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{{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 pluralization, "octopuses". This is correct and intentional, please maintain it. Other variants are discussed in the Etymology and pluralization 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 center 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}}</ref>}} The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The [[Siphon (mollusc)|siphon]] is used both for [[aquatic respiration|respiration]] and for [[aquatic locomotion#Jet propulsion|locomotion]], by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all [[invertebrate]]s.

Octopuses inhabit various regions of the [[ocean]], 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 a bundle of 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 the expulsion of [[octopus ink|ink]], the use of [[camouflage]] and [[deimatic display|threat displays]], the ability to jet quickly through the water and hide, and even 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 like 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]]'', inspiring other works such as [[Ian Fleming]]'s ''[[Octopussy and The Living Daylights|Octopussy]]''. Octopuses appear in Japanese erotic art, ''[[shunga]]''. They are eaten and considered a delicacy by humans in many parts of the world, especially the [[Mediterranean cuisine|Mediterranean]] and the Asian seas.

==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}}) for the common octopus.<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 grammatically 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 |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 still 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 known octopus species. Adults usually weigh around {{convert|15|kg|lb|abbr=on}}, with an arm span of up to {{convert|4.3|m|ft|abbr=on}}.<ref name="FONZ">{{cite web |url=http://nationalzoo.si.edu/Animals/Invertebrates/Facts/cephalopods/FactSheets/Pacificoctopus.cfm |title=Smithsonian National Zoological Park: Giant Pacific Octopus |publisher=Nationalzoo.si.edu |access-date=4 February 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140223084849/http://nationalzoo.si.edu/Animals/Invertebrates/Facts/cephalopods/FactSheets/Pacificoctopus.cfm |archive-date=23 February 2014 }}</ref> The largest specimen of this species to be scientifically documented was an animal with 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 for the giant Pacific octopus:<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]], ''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 }}</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 is [[Symmetry in biology|bilaterally symmetrical]] along its dorso-ventral (back to belly) axis; the head and [[Foot (mollusc)|foot]] are at one end of an elongated body and function as the anterior (front) of the animal. The head includes the mouth and brain. The foot has evolved into a set of flexible, prehensile [[cephalopod limb|appendages]], known as "arms", that surround the mouth and are attached to each other near their base by a webbed structure.<ref name=Ruppert/> The arms can be described based on side and sequence position (such as L1, R1, L2, R2) and divided into four pairs.{{sfnp|Wells|1978|pp=11–12}}<ref name=Ruppert/> 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? |first1=Byrne |last1=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 is known as the visceral hump; it contains most of the vital organs.{{sfnp|Mather|Anderson|Wood|2010|pp=13–15}}{{sfnp|Courage|2013|pp=40–41}} The mantle cavity has muscular walls and contains the gills; it is connected to the exterior by a funnel or [[Siphon (mollusc)|siphon]].<ref name=Ruppert/><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 |s2cid=84208773 }}</ref> The mouth of an octopus, located underneath the arms, has a sharp hard [[Cephalopod beak|beak]].{{sfnp|Courage|2013|pp=40–41}}

[[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 outer [[epidermis]] with mucous cells and sensory cells and a connective tissue [[dermis]] consisting largely of [[collagen]] fibres and various cells allowing colour change.<ref name=Ruppert/> Most of the body is made of soft tissue allowing it to lengthen, contract, and contort itself. The octopus can squeeze through tiny gaps; even the larger species can pass through an opening close to {{convert|2.5|cm|in|0|abbr=on}} in diameter.{{sfnp|Courage|2013|pp=40–41}} Lacking skeletal support, the arms work as [[muscular hydrostat]]s and contain longitudinal, transverse and circular muscles around a central axial nerve. They can extend and contract, twist to left or right, bend at any place in any direction or be held rigid.<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 anchor itself or to manipulate objects. Each sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called an [[Cephalopod limb|infundibulum]] and a central hollow cavity called an [[acetabulum (morphology)|acetabulum]], both of which are thick muscles covered in a protective chitinous cuticle. When a sucker attaches to a surface, the orifice between the two structures is sealed. The infundibulum provides adhesion while the acetabulum remains free, and 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> Each of the eight arms senses and responds to light, allowing the octopus to control the 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}}</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 eyes of the octopus are large and at the top of the head. They are similar in structure to those of a fish, and are enclosed in a [[cartilage|cartilaginous]] capsule fused to the cranium. 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 is suspended behind the pupil; photoreceptive [[Retina|retinal cells]] cover the back of the eye. The pupil can be adjusted in size; a retinal pigment screens incident light in bright conditions.<ref name=Ruppert/>

Some species differ in form from the typical octopus body shape. [[Basal (phylogenetics)|Basal]] species, the [[Cirrina]], have stout gelatinous bodies with webbing that reaches near the tip of their arms, and two large [[Cephalopod fin|fins]] above the eyes, supported by an [[cirrate shell|internal shell]]. Fleshy papillae or [[cirrus (biology)|cirri]] are found along the bottom of the arms, and the eyes are more developed.<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. Octopuses have three hearts; a systemic or main heart that circulates blood around the body and two branchial or gill hearts that pump it through each of the two gills. The systemic heart becomes inactive when the animal is swimming. Thus the octopus tires quickly and prefers to crawl.{{sfnp|Wells|1978|pp=31–35}}{{sfnp|Courage|2013|pp=42–43}} Octopus blood contains the [[copper]]-rich protein [[haemocyanin]] to transport oxygen. This makes the blood very [[viscosity|viscous]] and it requires considerable pressure to pump it around the body; octopuses' [[blood pressure]]s can exceed {{convert|75|mmHg|kPa|-1|abbr=on}}.{{sfnp|Wells|1978|pp=31–35}}{{sfnp|Courage|2013|pp=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]]. The haemocyanin is dissolved in the plasma instead of being carried within blood cells and gives the blood a bluish colour.{{sfnp|Wells|1978|pp=31–35}}{{sfnp|Courage|2013|pp=42–43}}

