Editing Squid (section)

==Description==
[[File:Composite diagram illustrating basic squid features, ventral aspect.jpg|thumb|upright=1.35|Basic squid features (ventral aspect)]]

Squid are soft-bodied molluscs whose forms evolved to adopt an active predatory lifestyle. The head and foot of the squid are at one end of a long body, and this end is functionally [[anterior]], leading the animal as it moves through the water. A set of eight arms and two distinctive tentacles surround the mouth; each appendage takes the form of a [[muscular hydrostat]] and is flexible and prehensile, usually bearing disc-like suckers.<ref name=Ruppert/>

The suckers may lie directly on the arm or be stalked. Their rims are stiffened with chitin and may contain minute toothlike denticles. These features, as well as strong musculature, and a small ganglion beneath each sucker to allow individual control, provide a very powerful adhesion to grip prey. Hooks are present on the arms and tentacles in some species, but their function is unclear.<ref name=Hanlon/> The two tentacles are much longer than the arms and are retractile. Suckers are limited to the spatulate tip of the tentacle, known as the [[Manus (anatomy)|manus]].<ref name=Ruppert/>

In the mature male, the outer half of one of the left arms is [[hectocotylus|hectocotylised]]''&nbsp;''– and ends in a copulatory pad rather than suckers. This is used for depositing a [[spermatophore]] inside the mantle cavity of a female. A ventral part of the foot has been converted into a [[siphon (mollusc)|funnel]] through which water exits the mantle cavity.<ref name=Ruppert/>

The main body mass is enclosed in the mantle, which has a [[Cephalopod fin|swimming fin]] along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and internal. The visceral mass, which is covered by a thin, membranous [[Squamous epithelium|epidermis]], forms a cone-shaped posterior region known as the "visceral hump". The mollusc shell is reduced to an internal, longitudinal chitinous "pen" in the functionally dorsal part of the animal; the pen acts to stiffen the squid and provides attachments for muscles.<ref name=Ruppert/>

On the functionally ventral part of the body is an opening to the mantle cavity, which contains the [[gill]]s (ctenidia) and openings from the excretory, digestive and [[reproductive system]]s. An inhalant siphon behind the funnel draws water into the mantle cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion.<ref name=Johnson1972>{{Cite journal | last1=Johnson | first1=W. | last2=Soden | first2=P. D. | last3=Trueman | first3=E. R. | year=1972 | title=A Study in Jet Propulsion: An analysis of the motion of the squid, ''Loligo vulgaris'' | journal=Journal of Experimental Biology | volume=56 | issue=1 | pages=155–165| doi=10.1242/jeb.56.1.155 | bibcode=1972JExpB..56..155J }}</ref> In this form of locomotion, water is sucked into the mantle cavity and expelled out of the funnel in a fast, strong jet. The direction of travel is varied by the orientation of the funnel.<ref name=Ruppert/> Squid are strong swimmers and certain species can "fly" for short distances out of the water.<ref>{{cite magazine |last=Jabr |first=F. |date=2 August 2010 |url=https://www.scientificamerican.com/article/can-squid-fly/ |title=Fact or Fiction: Can a Squid Fly Out of the Water? |magazine=[[Scientific American]]}}</ref>

===Camouflage===
Squid make use of different kinds of camouflage, namely [[active camouflage]] for background matching (in shallow water) and counter-illumination. This helps to [[Anti-predator adaptation|protect them from their predators]] and allows them to approach their prey.{{sfn|Cott|1940|p=32}}<ref name="Young Roper 1976"/>

