https://web.archive.org/web/20180716165547/http://www.thecephalopodpage.org/vampy.php |url-status=dead }}</ref>

The [[cladogram]]s are based on Sanchez et al., 2018, who created a [[molecular phylogeny]] based on [[mitochondria]]l and [[nuclear DNA]] marker sequences.<ref name="Sanchez et al, 2018">{{cite journal |last1=Sanchez |first1=Gustavo |last2=Setiamarga |first2=Davin H. E. |last3=Tuanapaya |first3=Surangkana |last4=Tongtherm |first4=Kittichai |last5=Winkelmann |first5=Inger E. |last6=Schmidbaur |first6=Hannah |last7=Umino |first7=Tetsuya |last8=Albertin |first8=Caroline |last9=Allcock |first9=Louise |last10=Perales-Raya |first10=Catalina |last11=Gleadall |first11=Ian |last12=Strugnell |first12=Jan M. |last13=Simakov |first13=Oleg |last14=Nabhitabhata |first14=Jaruwat |title=Genus-level phylogeny of cephalopods using molecular markers: current status and problematic areas |journal=PeerJ  |volume=6 |year=2018 |doi=10.7717/peerj.4331 |pmid=29456885 |pmc=5813590 |page=e4331 |doi-access=free }}</ref> The position of the Eledonidae is from Ibáñez et al., 2020, with a similar methodology.<ref name="Ibáñez Fenwick 2020">{{cite journal |last1=Ibáñez |first1=Christian M. |last2=Fenwick |first2=Mark |last3=Ritchie |first3=Peter A. |last4=Carrasco |first4=Sergio A. |last5=Pardo-Gandarillas |first5=M. Cecilia |title=Systematics and Phylogenetic Relationships of New Zealand Benthic Octopuses (Cephalopoda: Octopodoidea) |journal=Frontiers in Marine Science |volume=7 |year=2020 |page=182 |issn=2296-7745 |doi=10.3389/fmars.2020.00182|doi-access=free |bibcode=2020FrMaS...7..182I }}</ref> Divergence dates are from Kröger et al., 2011 and Fuchs et al., 2019.<ref name="Kröger 2011"/><ref name="Fuchs Iba 2019"/>

{{clade
|label1=[[Cephalopods]]|sublabel1=530 mya<!--<ref name="Kröger 2011"/>-->
|1={{clade
   |label1=[[Nautiloid]]s
   |1=''[[Nautilus (genus)|Nautilus]]'' [[File:Nautilus belauensis profile (white background).jpg|70px|alt=A spiral nautilus in a blue sea]]
   |label2=[[Coleoidea|Coleoids]]|sublabel2=416 mya<!--<ref name="Kröger 2011"/>-->
   |2={{clade
      |label1=[[Decabrachia]] 
      |1=[[Squid|Squids and cuttlefish]] [[File:Berryteuthis_magister.jpg|70px|alt=A squid]]
      |label2=Vampyropoda|sublabel2=276 mya<!--<ref name="Kröger 2011"/>--> 
      |2={{clade
         |label1=[[Vampyromorphida]] 
         |1=[[File:Vampyroteuthis_infernalis.jpg|90px|alt=A strange blood-red octopus, its arms joined by a web]]
         |label2='''Octopods'''|sublabel2=155 mya
         |2=[[File:Octopus vulgaris Merculiano.jpg|70px|alt=A brown octopus with wriggly arms]]
         }}
      }}
   }}
}}

The molecular analysis of the octopods shows that the suborder Cirrina (Cirromorphida) and the superfamily Argonautoidea are [[paraphyletic]] and are broken up; these names are shown in quotation marks and italics on the cladogram.