The systemic heart has muscular contractile walls and consists of a single ventricle and two atria, one for each side of the body. The blood vessels consist of arteries, capillaries and veins and are lined with a cellular [[endothelium]] which is quite unlike that of most other [[invertebrate]]s. The blood circulates through the aorta and capillary system, to the vena 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/>

===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 involves drawing water into the mantle cavity through an aperture, passing it through the gills, and expelling it through the siphon. The ingress of water is achieved by contraction of radial muscles in the mantle wall, and flapper valves shut when strong circular muscles force the water out 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 expand the respiratory chamber.{{sfnp|Wells|1978|pp=24–26}} The [[lamella (surface anatomy)|lamella]] structure of the gills allows for a 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 }}</ref> Water flow over the gills correlates with locomotion, and an octopus can propel its body when it expels water out of its siphon.{{sfnp|Wells|1978|pp=24–26}}<ref name="Schmidt"/>

The thin skin of the octopus absorbs additional oxygen. When resting, around 41% of an octopus's oxygen absorption is through the skin. This decreases to 33% when it swims, as more water flows over the gills; skin oxygen uptake also increases. When it is resting after a meal, absorption through the skin can drop to 3% of its total oxygen uptake.<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}}</ref>

===Digestion and excretion===
The digestive system of the octopus begins with the [[buccal mass]] which consists of the mouth with its [[chitin]]ous beak, the pharynx, [[radula]] and salivary glands.{{sfnp|Wells|1978|pp=73–79}} The radula is a spiked, muscular tongue-like organ with multiple rows of tiny teeth.{{sfnp|Courage|2013|pp=40–41}} Food is broken down and is forced into the oesophagus 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 by numerous membranes. The tract consists of a [[crop (anatomy)|crop]], where the food is stored; a stomach, where food is ground down; a [[caecum]] where the now sludgy food is sorted into fluids and particles and which plays an important role in absorption; the [[hepatopancreas|digestive gland]], where liver cells break down and absorb the fluid and become "brown bodies"; and the intestine, where the accumulated waste is turned into faecal ropes by secretions and blown out of the funnel via the rectum.{{sfnp|Wells|1978|pp=73–79}}

During [[osmoregulation]], fluid is added to the [[Pericardium|pericardia]] of the branchial hearts. The octopus has two [[nephridium|nephridia]] (equivalent to vertebrate kidneys) which are associated with the branchial hearts; these and their associated ducts connect the pericardial cavities with the mantle cavity. Before reaching the branchial heart, each branch of the [[vena cava]] expands to form renal appendages which are in direct contact with the thin-walled nephridium. The urine is first formed in the pericardial cavity, and is modified by excretion, chiefly of ammonia, and selective absorption from the renal appendages, as it is passed along the associated duct and through the nephridiopore into the mantle cavity.<ref name=Ruppert/>{{sfnp|Wells|1978|pp=54–56}}

[[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 (along with [[cuttlefish]]) have the highest [[brain-to-body mass ratio]]s of all invertebrates;<ref name="Albertin Simakov 2015"/> this is greater than that of many vertebrates.<ref name="Pilleri1984">{{cite book |last=Pilleri |first=Georg |title=Investigations on Cetacea |url=https://books.google.com/books?id=TZFCAQAAMAAJ |access-date=30 July 2018 |volume=16–17 |year=1984 |publisher=Hirnanatomisches Institut der Universität |page=161}}</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> The [[nervous system]] is complex, only part of which is localised in its brain, which is contained in a cartilaginous capsule.<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> Two-thirds of an octopus's [[neuron]]s are in the nerve cords of its arms. This allows their arms to perform complex [[reflex]] actions without input from the brain.<ref>{{cite journal |pmid=15829594 |doi=10.1152/jn.00684.2004 |volume=94 |issue=2 |first1=Y. |last1=Yekutieli |first2=R. |last2=Sagiv-Zohar |first3=R. |last3=Aharonov |first4=Y. |last4=Engel |first5=B. |last5=Hochner |first6=T. |last6=Flash |title=Dynamic model of the octopus arm. I. Biomechanics of the octopus reaching movement |year=2005 |journal=Journal of Neurophysiology |pages=1443–1458 |s2cid=14711055 }}</ref> Unlike vertebrates, the complex motor skills of octopuses 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> The nervous system of cephalopods is the most complex of all invertebrates.<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=https://books.google.com/books?id=dW5e6FHOH-4C&pg=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> The giant [[nerve]] fibers of the cephalopod [[Mantle (mollusc)|mantle]] have been widely used for many years as experimental material in [[neurophysiology]]; their large diameter (due to lack of [[myelination]]) makes them relatively easy to study compared with other animals.<ref>{{cite journal | last1=Tasaki | first1=I. | last2=Takenaka | first2=T. | title=Resting and Action Potential of Squid Giant Axons Intracellularly Perfused with Sodium-Rich Solutions | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=50 | issue=4 | date=1963 | issn=0027-8424 | pmid=14077488 | pmc=221236 | doi=10.1073/pnas.50.4.619 | pages=619–626| doi-access=free | bibcode=1963PNAS...50..619T }}</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"/> and can distinguish the [[polarization (waves)|polarisation]] of light. [[Colour vision]] appears to vary from species to species, for example, being present in ''O. aegina'' but absent in ''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 camouflages them; the 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 |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 }}</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 sacrifices 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 brain 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. They provide information on the position of the body relative to gravity and can detect angular acceleration. An [[autonomic nervous system|autonomic]] response keeps the octopus's eyes oriented so that the pupil is always horizontal.<ref name=Ruppert/> Octopuses may also use the statocyst to hear sound. 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 }}</ref>