The skin is covered in controllable [[chromatophore]]s of different colours, enabling the squid to match its coloration to its surroundings.{{sfn|Cott|1940|p=32}}<ref>{{cite journal |author1=Gilmore, R. |author2=Crook, R. |author3=Krans, J. L. |year=2016 |title=Cephalopod Camouflage: Cells and Organs of the Skin |journal=Nature Education |volume=9 |issue=2 |page=1 |url=https://www.nature.com/scitable/topicpage/cephalopod-camouflage-cells-and-organs-of-the-144048968}}</ref> The play of colours may in addition distract prey from the squid's approaching tentacles.{{sfn|Cott|1940|p=383}} The skin also contains light reflectors called [[iridophore]]s and [[leucophore]]s that, when activated, in [[millisecond]]s create changeable skin patterns of polarized light.<ref name="Mäthger reflections">{{cite journal | last1=Mäthger | first1=L. M. | last2=Shashar | first2=N. | last3=Hanlon | first3=R. T. | title=Do cephalopods communicate using polarized light reflections from their skin? | journal=Journal of Experimental Biology | volume=212 | issue=14 | date=2009 | doi=10.1242/jeb.020800 | pmid=19561202 | pages=2133–2140| doi-access=free | bibcode=2009JExpB.212.2133M }}</ref><ref name="Mäthger mech">{{cite journal | last1=Mäthger | first1=Lydia M | last2=Denton | first2=Eric J | last3=Marshall | first3=N. Justin | last4=Hanlon | first4=Roger T | title=Mechanisms and behavioural functions of structural coloration in cephalopods | journal=Journal of the Royal Society Interface | volume=6 | issue=suppl_2 | pages=S149–63 | date=2009 | doi=10.1098/rsif.2008.0366.focus| pmid=19091688 | pmc=2706477 }}</ref> Such skin camouflage may serve various functions, such as communication with nearby squid, prey detection, navigation, and orientation during hunting or seeking shelter.<ref name="Mäthger reflections"/> Neural control of the iridophores enabling rapid changes in skin [[iridescence]] appears to be regulated by a [[cholinergic]] process affecting [[reflectin]] [[protein]]s.<ref name="Mäthger mech"/>

Some [[mesopelagic]] squid such as the [[firefly squid]] (''Watasenia scintillans'') and the [[midwater squid]] (''Abralia veranyi'') use counter-illumination camouflage, generating light to match the downwelling light from the ocean surface.<ref name="Young Roper 1976">{{cite journal | last1=Young | first1=R. | last2=Roper | first2=C. | title=Bioluminescent countershading in midwater animals: evidence from living squid | journal=Science | volume=191 | issue=4231 | date=March 1976 | doi=10.1126/science.1251214 | pmid=1251214 | pages=1046–1048| bibcode=1976Sci...191.1046Y }}</ref><ref name="Jones 2004">{{cite journal |author1=Jones, B. W. |author2=Nishiguchi, M. K. |s2cid=86576334 |year=2004 |title=Counterillumination in the Hawaiian bobtail squid, ''Euprymna scolopes'' Berry (Mollusca: Cephalopoda) |journal=[[Marine Biology (journal)|Marine Biology]] |volume=144 |issue=6 |pages=1151–1155 |doi=10.1007/s00227-003-1285-3 |bibcode=2004MarBi.144.1151J |url=http://www.medmicro.wisc.edu/labs/mcfall_ruby_papers/pdf/2004/Jones_Nishiguchi_2004_Biol.pdf |url-status=live |archive-url=https://web.archive.org/web/20100611082606/http://www.medmicro.wisc.edu/labs/mcfall_ruby_papers/pdf/2004/Jones_Nishiguchi_2004_Biol.pdf |archive-date=11 June 2010}}</ref><ref>{{cite journal |url=http://www.ingentaconnect.com/content/umrsmas/bullmar/1983/00000033/00000004/art00003 |title=Oceanic Bioluminescence: an Overview of General Functions |author=Young, Richard Edward |journal=Bulletin of Marine Science |year=1983 |volume=33 |issue=4 |pages=829–845}}</ref> This creates the effect of [[countershading]], making the underside lighter than the upperside.<ref name="Young Roper 1976"/>