{{clade
|label1='''Octopoda'''
|1={{clade
   |label1="''[[Cirrina|Cirromorphida]]''" part
   |1={{clade
      |1=[[Cirroteuthidae]] [[File:CirrothaumaMurDraw2.jpg|40px]]
      |2=[[Stauroteuthidae]] [[File:Stauroteuthis syrtensis (main).jpg|65px]]
      }}
   |2={{clade
      |label1="''[[Cirrina|Cirromorphida]]''" part
      |1={{clade
         |1=[[Opisthoteuthidae]] [[File:Opisthoteuthis californiana (white background).jpg|45px]]
         |2=[[Cirroctopodidae]] [[File:Cirroctopus mawsoni Vent.jpg|45px]]
         }}
      |label2=[[Octopoda|Octopodida]]
      |2={{clade
         |label1="''[[Argonautoidea]]''" part
         |1={{clade
            |1=[[Tremoctopodidae]] [[File:Pelagic octopus Tremoctopus.jpg|45px]]
            |2=[[Alloposidae]] [[File:Haliphron atlanticus (70 mm ML).jpg|60px]]
            }}
         |2={{clade
            |1={{clade
               |label1="''[[Argonautoidea]]''" part
               |1={{clade
                  |1=[[Argonautidae]] [[File:Argonauta_argo_Merculiano.jpg|55px]]
                  |2=[[Ocythoidae]] [[File:Ocythoe tuberculata (Merculiano).jpg|50px]]
                  }}
               |label2=[[Octopodoidea]]
               |2={{clade
                  |1=[[Eledonidae]] [[File:Eledone cirrhosa1.jpg|75px]]
                  |2={{clade
                     |1=[[Bathypolypodidae]] [[File:Bathypolypus valdiviae.jpg|75px]]
                     |2={{clade
                        |1=[[Enteroctopodidae]] [[File:E zealandicus (white background).jpg|55px]]
                        |2={{clade
                           |1=[[Octopodidae]] [[File:Octopus vulgaris Merculiano.jpg|55px]]
                           |2=[[Megaleledonidae]] [[File:Graneledone boreopacifica (white background).jpg|55px]]
                           }}
                        }}
                     }}
                  }}
               }}
            |2={{clade
               |1=[[Bolitaenidae]] [[File:Eledonella pygmaea.jpg|75px]]
               |2={{clade
                  |1=[[Amphitretidae]] [[File:Amphitretus pelagicus.jpg|70px]]
                  |2=[[Vitreledonellidae]] [[File:Vitreledonella richardi (white background).jpg|50px]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

===RNA editing and the genome===

Octopuses, like other coleoid cephalopods but unlike more [[Basal (phylogenetics)|basal]] cephalopods or other molluscs, are capable of greater [[RNA editing]], changing the [[nucleic acid sequence]] of the [[primary transcript]] of RNA molecules, than any other organisms. Much editing is done in the nervous system, particularly for [[Membrane potential#Cell excitability|excitability]] and neuronal morphology. Coleoids rely mostly on [[ADAR]] enzymes for RNA editing, which requires large, [[double-stranded RNA]] structures. The many editing sites are [[Conserved sequence|conserved]] in the coleoid genome and the mutation rates for the sites are hampered. Hence, greater transcriptome plasticity has come at the cost of slower genome evolution.<ref>{{cite journal |last1=Liscovitch-Brauer |first1=N. |last2=Alon |first2=S. |last3=Porath |first3=H. T. |last4=Elstein |first4=B. |last5=Unger |first5=R. |last6=Ziv |first6=T. |last7=Admon |first7=A. |last8=Levanon |first8=E. Y. |last9=Rosenthal |first9=J. J. C. |last10=Eisenberg |first10=E. |year=2017 |title=Trade-off between transcriptome plasticity and genome evolution in cephalopods |journal=Cell |volume=169 |issue=2 |pages=191–202 |doi=10.1016/j.cell.2017.03.025 |pmid=28388405 |pmc=5499236}}</ref>

The octopus genome is unremarkably [[bilaterian]] except for large developments of two gene families: [[protocadherin]]s, which regulate the development of neurons; and the [[C2H2 zinc-finger]] transcription factors. Many novel genes in both cephalopods generally and octopus specifically manifest in the animals' skin, suckers, and nervous system.<ref name="Albertin Simakov 2015"/>