Octopuses have an excellent [[somatosensory system]]. Their suction cups are equipped with [[chemoreceptors]] so they can [[taste]] what they touch. 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 observe the arms visually 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]] of an octopus is located under the digestive gland. A gland attached to the sac produces the [[octopus ink|ink]], and the sac stores it. The sac is close enough to the funnel for the octopus to shoot out the ink with a water jet. Before it leaves the funnel, the ink passes through glands which mix it with mucus, creating a thick, dark blob which allows the animal to escape from a predator.{{sfnp|Mather|Anderson|Wood|2010|p=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 are [[Gonochorism|gonochoric]] and have a single, posteriorly-located gonad which is associated with the [[coelom]]. The [[testis]] in males and the [[ovary]] in females bulges into the [[gonocoel]] and the [[gamete]]s are released here. The gonocoel is connected by the [[gonoduct]] to the [[mantle cavity]], which it enters at the [[gonopore]].<ref name=Ruppert/> An [[optic gland]] creates hormones that cause the octopus to mature and age and stimulate gamete production. The gland may be triggered by environmental conditions such as temperature, light and nutrition, which thus control the timing of reproduction and lifespan.{{sfnp|Mather|Anderson|Wood|2010|p=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>

When octopuses reproduce, the male uses a specialised arm called a [[hectocotylus]] to transfer [[spermatophore]]s (packets of sperm) from the terminal organ of the reproductive tract (the cephalopod "penis") into the female's mantle cavity.<ref name=TOL>{{cite web |last1=Young |first1=R. E. |last2=Vecchione |first2=M. |last3=Mangold |first3=K. M. |year=1999 |url=http://tolweb.org/accessory/Cephalopoda_Glossary?acc_id=587 |title=Cephalopoda Glossary |website=Tree of Life web project}}</ref> The hectocotylus in [[benthic]] octopuses is usually the third right arm, which has a spoon-shaped depression and modified suckers near the tip. In most species, fertilisation occurs in the mantle cavity.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |url={{google books |plainurl=y |id=W2v0kQEACAAJ |page=363}}|year=2008 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=363–364 }}</ref>

The reproduction of octopuses has been studied in only a few species. One such species is the [[giant Pacific octopus]], in which courtship is accompanied, especially in the male, by changes in skin texture and colour. 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 for the species, which in the giant Pacific octopus is 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, and it is stored internally by the female.<ref name=Ruppert/>

[[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]]

About forty days after mating, the female giant Pacific octopus attaches strings of small fertilised eggs (10,000 to 70,000 in total) to rocks in a crevice or under an overhang. Here she guards and cares for them for about 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}} The female aerates them and keeps them clean; if left untended, many will die.<ref name=Scheel>{{cite web |last=Scheel |first=David |title=Giant Octopus: Fact Sheet |url=http://marine.alaskapacific.edu/octopus/factsheet.html |publisher=Alaska Pacific University |access-date=9 April 2017 |url-status=dead |archive-url=https://web.archive.org/web/20121115121756/http://marine.alaskapacific.edu/octopus/factsheet.html |archive-date=15 November 2012 }}</ref> She does not feed during this time and dies soon after. Males become [[senescence|senescent]] and die a few weeks after mating.{{sfnp|Mather|Anderson|Wood|2010|p=147}}

The eggs have large yolks; [[cleavage (embryo)|cleavage]] (division) is superficial and a [[germinal disc]] develops at the pole. During [[gastrulation]], the margins of this grow down and surround the yolk, forming a yolk sac, which eventually forms part of the gut. The dorsal side of the disc grows upward and forms the embryo, with a shell gland on its dorsal surface, gills, mantle and eyes. The arms and funnel develop as part of the foot on the ventral side of the disc. The arms later migrate upward, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo develops.<ref name=Ruppert/>

[[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 and are [[plankton]]ic for weeks to months, depending on the species and water temperature. They feed on [[copepod]]s, [[arthropod]] larvae and other [[zooplankton]], eventually settling on the ocean floor and developing directly into adults with no distinct [[Metamorphosis|metamorphoses]] that are present in other groups of [[mollusc]] larvae.<ref name=Ruppert/> Octopus species that produce larger eggs – including the [[Southern blue-ringed octopus|southern blue-ringed]], [[Caribbean reef octopus|Caribbean reef]], [[California two-spot octopus|California two-spot]], ''[[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> and deep sea octopuses – instead hatch as benthic animals similar to the adults.<ref name = "AZAmanual" />{{rp|74–75}}