Counter-illumination is also used by the [[Euprymna scolopes|Hawaiian bobtail squid]] (''Euprymna scolopes''), which has [[Aliivibrio fischeri|symbiotic bacteria]] (''Aliivibrio fischeri'') that produce light to help the squid avoid nocturnal predators.<ref name="Nyholm2004">{{cite journal | last1=Nyholm | first1=S. V. | last2=McFall-Ngai | first2=M. J. | s2cid=21583331 | title=The winnowing: establishing the squid-''Vibrio'' symbiosis | journal=Nature Reviews Microbiology | volume=2 | issue=8 | date=August 2004 | doi=10.1038/nrmicro957 | pmid=15263898 | pages=632–642}}</ref> This light shines through the squid's skin on its underside and is generated by a large and complex two-lobed light organ inside the squid's mantle cavity. From there, it escapes downwards, some of it travelling directly, some coming off a reflector at the top of the organ (dorsal side). Below there is a kind of [[iris (eye)|iris]], which has branches (diverticula) of its [[ink sac]], with a lens below that; both the reflector and lens are derived from [[mesoderm]]. The squid controls light production by changing the shape of its iris or adjusting the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted.<ref name="Jones 2004"/> Light production shows a correlation with intensity of down-welling light, but it is about one third as bright; the squid can track repeated changes in brightness. Because the Hawaiian bobtail squid hides in sand during the day to avoid predators, it does not use counter-illumination during daylight hours.<ref name="Jones 2004"/>

<gallery mode="packed">
File:Chromatophores.jpg|upright=1.25|Controllable [[chromatophore]]s of different colours in the skin of a squid allow it to change its coloration<!--this is a common British usage, e.g.{{sfn|Cott|1940}} book title--> and patterns rapidly, whether for camouflage or signalling.
File:Squid Counterillumination.png|Principle of [[counter-illumination]] camouflage of the firefly squid, ''[[Watasenia scintillans]]''. When seen from below by a [[predator]], the animal's light helps to match its brightness and colour to the sea surface above.
</gallery>

===Predator distraction with ink===
[[File:Loligosepia aalensis 01.jpg|thumb|Fossil ''Loligosepia aalensis'' from the lower Jurassic; the [[ink sac]] is still full of black eu[[melanin]] pigment.]]

Squid distract attacking predators by ejecting a cloud of [[Cephalopod ink|ink]], giving themselves an opportunity to escape.{{sfn|Cott|1940|p=381}}<ref name=Derby>{{cite journal |last1=Derby |first1=Charles D. |title=Escape by Inking and Secreting: Marine Molluscs Avoid Predators Through a Rich Array of Chemicals and Mechanisms |journal=The Biological Bulletin |date=December 2007 |volume=213 |issue=3 |pages=274–289 |doi=10.2307/25066645 |pmid=18083967 |jstor=25066645|s2cid=9539618 }}</ref> The ink gland and its associated ink sac empties into the rectum close to the anus, allowing the squid to rapidly discharge black ink into the mantle cavity and surrounding water.<ref name=Hanlon>{{cite book|author1=Hanlon, Roger T. |author2=Messenger, John B. |title=Cephalopod Behaviour|url=https://books.google.com/books?id=Nxfv6xZZ6WYC&pg=PA25 |year=1998 |publisher=Cambridge University Press|isbn=978-0-521-64583-6 |pages=25–26}}</ref> The ink is a suspension of [[melanin]] particles and quickly disperses to form a dark cloud that obscures the escape manoeuvres of the squid. Predatory fish may also be deterred by the alkaloid nature of the discharge which may interfere with their [[chemoreceptor]]s.<ref name=Ruppert/>

===Nervous system and sense organs===
{{see|Cephalopod intelligence}}

Cephalopods have the most highly developed nervous systems among [[invertebrate]]s. Squids have a complex brain in the form of a nerve ring encircling the [[oesophagus]], enclosed in a cartilaginous [[cranium]]. Paired cerebral [[ganglia]] above the oesophagus receive sensory information from the eyes and [[statocyst]]s, and further ganglia below control the muscles of the mouth, foot, mantle and viscera. [[Squid giant axon|Giant axons]] up to {{cvt|1|mm|in|2}} in diameter convey nerve messages with great rapidity to the circular muscles of the mantle wall, allowing a synchronous, powerful contraction and maximum speed in the jet propulsion system.<ref name=Ruppert/>