==Relationship to humans==
===Cultural significance===
[[File:AMI - Oktopusvase.jpg| thumb|upright|[[Minoan civilization|Minoan]] clay vase with octopus decoration, c. 1500 BC|alt=An ancient nearly spherical vase with 2 handles by the top, painted all over with an octopus decoration in black]]
Ancient seafaring people were aware of the octopus, as evidenced by artworks and designs. It was depicted on coins during the [[Minoan civilization]] possibly as early as 1650 BCE and on pottery in [[Mycenaean Greece]] around between 1200 and 1100 BCE. A Hawaiian [[creation myth]] suggests that the octopus is the lone survivor of a previous age. The legendary sea monster, the [[kraken]] is conceived as octopus-like.<ref name=Courage/>{{rp|1, 4–5}} Similarly, [[Medusa]] was compared to an octopus, with her snake-hair resembling the creature's arms.<ref name=Schweid>{{cite book|last=Schweid|first=R|year=2013|title=Octopus|publisher=Reaktion Books|isbn=978-1-78023-177-8}}</ref>{{rp|133}}  The [[Akkorokamui]] is a gigantic octopus-like [[monster]] from [[Ainu people|Ainu]] folklore, worshipped in [[Shinto]].<!--<ref name="Batchelor">{{cite book |last=Batchelor |first=John |title=The Ainu and Their Folklore |url=https://archive.org/details/b29010664 |location=London |publisher=The Religious Tract Society |year=1901 |page=NEEDED; it may be there but not obvs and not in index }}</ref>--><ref>{{cite journal |last1=Srinivasan |first1=A. |title=The Sucker, the Sucker! [Review] |url=https://discovery.ucl.ac.uk/id/eprint/10024715/3/Srinivasan_Octopuses.pdf |journal=[[London Review of Books]] |volume=39 |issue=17 |pages=23–25 |date=2017}}</ref> 

In the [[Asuka period|Asuka-era]] Japanese legend ''Taishokan'', a female diver battles an octopus to recover a stolen jewel, which became the inspiration for [[woodblock printing]]s. Similarly, in the 1973 novel ''[[Gravity's Rainbow]]'' an octopus named Grigori attacks a woman on the beach. A battle with an octopus plays a significant role in [[Victor Hugo]]'s 1866 book ''Travailleurs de la mer'' (''[[Toilers of the Sea]]''). The octopus continues to be depicted as antagonistic in films such as ''[[Wake of the Red Witch]]'' (1948).<ref name=Schweid/>{{rp|129–131, 138–139, 145–147}} 

In [[political cartoon]]s, octopuses have been used to symbolise empires and large organizations, the arms representing long reach. Octopus also had an erotic appeal. Japanese erotic art, ''[[shunga]]'', includes [[ukiyo-e]] woodblock prints such as [[Katsushika Hokusai]]'s 1814 print ''Tako to ama'' (''[[The Dream of the Fisherman's Wife]]''), in which a woman is sexually intertwined with a large and a small octopus. This art style would inspire [[Pablo Picasso]]'s 1903 drawing ''An Erotic Drawing: Woman and Octopus''.<ref name=Schweid/>{{rp|126–128}} Some individual octopuses gained celebrity status, notably [[Paul the Octopus]] who predicted the winners of the [[2010 FIFA World Cup]].<ref name=Courage/>{{rp|3–4}}

===Danger to humans===
{{further information|Cephalopod attack}}
[[File:Colossal octopus by Pierre Denys de Montfort.jpg|thumb|upright|Pen and [[Wash (visual arts)|wash]] drawing of an imagined [[colossal octopus]] attacking a ship, by the [[malacologist]] [[Pierre Denys de Montfort|Pierre de Montfort]], 1801|alt=Coloured drawing of a huge octopus rising from the sea and attacking a sailing ship's three masts with its spiralling arms]]