In the [[Argonaut (animal)|argonaut]] (paper nautilus), the female secretes a fine, fluted, papery shell in which the eggs are deposited and in which she also resides while floating in mid-ocean. In this she broods the young, and it also serves as a buoyancy aid allowing her to adjust her depth. The male argonaut is minute by comparison and has no shell.<ref>{{cite magazine |url=https://www.wired.com/2015/01/absurd-creature-of-the-week-argonaut/ |title=Absurd Creature of the Week: The Beautiful Octopus Whose Sex Is All About Dismemberment |last=Simon |first=Matt |date=16 January 2015 |magazine=Wired: Science |access-date=20 May 2017}}</ref>

===Lifespan===

Octopuses have short lifespans, and some species complete their lifecycles in only six months. The [[Giant Pacific octopus]], one of the two largest species of octopus, usually lives for three to five years. Octopus lifespan is limited by reproduction.<ref name=NewScientist/> For most octopuses, the last stage of their life is called senescence. It is the breakdown of cellular function without repair or replacement. For males, this typically begins after mating. Senescence may last from weeks to a few months, at most. For females, it begins when they lay a clutch of eggs. Females will spend all their time aerating and protecting their eggs until they are ready to hatch. During senescence, an octopus does not feed and quickly weakens. Lesions begin to form and the octopus literally degenerates. Unable to defend themselves, octopuses often fall prey to 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> This makes most octopuses effectively [[semelparous]]. The [[larger Pacific striped octopus]] (LPSO) is an exception, as it can reproduce repeatedly over a life of around two years.<ref name=NewScientist>{{cite news |title=Octopuses were thought to be solitary until a social species turned up |work=New Scientist |date=Dec 21, 2019 |last=Hooper |first=Rowan |url=https://www.newscientist.com/article/mg24432610-400-octopuses-were-thought-to-be-solitary-until-a-social-species-turned-up/}}</ref>

Octopus reproductive organs mature due to the [[hormonal]] influence of the optic gland but result in the inactivation of their digestive glands. Unable to feed, the octopus typically dies of starvation.<ref name="auto"/> Experimental removal of both optic glands after spawning was found to result in the cessation of [[broodiness]], the resumption of feeding, increased growth, and greatly extended lifespans. It has been proposed that the naturally short lifespan may be functional to prevent rapid overpopulation.<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 live in every ocean, and different species have adapted to different [[marine habitat]]s. As juveniles, common octopuses inhabit shallow [[tide pool]]s. The Hawaiian day octopus (''[[Octopus cyanea]]'') lives on coral reefs; [[Argonaut (animal)|argonauts]] drift in [[Pelagic zone|pelagic waters]]. ''[[Abdopus aculeatus]]'' mostly lives in near-shore [[seagrass]] beds. Some species are adapted to the cold, ocean depths. The spoon-armed octopus (''[[Bathypolypus arcticus]]'') is found at depths of {{convert|1000|m|ft|abbr=on}}, and ''[[Vulcanoctopus hydrothermalis]]'' lives near [[hydrothermal vent]]s at {{convert|2000|m|ft|abbr=on}}.{{sfnp|Mather|Anderson|Wood|2010|pp=13–15}} 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 are known to live at [[bathyal]] and [[abyssal]] depths, there is only a single indisputable record of an octopus 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> 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>

==Behaviour and ecology==

Most species are solitary when not mating,<ref>{{cite magazine |url=http://www.nationalgeographic.com/magazine/2016/04/basic-instincts-octopus-mating/ |archive-url=https://web.archive.org/web/20160329050203/http://www.nationalgeographic.com/magazine/2016/04/basic-instincts-octopus-mating/ |url-status=dead |archive-date=29 March 2016 |magazine=National Geographic |date=April 2016| first=Patricia |last=Edmonds |title=What's Odd About That Octopus? It's Mating Beak to Beak}}</ref> though a few are known to occur in high densities and with frequent interactions, signaling, mate defending and eviction of individuals from dens. This is likely the result of abundant food supplies combined with limited 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 LPSO has been described as particularly social, living in groups of up to 40 individuals.<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 outcrops or other hard structures, though some species burrow into sand or mud. Octopuses are not [[territory (animal)|territorial]] but generally remain in a home range; they may leave in search of food. They can [[Animal navigation|navigate]] back 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.{{sfnp|Courage|2013|pp=45–46}}

Octopuses bring captured prey to the den, where they can eat it safely. Sometimes the octopus catches more prey than it can eat, and the den is often surrounded by a [[midden]] of dead and uneaten food items. Other creatures, such as fish, [[crab]]s, molluscs and [[echinoderm]]s, often share the den with the octopus, either because they have arrived as [[scavenger]]s, or because they have survived capture.<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]]

Nearly all octopuses are predatory; bottom-dwelling octopuses eat mainly [[crustacean]]s, [[polychaete worm]]s, and other molluscs such as [[whelk]]s and [[clam]]s; open-ocean octopuses eat mainly prawns, fish and other cephalopods.<ref name="feeding"/> Major items in the diet of the giant Pacific octopus include [[Bivalvia|bivalve molluscs]] such as the cockle ''[[Clinocardium nuttallii]]'', clams and scallops and crustaceans such as crabs and [[Majoidea|spider crabs]]. Prey that it is likely to reject include [[Naticidae|moon snails]] because they are too large and [[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 journal|last1=Collins|first1=Martin A.|last2=Villanueva|first2=Roger|date=June 2006|title=Taxonomy, ecology and behaviour of the cirrate octopods|journal=Oceanography and Marine Biology|series=Oceanography and Marine Biology - an Annual Review|volume=44|pages=277–322|doi=10.1201/9781420006391.ch6|url=https://www.researchgate.net/publication/266220687|access-date=5 February 2024 |isbn=978-0-8493-7044-1}}</ref>