The paired eyes, on either side of the head, are housed in capsules fused to the cranium. Their structure is very similar to that of a fish eye, with a globular [[Lens (anatomy)|lens]] that has a [[depth of focus]] from {{convert|3|cm|in|1|abbr=on}} to infinity. The image is focused by changing the position of the lens, as in a camera or [[telescope]], rather than changing the shape of the lens, as in the [[human eye]]. Squid adjust to changes in light intensity by expanding and contracting the slit-shaped [[pupil]].<ref name=Ruppert/> Deep sea squids in the family [[Histioteuthidae]] have eyes of two different types and orientation. The large left eye is tubular in shape and looks upwards, presumably searching for the silhouettes of animals higher in the [[water column]]. The normally-shaped right eye points forwards and downwards to detect prey.<ref>{{cite web | author1 = Young, Richard E. | author1-link = Richard E. Young | author2 = Vecchione, Michael | author2-link = Michael Vecchione | year = 2013 | title = ''Histioteuthidae'' Verrill, 1881. | url = http://tolweb.org/Histioteuthidae/19782/2013.11.03 | publisher = The [[Tree of Life Web Project]] |access-date=9 December 2018}}</ref>

The statocysts are involved in maintaining balance and are analogous to the inner ear of fish. They are housed in cartilaginous capsules on either side of the cranium. They provide the squid with information on its body position in relation to gravity, its orientation, acceleration and rotation, and are able to perceive incoming vibrations. Without the statocysts, the squid cannot maintain equilibrium.<ref name=Ruppert/> Squid appear to have limited hearing,<ref>{{cite news|url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8095000/8095977.stm|title=The cephalopods can hear you |publisher=BBC|date=15 June 2009|first=Matt|last=Walker|access-date=2 April 2010}}</ref> but the head and arms bear lines of hair-cells that are weakly sensitive to water movements and changes in pressure, and are analogous in function to the [[lateral line]] system of fish.<ref name=Ruppert/>

===Reproductive system===
[[File:Onykia ingens with erect penis.jpg|thumb|left|Male ''[[Onykia ingens]]'' with penis erected to {{convert|67|cm|in|0|abbr=on}}]]

The sexes are separate in squid, with a single [[gonad]] in the posterior part of the body. Fertilisation is external and usually takes place in the mantle cavity of the female. The male has a [[testis]] from which sperm pass into a single gonoduct where they are rolled together into a long bundle, or spermatophore. The gonoduct is elongated into a "penis" that extends into the mantle cavity and through which spermatophores are ejected. In shallow water species, the penis is short, and the spermatophore is removed from the mantle cavity by a tentacle of the male, which is specially adapted for the purpose and known as a [[hectocotylus]], and placed inside the mantle cavity of the female during mating.<ref name=Ruppert/>

[[File:Uroteuthis duvauceli hectocotylus.jpg|thumb|upright=1.2|[[Hectocotylus]] of ''[[Uroteuthis duvauceli]]'': one tentacle of the male is adapted for transferring the [[spermatophore]].]]

The female has a large translucent [[ovary]], situated towards the posterior of the visceral mass. From here, eggs travel along the gonocoel, where there are a pair of white [[nidamental gland]]s, which lie anterior to the gills. Also present are red-spotted accessory nidamental glands containing [[Symbiosis|symbiotic]] bacteria; both organs are associated with nutrient manufacture and forming shells for the eggs. The gonocoel enters the mantle cavity at the [[gonopore]], and in some species, receptacles for storing spermatophores are located nearby, in the mantle wall.<ref name=Ruppert/>