Octopuses generally avoid humans, but some conflictual incidents have been verified. For example, a {{convert|2.4|metre|feet|0|adj=on}} Pacific octopus, said to be nearly perfectly camouflaged, "lunged" at a diver and "wrangled" over his camera before it let go. Another diver recorded the encounter on video.<ref>{{cite news |last=Ross |first=Philip |title=8-Foot Octopus Wrestles Diver Off California Coast, Rare Encounter Caught on Camera |url=http://www.ibtimes.com/8-foot-octopus-wrestles-diver-calif-coast-rare-encounter-caught-camera-video-1556415 |work=[[International Business Times]] |date=18 February 2014}}</ref> All species are venomous, but only blue-ringed octopuses have venom that is lethal to humans.<ref>{{cite web|title=Eight Strange and Wonderful Facts About Octopuses|website=Shedd Aquarium|date=6 September 2023 |url=https://www.sheddaquarium.org/stories/eight-strange-and-wonderful-facts-about-octopuses#:~:text=Venom,Australia%20is%20dangerous%20to%20humans|accessdate=4 April 2025}}</ref> Blue-ringed octopuses rank amongst the most dangerous marine animals; their bites are reported each year across the animals' range from Australia to the eastern Indo-Pacific Ocean. They bite only when provoked or accidentally touched; bites are small and usually painless. The venom appears to be able to penetrate the skin without a puncture, given prolonged contact. It contains [[tetrodotoxin]], which causes paralysis by blocking the transmission of [[nerve impulse]]s to the muscles. This causes death by respiratory failure leading to [[Cerebral hypoxia|cerebral anoxia]]. No antidote is known, but if breathing can be kept going artificially, patients recover within 24 hours.<ref>{{cite web |title=Blue-ringed Octopuses, ''Hapalochlaena maculosa'' |url=http://marinebio.org/species.asp?id=403 |url-status=dead |archive-url=https://archive.today/20120524092455/http://marinebio.org/species.asp?id=403 |archive-date=2012-05-24 |access-date=12 April 2017 |publisher=The MarineBio Conservation Society}}</ref><ref>{{cite web |last1=Caldwell |first1=Roy <!--University of California at Berkeley--> |title=What makes blue-rings so deadly? Blue-ringed octopus have tetrodotoxin |url=http://www.thecephalopodpage.org/bluering2.php |website=The Cephalopod Page |access-date=12 April 2017 |archive-date=18 July 2018 |archive-url=https://web.archive.org/web/20180718144602/http://www.thecephalopodpage.org/bluering2.php |url-status=dead }}</ref> Bites have been recorded from captive octopuses of other species; they leave temporary swellings.<ref name=Wells/>{{rp|68}}

===As a food source===
{{Main|Octopus as food}}
[[File:Tako nigiri 2.jpg|thumb|Octopus [[sushi]]]]
Octopuses [[fisheries|are fished]] around the world and between 1988 and 1995, catches varied between 245,320 and 322,999 metric tons.<ref name=gillespie>{{cite web |last1=Gillespie |first1=G. E. |last2=Parker |first2=G. |last3=Morrison |first3=J. |date=1998 |title=A Review of Octopus Fisheries Biology and British Columbia Octopus Fisheries |publisher=Canadian Stock Assessment Secretariat |url=https://waves-vagues.dfo-mpo.gc.ca/Library/227412.pdf}}</ref> The world catch peaked in 2007 at 380,000 tons, and had fallen by a tenth by 2012.<ref>{{cite web |last1=Rocliffe |first1=S. |last2=Harris |first2=A. |url=https://blueventures.org/publication/status-octopus-fisheries-western-indian-ocean/ |title=The status of octopus fisheries in the Western Indian Ocean |year=2016 |access-date=18 June 2017}}</ref> Methods to capture octopuses include pots, [[fish trap|trapping]], [[trawling|trawling]], snaring, drift fishing, spearing, hooking and catching by hands.<ref name=gillespie/> Octopuses are also [[bycatch]].<ref>{{cite journal |last1=Sauer |first1=Warwick H. H. |last2=Gleadall |first2=Ian G. |display-authors=etal |date=6 December 2019 |title=World Octopus Fisheries |journal=Reviews in Fisheries Science & Aquaculture |publisher=[[Taylor & Francis]] |volume=29 |issue=3 |pages=279–429 |doi=10.1080/23308249.2019.1680603 |issn=2330-8249 |s2cid=210266167 |hdl-access=free |hdl=10261/227068}}</ref>