A benthic (bottom-dwelling) octopus typically moves among the rocks and feels through the crevices. The creature may make a jet-propelled pounce on prey and pull it toward the mouth with its arms, the suckers restraining it. Small prey may be completely trapped by the webbed structure. Octopuses usually inject crustaceans like crabs with a paralysing [[saliva]] then dismember them with their beaks.<ref name="feeding">{{cite web |last1=Wassilieff |first1=Maggy |last2=O'Shea |first2=Steve |url=http://www.TeAra.govt.nz/en/octopus-and-squid/3 |title=Octopus and squid – Feeding and predation |website=Te Ara – the Encyclopedia of New Zealand |date=2 March 2009}}</ref>{{sfnp|Wells|1978|pp=74–75}} Octopuses feed on shelled molluscs either by forcing the valves apart, or by drilling a hole in the shell to inject a [[neurotoxin|nerve toxin]].<ref>{{cite journal |last=Wodinsky |first=Jerome |year=1969 |title=Penetration of the Shell and Feeding on Gastropods by ''Octopus'' |url=https://academic.oup.com/icb/article-pdf/9/3/997/602205/9-3-997.pdf |journal=American Zoologist |volume=9 |issue=3 |pages=997–1010 |doi=10.1093/icb/9.3.997 |doi-access=free }}</ref>{{sfnp|Wells|1978|pp=74–75}} It used to be thought that the hole was drilled 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. It takes about three hours for ''O. vulgaris'' to create a {{convert|0.6|mm|in|3|abbr=on}} hole. Once the shell is penetrated, the prey dies almost instantaneously, its muscles relax, and the soft tissues are easy for the octopus to remove. Crabs may also be treated in this way; 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'' has a reduced or non-existent radula and swallows prey whole.<ref name="marinebio"/> In the deep-sea genus ''[[Stauroteuthis]]'', some of the muscle cells that control the suckers in most species have been replaced with [[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|left|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, followed by swimming and crawling.<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 }}</ref> When in no hurry, they usually crawl on either solid or soft surfaces. Several arms are extended forward, some of the suckers adhere to the substrate and the animal hauls itself forward with its powerful arm muscles, while other arms may push rather than pull. As progress is made, other arms move ahead to repeat these actions and the original suckers detach. During crawling, the heart rate nearly doubles, and the animal requires ten or fifteen minutes to recover from relatively minor exercise.<ref name=Crowfootcrawling/>

Most octopuses swim by expelling a jet of water from the mantle through the siphon into the sea. The physical principle behind this is that the force required to accelerate the water through the orifice produces a reaction that propels the octopus in the opposite direction.<ref>{{cite journal |last1=Kassim |first1=I. |last2=Phee |first2=L. |last3=Ng |first3=W. S. |last4=Gong |first4=F. |last5=Dario |first5=P. |last6=Mosse |first6=C. A. |year=2006 |title=Locomotion techniques for robotic colonoscopy |journal=IEEE Engineering in Medicine and Biology Magazine |volume=25 |issue=3 |pages=40–56 |doi=10.1109/MEMB.2006.1636351 |pmid=16764431 |s2cid=9124611 }}</ref> 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 held out sideways, and 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"/>

[[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 8-cornered web]]

Cirrate octopuses cannot produce jet propulsion and rely on their fins for swimming. They have neutral buoyancy and drift through the water with the fins extended. 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]]. This moves the body slowly.<ref name="marinebio"/>

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"/>{{sfnp|Mather|Anderson|Wood|2010|p=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 ungainly gait supported by its remaining arms held rigid.<ref name=Finn/>

===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=http://www.nwf.org/News-and-Magazines/National-Wildlife/Animals/Archives/1997/Armed-But-Not-Dangerous.aspx |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> Young octopuses learn nothing from their parents, as adults provide no [[parental care]] beyond tending to their eggs until the young octopuses hatch.<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}}

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"/> Octopuses have also been observed in what has been described as [[play (animal behaviour)|play]]: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them.<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}}</ref> Octopuses often break out of their 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> 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>

===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]] when hunting and to avoid predators. To do this, they use specialised skin cells that change the appearance of the skin by adjusting its colour, opacity, or reflectivity. [[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}}</ref> This colour-changing ability is also used to communicate with or warn other octopuses.{{sfnp|Mather|Anderson|Wood|2010|pp=90–97}}

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 spiky appearance of algae; in others, skin anatomy is limited to relatively uniform shades of one colour with limited skin texture. Octopuses that are diurnal and live in shallow water have evolved more complex skin than their nocturnal and deep-sea counterparts.{{sfnp|Mather|Anderson|Wood|2010|pp=90–97}}