In shallow-water species of the [[continental shelf]] and [[epipelagic]] or [[mesopelagic zone]]s, it is frequently one or both of arm pair IV of males that are modified into hectocotyli.<ref>{{cite web |url=http://tolweb.org/accessory/Cephalopoda_Glossary?acc_id=587 |title=Hectocotylus |author1=Young, R. E. |author2=Vecchione, M. |author3=Mangold, K. M.|work=Cephalopoda Glossary|publisher=Tree of Life Web Project |access-date=14 December 2018}}</ref> However, most deep-sea squid lack hectocotyl arms and have longer penises; Ancistrocheiridae and Cranchiinae are exceptions.<ref name="Arkhipkin 2010"/> [[Giant squid]] of the genus ''Architeuthis'' are unusual in that they possess both a large penis and modified arm tips, although whether the latter are used for spermatophore transfer is uncertain.<ref name="Arkhipkin 2010"/> Penis elongation has been observed in the deep-water species ''[[Onykia ingens]]''; when erect, the penis may be as long as the mantle, head, and arms combined.<ref name="Arkhipkin 2010" /><ref>{{cite news |last=Walker |first=M. |date=7 July 2010 |url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8792000/8792008.stm |title=Super squid sex organ discovered |work=BBC Earth News}}</ref> As such, deep-water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile [[barnacle]]s.<ref name="Arkhipkin 2010">{{cite journal |last1=Arkhipkin |first1=A. I. |first2=V. V. |last2=Laptikhovsky |year=2010 |title=Observation of penis elongation in ''Onykia ingens'': implications for spermatophore transfer in deep-water squid |journal=Journal of Molluscan Studies |doi=10.1093/mollus/eyq019 |volume=76 |issue=3 |pages=299–300|doi-access=free }}</ref>

===Digestive system===
[[File:Chtenopteryx sicula2.jpg|thumb|left|upright=1.1<!--format for low image-->|Ventral view of the viscera of a female ''[[Chtenopteryx sicula]]''|alt=Diagram labeling siphon, intestine, nidamental gland, accessory nidamental gland, renal pore, and branchial heart]]

Like all cephalopods, squids are predators and have complex digestive systems. The mouth is equipped with a sharp, horny [[Cephalopod beak|beak]] mainly made of chitin and [[cross-link]]ed proteins,<ref name="Tan Hoon Guerette Wei pp. 488–495">{{cite journal | last1=Tan | first1=YerPeng | last2=Hoon | first2=Shawn | last3=Guerette | first3=Paul A. | last4=Wei | first4=Wei | last5=Ghadban | first5=Ali | last6=Hao | first6=Cai | last7=Miserez | first7=Ali | last8=Waite | first8=J. Herbert | title=Infiltration of chitin by protein coacervates defines the squid beak mechanical gradient | journal=Nature Chemical Biology | volume=11 | issue=7 | year=2015  | doi=10.1038/nchembio.1833 | pmid=26053298 | pages=488–495 | s2cid=205303026 |quote=the beak contains two protein families. One family consists of chitin-binding proteins (DgCBPs) that physically join chitin chains, whereas the other family comprises highly modular histidine-rich proteins (DgHBPs).| url=http://www.escholarship.org/uc/item/3f21p9m3 }}</ref> which is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain minerals, unlike the teeth and jaws of many other organisms; the cross-linked proteins are histidine- and glycine-rich and give the beak a stiffness and hardness greater than most equivalent synthetic organic materials.<ref>{{cite journal |last1=Miserez |first1=A. |last2=Li |first2=Y. |last3=Waite |first3=H. |last4=Zok |first4=F. |year=2007 |title=Jumbo squid beaks: Inspiration for design of robust organic composites |journal=[[Acta Biomaterialia]] |volume=3 |issue=1 |pages=139–149 |doi=10.1016/j.actbio.2006.09.004 |pmid=17113369}}</ref> The stomachs of captured whales often have indigestible squid beaks inside. The mouth contains the [[radula]], the rough tongue common to all [[Mollusca|molluscs]] except [[bivalvia]], which is equipped with multiple rows of teeth.<ref name=Ruppert/> In some species, toxic [[saliva]] helps to control large prey; when subdued, the food can be torn in pieces by the beak, moved to the oesophagus by the radula, and swallowed.<ref>{{cite book |author1=Hanlon, Roger T. |author2=Messenger, John B. |title=Cephalopod Behaviour |url=https://books.google.com/books?id=Nxfv6xZZ6WYC&pg=PA48 |year=1998 |publisher=Cambridge University Press |isbn=978-0-521-64583-6 |page=48}}</ref>