Octopus is eaten in many cultures, such as those on the Mediterranean and Asian coasts.<ref>{{cite web |url=http://www.montereybayaquarium.org/animal-guide/octopus-and-kin/giant-pacific-octopus |title=Giant Pacific octopus |website=[[Monterey Bay Aquarium]] |date=2017 |access-date=1 August 2015 |archive-date=4 July 2018 |archive-url=https://web.archive.org/web/20180704213912/http://www.montereybayaquarium.org/animal-guide/octopus-and-kin/giant-pacific-octopus |url-status=dead }}</ref> The arms and other body parts are prepared in ways that vary by species and geography. Live octopuses or their wriggling pieces are consumed as ''[[san-nakji]]'' in Korean cuisine.<ref name="Guardian">{{cite news |url=https://www.theguardian.com/lifeandstyle/wordofmouth/2010/nov/10/live-and-let-dine |access-date=15 April 2015| last=Eriksen |first=L. |date=10 November 2010 |work=[[The Guardian]] |title=Live and let dine}}</ref><ref name="Newyorker">{{cite magazine |date=3 October 2014 |title=Why not eat octopus? |last=Killingsworth |first=Silvia |url=http://www.newyorker.com/tech/elements/eating-octopus |magazine=[[The New Yorker]] |access-date=15 April 2016}}</ref> If not prepared properly, however, the severed arms can choke the diner with their suction cups, causing at least one death in 2010.<ref>{{Cite web |last=Dodgson |first=Lindsay |date=2019-05-11 |title=Here's why eating a live octopus can be deadly |url=https://www.insider.com/eating-live-octopus-can-kill-you-2019-5 |website=Insider}}</ref> Animal welfare groups have objected to the live consumption of octopuses on the basis that they can experience pain.<ref name="Observer">{{cite news |url=https://www.theguardian.com/lifeandstyle/2010/may/30/food-restaurants-macho-eating-live |title=Macho foodies in New York develop a taste for notoriety |access-date=15 April 2015 |last=Ferrier |first=M. |date=30 May 2010 |work=[[The Guardian]]}}</ref>

===Science and technology===

In classical Greece, [[Aristotle]] (384–322 BC) [[Aristotle's biology|commented on]] their colour-changing abilities, both for camouflage and for [[signalling theory|signalling]], in his ''[[Historia animalium]]'': "The octopus&nbsp;... seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also [[deimatic behaviour|when alarmed]]."<ref>{{cite book |author=Aristotle |title=Historia animalium |orig-date=c. 350 BCE |volume=IX |page=622a: 2–10 |authorlink=Aristotle}} Cited in {{Cite book |last1=Borrelli |first1=Luciana |url=https://doi.org/10.36253/88-8453-376-7 |title=A Catalogue of Body Patterning in Cephalopoda |last2=Gherardi |first2=Francesca |last3=Fiorito |first3=Graziano |date=2006 |publisher=Firenze University Press |isbn=88-8453-377-5 |series=Cataloghi e collezioni |location=Firenze |doi=10.36253/88-8453-376-7 |authorlink2=Francesca}} {{cite web |date=2006 |title=Abstract |url=http://www.fupress.com/scheda.asp?IDV=487 |archive-url=https://web.archive.org/web/20180206145302/http://classics.mit.edu/Aristotle/history_anim.9.ix.html |archive-date=6 February 2018}}</ref> Aristotle noted that the octopus had a hectocotyl arm and suggested it might be used in reproduction. This claim was widely ignored until the 19th century. It was described in 1829 by the French zoologist [[Georges Cuvier]], who supposed it to be a parasitic worm, naming it as a new species, ''Hectocotylus octopodis''.<ref>{{Cite journal |last=Harman |first=Oren |date=2016-01-01 |title=The Lagoon: How Aristotle Invented Science |url=https://doi.org/10.1215/0961754x-3323121 |journal=Common Knowledge |volume=22 |issue=1 |pages=128 |doi=10.1215/0961754x-3323121 |issn=0961-754X}}</ref><ref>{{cite book |last=Leroi |first=Armand Marie |author-link=Armand Marie Leroi |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |publisher=Bloomsbury |date=2014 |isbn=978-1-4088-3622-4 |pages=71–72}}</ref><ref>{{cite web |title=The Cephalopoda |url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php|publisher=University of California Museum of Paleontology|access-date=27 March 2017}}</ref> Other zoologists thought it a spermatophore; the German zoologist [[Heinrich Müller (physiologist)|Heinrich Müller]] believed it was "designed" to detach during copulation. In 1856, the Danish zoologist [[Japetus Steenstrup]] demonstrated that it is used to transfer sperm, and only rarely detaches.<ref>{{cite book |last=Mann |first=T. |title=Spermatophores: Development, Structure, Biochemical Attributes and Role in the Transfer of Spermatozoa |url={{google books |plainurl=y |id=imPrCAAAQBAJ|page=28 }}|year=2012 |publisher=Springer |isbn=978-3-642-82308-4 |page=28}}</ref>