A "moving rock" trick involves the octopus 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=978-0-521-72370-1}}</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 may be preyed on by fishes, [[seabird]]s, [[sea otter]]s, [[pinniped]]s, [[cetacea]]ns, and other cephalopods.<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-->]].{{sfnp|Mather|Anderson|Wood|2010|pp=90–97}} An octopus may spend 40% of its time hidden away in its den. When the octopus is approached, it may extend an arm to investigate. 66% of ''Enteroctopus dofleini'' in one study had scars, with 50% having amputated 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 highly venomous blue-ringed octopus are hidden in muscular skin folds which contract when the animal is threatened, exposing 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 }}</ref> The [[Atlantic white-spotted octopus]] (''Callistoctopus macropus'') turns bright brownish red with oval white spots all over in a high contrast display.<ref>{{cite web |url=http://www.thecephalopodpage.org/MarineInvertebrateZoology/Octopusmacropus1.html |title=Grass octopus (''Octopus macropus'') |work=Marine Invertebrates of Bermuda |publisher=[[Bermuda Institute of Ocean Sciences]] |access-date=10 August 2018 |last1=Wigton |first1=Rachel |last2=Wood |first2=James B. |archive-url=https://web.archive.org/web/20160119135721/http://www.thecephalopodpage.org/MarineInvertebrateZoology/Octopusmacropus1.html |archive-date=19 January 2016 |url-status=dead |df=dmy-all }}</ref> 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>{{cite book |title=Cephalopod Behaviour |publisher=Cambridge University Press |last1=Hanlon |first1=R. T. |last2=Messenger |first2=J. B. |year=1998 |pages=80–81, 111 |edition=1st|isbn=978-0-521-64583-6}}</ref>

Once they have been seen by a predator, they commonly try to escape but can also create a distraction by ejecting an ink cloud from their ink sac. The ink is thought to reduce the efficiency of olfactory organs, which would aid evasion from predators that employ [[olfaction|smell]] for hunting, such as [[shark]]s. Ink clouds of some species might act as [[Pseudomorph#Pseudomorph in other fields|pseudomorphs]], or decoys that the predator attacks instead.<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 under attack, some octopuses can perform arm [[autotomy]], in a manner similar to the way [[skink]]s and other [[lizard]]s detach their tails. The crawling arm may distract would-be predators. Such severed arms remain sensitive to stimuli and move away from unpleasant sensations.<ref>{{cite web |url=http://blogs.scientificamerican.com/octopus-chronicles/2013/08/27/even-severed-octopus-arms-have-smart-moves/ |title=Even Severed Octopus Arms Have Smart Moves |first=Katherine |last=Harmon |date=27 August 2013 |work=Octopus Chronicles |publisher=Scientific American}}</ref> Octopuses can [[limb regeneration|replace lost limbs]].{{sfnp|Mather|Anderson|Wood|2010|p=85}}

Some octopuses, such as the [[mimic octopus]], can combine their highly 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===
The diseases and parasites that affect octopuses have been little studied, but 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 have been 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 that are 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 severe disease to the host. Octopuses have an [[innate immune system]]; their [[hemocyte (invertebrate immune system cell)|haemocytes]] respond to infection by [[phagocytosis]], encapsulation, infiltration, or cytotoxic activities to destroy or isolate the pathogens. The haemocytes play an important role in the recognition and elimination of foreign bodies and wound repair. Captive animals are more susceptible to pathogens than wild ones.<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 }}</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 }}</ref>

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

The scientific name Octopoda was first coined and 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"/> The Octopoda consists of around 300 known species{{sfnp|Mather|Anderson|Wood|2010|p=145}} and were historically divided into two suborders, the [[Incirrina]] and the Cirrina.<ref name="Corporation2004"/> More recent evidence suggests Cirrina is merely 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 evolved from a mollusc resembling the [[Monoplacophora]] in the [[Cambrian]] some 530 million years ago. The Coleoidea diverged from the nautiloids in the [[Devonian]] some 416 million years ago. In turn, the coleoids (including the squids and octopods) brought their shells inside the body and some 276 million years ago, during the [[Permian]], split into the Vampyropoda and the 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}}</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> 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}}</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 |issn=2296-7745 |doi=10.3389/fmars.2020.00182|doi-access=free }}</ref> Dates of divergence 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 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. Editing is concentrated in the nervous system, and affects proteins involved in neural excitability and neuronal morphology. More than 60% of RNA transcripts for coleoid brains are recoded by editing, compared to less than 1% for a human or [[Drosophila|fruit fly]]. Coleoids rely mostly on [[ADAR]] enzymes for RNA editing, which requires large [[double-stranded RNA]] structures to flank the editing sites. Both the structures and editing sites are conserved in the coleoid genome and the mutation rates for the sites are severely hampered. Hence, greater transcriptome plasticity has come at the cost of slower genome evolution.{{sfnp|Courage|2013|pp=46–49}}<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 genes specific to cephalopods are expressed in the animals' skin, suckers, and nervous system.<ref name="Albertin Simakov 2015"/>

