Editing Octopus (section)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

===Ink sac===

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