[[File:OCTOPUS arm1.JPG|thumb|Flexible [[biomimetic]] 'Octopus' [[robotics]] arm. The BioRobotics Institute, [[Scuola Superiore Sant'Anna]], [[Pisa]], 2011<ref name="Laschi Cianchetti 2012">{{cite journal |last1=Laschi |first1=Cecilia|author1-link=Cecilia Laschi |last2=Cianchetti |first2=Matteo |last3=Mazzolai |first3=Barbara |last4=Margheri |first4=Laura |last5=Follador |first5=Maurizio |last6=Dario |first6=Paolo |title=Soft Robot Arm Inspired by the Octopus |journal=Advanced Robotics |volume=26 |issue=7 |year=2012 |pages=709–727 |issn=0169-1864 |doi=10.1163/156855312X626343|s2cid=6104200 }}</ref>]]

Octopuses offer many [[model organism|possibilities in biological research]]; the California two-spot octopus had its genome sequenced, allowing exploration of its molecular adaptations.<ref name="Albertin Simakov 2015">{{cite journal |last1=Albertin |first1=Caroline B. |last2=Simakov |first2=Oleg |last3=Mitros |first3=Therese |last4=Wang |first4=Z. Yan |last5=Pungor |first5=Judit R. |last6=Edsinger-Gonzales |first6=Eric |last7=Brenner |first7=Sydney |last8=Ragsdale |first8=Clifton W. |last9=Rokhsar |first9=Daniel S. |title=The octopus genome and the evolution of cephalopod neural and morphological novelties |journal=Nature |volume=524 |issue=7564 |year=2015 |pages=220–224 |issn=0028-0836 |doi=10.1038/nature14668|pmid=26268193 |pmc=4795812 |bibcode=2015Natur.524..220A |doi-access=free }}</ref> Having [[convergent evolution|independently evolved]] mammal-like intelligence, octopuses were compared by the philosopher [[Peter Godfrey-Smith]], who studied the nature of intelligence,<ref>{{cite book |last=Godfrey-Smith |first=Peter |author-link=Peter Godfrey-Smith |title=[[Other Minds: The Octopus, the Sea, and the Deep Origins of Consciousness]] |date=2018 |publisher=William Collins |isbn=978-0-00-822629-9 |pages=77–105, 137–157}}</ref> to hypothetical [[Extraterrestrial intelligence|intelligent extraterrestrials]].<ref>{{cite web |last=Baer |first=Drake |date=20 December 2016 |title=Octopuses Are 'the Closest We Will Come to Meeting an Intelligent Alien' |publisher=Science of Us |access-date=26 April 2017 |url=http://nymag.com/scienceofus/2016/12/octopuses-are-intelligent-aliens.html}}</ref> Their intelligence and flexible bodies enable them to escape from supposedly secure tanks in [[public aquarium]]s.<ref>{{Cite news |url=https://www.washingtonpost.com/news/animalia/wp/2016/04/13/octopus-slips-out-of-aquarium-tank-crawls-across-floor-escapes-down-pipe-to-ocean/ |title=Octopus slips out of aquarium tank, crawls across floor, escapes down pipe to ocean |last=Brulliard |first=Karin |date=13 April 2016 |newspaper=The Washington Post |access-date=20 February 2017}}</ref>