==Relationship to humans==

===In art, literature, and mythology===
[[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. For example, a stone carving found in the archaeological recovery from Bronze Age [[Minoan civilization|Minoan Crete]] at [[Knossos]] (1900–1100 BC) depicts a fisherman carrying an octopus.<ref>{{cite web |last=Hogan |first=C. Michael |date=22 December 2007 |url=http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes |title=Knossos fieldnotes |website=The Modern Antiquarian}}</ref> The terrifyingly powerful [[Gorgon]] of [[Greek mythology]] may have been inspired by the octopus or squid, the octopus itself representing the severed head of [[Medusa]], the beak as the protruding tongue and fangs, and its tentacles as the snakes.<ref>{{cite book |url={{google books|plainurl=y|id=OnHO4orvz18C}} |title=Medusa: Solving the Mystery of the Gorgon |last=Wilk |first=Stephen R. |date=2000 |publisher=Oxford University Press |isbn=978-0-19-988773-6}}</ref> The [[Kraken]] are legendary sea monsters of giant proportions said to dwell off the coasts of Norway and Greenland, usually portrayed in art as giant octopuses attacking ships. [[Carl Linnaeus|Linnaeus]] included it in the first edition of his 1735 ''[[Systema Naturae]]''.<ref>{{cite book |last=Linnaeus |first=Carl |author-link=Carl Linnaeus |title=Systema Naturae |publisher=Laurentius Salvius |year=1735 |url={{google books |plainurl=y |id=WfQTAAAAQAAJ|page=82}} }}</ref><ref name=metropolitana>{{cite book |first1=Edward |last1=Smedley |first2=Hugh James |last2=Rose |first3=Henry John |last3=Rose |title=Encyclopaedia Metropolitana, Or, Universal Dictionary of Knowledge: Comprising the Twofold Advantage of a Philosophical and an Alphabetical Arrangement, with Appropriate Engravings |url={{google books |plainurl=y |id=3X1GAQAAIAAJ|page=255}} |year=1845 |publisher=B. Fellowes |pages=255–}}</ref> One translation of the Hawaiian [[creation myth]] the [[Kumulipo]] suggests that the octopus is the lone survivor of a previous age.<ref name=Dixon>{{cite book |volume=9 |last=Dixon |first=Roland Burrage |author-link=Roland Burrage Dixon |title=Oceanic |series=The Mythology of All Races |url={{google books |plainurl=y |id=gLIIAQAAIAAJ&pg=PP2}} |date=1916 |publisher=Marshall Jones Company |pages=2–}}</ref><ref>{{Cite book|url=https://archive.org/details/bub_gb_s4EBAAAAQAAJ |title=Die heilige Sage der Polynesier: Kosmogonie und Theogonie |last=Bastian |first=Adolf |date=1881 |publisher=F. A. Brockhaus |others=Oxford University  |location=Leipzig |pages=[https://archive.org/details/bub_gb_s4EBAAAAQAAJ/page/n127 107]–108}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=7Ir_cgqw_9QC |title=The Kumulipo: A Hawaiian Creation Chant |last=Beckwith|first=Martha Warren|year=1981|publisher=University of Hawaii Press |isbn=978-0-8248-0771-9 |pages=52–53}}</ref> 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>

A battle with an octopus plays a significant role in [[Victor Hugo]]'s 1866 book ''Travailleurs de la mer'' (''[[Toilers of the Sea]]'').<ref>{{cite web |title=[Review] The Toilers of the Sea by Victor Hugo & Translated by James Hogarth |url=https://www.kirkusreviews.com/book-reviews/victor-hugo/the-toilers-of-the-sea/ |publisher=[[Kirkus Reviews]] |access-date=14 May 2021 |date=2002}}</ref> [[Ian Fleming]]'s 1966 short story collection ''[[Octopussy and The Living Daylights]]'', and the 1983 [[Octopussy|''James Bond'' film]] were partly inspired by Hugo's book.<ref>{{cite journal |last1=Cohen-Vrignaud |first1=Gerard |title=On Octopussies, or the Anatomy of Female Power |journal=Differences |date=2012 |volume=23 |issue=2 |pages=32–61 |doi=10.1215/10407391-1533520 }}</ref> 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 an [[ama diver]] is sexually intertwined with a large and a small octopus.<ref name=Helsinki>{{cite book |first1=Sointu |last1=Fritze |first2=Saara |last2=Suojoki |title=Forbidden Images: Erotic Art from Japan's Edo Period |url={{google books |plainurl=y |id=QRtmAAAACAAJ|pate=23}} |year=2000 |publisher=Helsingin kaupungin taidemuseo |isbn=978-951-8965-54-4| language=fi |pages=23–28 }}</ref><ref>{{cite book |title=Japanese Erotic Fantasies: Sexual Imagery of the Edo Period |last1=Uhlenbeck |first1=Chris |last2=Winkel |first2=Margarita |last3=Tinios |first3=Ellis |last4=Newland |first4=Amy Reigle |year=2005 |publisher=Hotei |isbn=978-90-74822-66-4 |page=161}}</ref> The print is a forerunner of [[tentacle erotica]].<ref name=Briel>{{cite book |last=Briel |first=Holger |date=2010 |chapter=The Roving Eye Meets Traveling Pictures: The Field of Vision and the Global Rise of Adult Manga |editor1-last=Berninger |editor1-first=Mark |editor2-last=Ecke |editor2-first=Jochen |editor3-last=Haberkorn |editor3-first=Gideon |title=Comics As a Nexus of Cultures: Essays on the Interplay of Media, Disciplines |url={{google books |plainurl=y |id=e-aWWTZeergC|page=203}}| publisher=McFarland |isbn=978-0-7864-3987-4 |page=203}}</ref> The biologist [[P. Z. Myers]] noted in his science blog, ''[[Pharyngula (blog)|Pharyngula]]'', that octopuses appear in "extraordinary" graphic illustrations involving women, tentacles, and bare breasts.<ref>{{cite web |last=Myers |first=Paul Zachary |author-link1=PZ Myers |title=Extraordinary Octopus Illustrations |url=http://scienceblogs.com/pharyngula/2011/05/17/extraordinary-octopus-illustra/ |website=[[Pharyngula]] |access-date=18 March 2017 |date=17 May 2017}}</ref><ref>{{cite web |last=Myers |first=Paul Zachary |author-link=PZ Myers |title=Definitely not safe for work |url=http://scienceblogs.com/pharyngula/2006/10/29/definitely-not-safe-for-work/ |website=[[Pharyngula]] |access-date=18 March 2017 |date=29 October 2006}}</ref>

Since it has numerous arms emanating from a common centre, the octopus is often used as a symbol for a powerful and manipulative organisation, company, or country.<ref>{{cite web |last=Smith |first=S. |url=http://www.imediaethics.org/why-mark-zuckerberg-octopus-cartoon-evokes-nazi-propaganda-german-paper-apologizes/ |title=Why Mark Zuckerberg Octopus Cartoon Evokes 'Nazi Propaganda,' German Paper Apologizes |publisher=iMediaEthics |date=26 February 2010 |access-date=31 May 2017}}</ref>

The Beatles song "[[Octopus's Garden]]", on the band's 1969 album ''[[Abbey Road]],'' was written by [[Ringo Starr]] after he was told about how octopuses travel along the sea bed picking up stones and shiny objects with which to build gardens.<ref>{{Cite web |title=Beatles Songwriting & Recording Database: Abbey Road |url=http://www.beatlesinterviews.org/dba11road.html |access-date=2024-04-02 |website=www.beatlesinterviews.org}}</ref>

===Danger to humans===
[[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 [[cephalopod attack|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 name="Fry Roelants 2009">{{cite journal |last1=Fry |first1=B. G. |last2=Roelants |first2=K. |last3=Norman |first3=J. A. |title=Tentacles of Venom: Toxic Protein Convergence in the Kingdom Animalia |journal=Journal of Molecular Evolution |volume=68 |issue=4 |year=2009 |pages=311–321 |issn=0022-2844 |doi=10.1007/s00239-009-9223-8|pmid=19294452 |bibcode=2009JMolE..68..311F |s2cid=13354905 }}</ref> Blue-ringed octopuses are among the deadliest animals in the sea, 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 stepped upon; 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}}</ref> Bites have been recorded from captive octopuses of other species; they leave swellings which disappear in a day or two.{{sfnp|Wells|1978|pp=68}}

===As a food source===
{{Main|Octopus as food}}

Octopus [[fisheries]] exist around the world with total catches varying between 245,320 and 322,999 metric tons from 1986 to 1995.<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=http://www.dfo-mpo.gc.ca/CSAS/Csas/DocREC/1998/98_087_e.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|traps]], [[trawling|trawls]], snares, drift fishing, spearing, hooking and hand collection.<ref name=gillespie/> Octopuses have a food conversion efficiency greater than that of chickens, making [[octopus aquaculture]] a possibility.<ref>{{cite news |last=Wells |first=Martin |year=1983 |title=Cephalopods do it differently |volume=100 |pages=333–334 |work=New Scientist |number=1382 |url={{google books |plainurl=y |id=qmQ8V0Htqu0C|page=333}} |issn=0262-4079}}</ref> Octopuses compete with human fisheries targeting other species, and even rob traps and nets for their catch; they may, themselves, be caught as [[bycatch]] if they cannot get away.<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 ''[[ikizukuri]]'' in Japanese cuisine and ''[[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 still 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>

===In science and technology===

In classical Greece, [[Aristotle]] (384–322 BC) [[Aristotle's biology|commented on]] the colour-changing abilities of the octopus, 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>[[Aristotle]] (c. 350 BC). ''[[Historia animalium]]''. IX, 622a: 2–10. Cited in Borrelli, Luciana; [[Francesca Gherardi|Gherardi, Francesca]]; Fiorito, Graziano (2006). ''A catalogue of body patterning in Cephalopoda''. Firenze University Press. {{ISBN|978-88-8453-377-7}}. [http://www.fupress.com/scheda.asp?IDV=487 Abstract] {{Webarchive|url=https://web.archive.org/web/20180206145302/http://classics.mit.edu/Aristotle/history_anim.9.ix.html |date=6 February 2018 }}</ref> Aristotle noted that the octopus had a hectocotyl arm and suggested it might be used in sexual reproduction. This claim was widely disbelieved 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 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 |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]], including their ability to regenerate limbs, change the colour of their skin, behave intelligently with a distributed nervous system, and make use of 168 kinds of [[protocadherin]]s (humans have 58), the proteins that guide the connections neurons make with each other. The California two-spot octopus has 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 have been compared by the philosopher [[Peter Godfrey-Smith]], who has 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 problem-solving skills, along with their mobility and lack of rigid structure enable them to escape from supposedly secure tanks in laboratories and [[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, octopuses are listed in some countries as [[animal testing|experimental animals]] on which surgery may not be performed without [[anesthesia]], a protection usually extended only to vertebrates. 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 is 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}}

===Bibliography===

* {{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}}
* {{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 |page=13}} |publisher=[[Timber Press]] |isbn=978-1-60469-067-5}}
* {{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}}

==Further reading==
* {{cite web |title=Studying the creativity and intelligence of the octopus |date=30 Aug 2020 |website=CBS News |url=https://www.cbsnews.com/news/studying-the-creativity-and-intelligence-of-the-octopus-8-30-2020/}}
* {{cite web |title=Untangling the Mysteries of the Octopus |date=12 Jan 2020 |website=CBS News |format=Video 7:10 |url=https://www.msn.com/en-us/news/us/untangling-the-mysteries-of-the-octopus/vi-BB18wlJr?ocid=spartan-ntp-feeds}}

==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]]

{{Commercial fish topics}}
{{Commercial molluscs}}
{{Edible molluscs}}
{{Taxonbar|from=Q40152}}
{{Authority control}}

[[Category:Octopuses| ]]
[[Category:Articles containing video clips]]
[[Category:Commercial molluscs]]
[[Category:Extant Pennsylvanian first appearances]]
[[Category:Tool-using animals]]