Due to their intelligence, many argue that octopuses should be given protections when used for experiments.<ref name=protections>{{cite journal|author=Reardon, Sara|title=Octopuses used in research could receive same protections as monkeys|journal=Nature|accessdate=6 April 2025|date=15 September 2023|doi=10.1038/d41586-023-02887-w |pmid=37714985 |url=https://www.nature.com/articles/d41586-023-02887-w}}</ref> In the UK from 1993 to 2012, the common octopus (''Octopus vulgaris'') was the only invertebrate protected under the [[Animals (Scientific Procedures) Act 1986]].<ref>{{cite web |title=The Animals (Scientific Procedures) Act (Amendment) Order 1993 |url=http://www.legislation.gov.uk/uksi/1993/2103/article/3/made#text%3D%22Octopus%22 |publisher=The National Archives |access-date=18 February 2015}}</ref> In 2012, this legislation was extended to include all cephalopods<ref>{{cite web |title=The Animals (Scientific Procedures) Act 1986 Amendment Regulations 2012 |url=http://www.legislation.gov.uk/uksi/2012/3039/regulation/3/made |publisher=The National Archives |access-date=18 February 2015}}</ref> in accordance with a general [[European Union|EU]] directive.<ref name="EUdirective">{{cite web |title=Directive 2010/63/EU of the European Parliament and of the Council |url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:276:0033:0079:EN:PDF |publisher=Official Journal of the European Union |access-date=18 February 2015 |at=Article 1, 3(b) (see page 276/39)}}</ref> 

Some [[robotics]] research is exploring [[biomimicry]] of octopus features. Octopus arms can move and sense largely autonomously without intervention from the animal's central nervous system. In 2015 a team in Italy built soft-bodied robots able to crawl and swim, requiring only minimal computation.<ref>{{cite web |title=PoseiDRONE |url=http://sssa.bioroboticsinstitute.it/projects/PoseiDRONE |publisher=The BioRobotics Institute, Scuola Superiore Sant'Anna |access-date=14 May 2021 |archive-date=15 May 2021 |archive-url=https://web.archive.org/web/20210515115844/http://sssa.bioroboticsinstitute.it/projects/PoseiDRONE |url-status=dead }}</ref><ref name="Laschi 2015">{{cite book |last1=Laschi |first1=Cecilia |title=Soft Robotics |chapter=Soft Robotics Research, Challenges, and Innovation Potential, Through Showcases |year=2015 |pages=255–264 |doi=10.1007/978-3-662-44506-8_21|isbn=978-3-662-44505-1 }}</ref> In 2017, a German company made an arm with a soft [[pneumatic]]ally controlled [[silicone]] gripper fitted with two rows of suckers. It was able to grasp objects such as a metal tube, a magazine, or a ball, and to fill a glass by pouring water from a bottle.<ref>{{cite magazine |last=Burgess |first=Matt |title=This robotic octopus tentacle isn't creepy at all |url=https://www.wired.co.uk/article/octopus-robot-tentacle |magazine=[[Wired (magazine)|Wired]] |date=27 March 2017}}</ref>

==See also==
* {{annotated link|My Octopus Teacher}}

== Notes ==
{{Notelist}}

==References==
{{Reflist |30em}}

==External links==
{{Wikibooks |Dichotomous Key |Octopoda}}
{{Commons}}
* [http://www.eol.org/pages/2315/overview Octopuses – Overview] at the ''[[Encyclopedia of Life]]''
* [http://tolweb.org/tree?group=Octopoda Octopoda] {{Webarchive|url=https://web.archive.org/web/20200929203838/http://tolweb.org/tree?group=Octopoda |date=29 September 2020 }} at the [[Tree of Life Web Project]]
*[https://hakaimagazine.com/features/can-we-really-be-friends-octopus/ "Can We Really Be Friends with an Octopus?"] at [[Hakai Magazine]], January 11, 2022


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[[Category:Octopuses| ]]
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[[Category:Commercial molluscs]]
[[Category:Extant Pennsylvanian first appearances]]
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