The food [[bolus (digestion)|bolus]] is moved along the gut by waves of muscular contractions ([[peristalsis]]). The long oesophagus leads to a muscular [[stomach]] roughly in the middle of the visceral mass. The [[Hepatopancreas|digestive gland]], which is equivalent to a vertebrate liver, diverticulates here, as does the [[pancreas]], and both of these empty into the [[Cecum|caecum]], a pouch-shaped sac where most of the absorption of nutrients takes place.<ref name=Ruppert/> Indigestible food can be passed directly from the stomach to the [[rectum]] where it joins the flow from the caecum and is voided through the [[anus]] into the mantle cavity.<ref name=Ruppert/> Cephalopods are short-lived, and in mature squid, priority is given to reproduction;<ref>{{cite news |title=Octopuses and the Puzzle of Aging |author=Godfrey-Smith, Peter |author-link=Peter Godfrey-Smith |url=https://www.nytimes.com/2016/12/02/opinion/sunday/octopuses-and-the-puzzle-of-aging.html |newspaper=[[The New York Times]] |date=2 December 2016 |access-date=12 December 2018}}</ref> the female ''[[Onychoteuthis banksii]]'' for example, sheds its feeding tentacles on reaching maturity, and becomes flaccid and weak after spawning.<ref>{{cite iucn |author=Barratt, I. |author2=Allcock, L. |year=2014 |title=''Onychoteuthis banksii'' |volume=2014 |page=e.T163375A1003448 |doi=10.2305/IUCN.UK.2014-1.RLTS.T163375A1003448.en |access-date=28 April 2025}}</ref><ref>{{cite journal |author=Bolstad, K. S. |year=2008 |title=Two New Species and a Review of the Squid Genus ''Onychoteuthis'' Lichtenstein, 1818 (Oegopsida: Onychoteuthidae) from the Pacific Ocean |journal=Bulletin of Marine Science |volume=83 |issue=3 |pages=481–529 |url=https://www.ingentaconnect.com/content/umrsmas/bullmar/2008/00000083/00000003/art00005 }}</ref>

===Cardiovascular and excretory systems===
The squid mantle cavity is a seawater-filled sac containing three hearts and other organs supporting circulation, respiration, and [[excretion]].<ref name="hanlon">{{cite book |author1=Roger Hanlon |author2=Mike Vecchione |author3=Louise Allcock |title=Octopus, Squid, and Cuttlefish: A Visual, Scientific Guide to the Oceans' Most Advanced Invertebrates |date=1 October 2018 |publisher=University of Chicago Press |isbn=978-0-226-45956-1 |url=https://books.google.com/books?id=YtmUswEACAAJ&pg=PA33 |access-date=12 December 2018}}</ref> Squid have a main [[systemic circulation|systemic]] heart that pumps blood around the body as part of the general [[circulatory system]], and two [[branchial heart]]s. The systemic heart consists of three chambers, a lower [[Ventricle (heart)|ventricle]] and two upper [[Atrium (heart)|atria]], all of which can contract to propel the blood. The branchial hearts pump blood specifically to the gills for oxygenation, before returning it to the systemic heart.<ref name=hanlon/> The blood contains the [[copper]]-rich protein [[hemocyanin]], which is used for [[oxygen transport]] at low ocean temperatures and low oxygen concentrations, and makes the oxygenated blood a deep, blue color.<ref name=hanlon/> As systemic blood returns via two [[venae cavae]] to the branchial hearts, excretion of [[urine]], [[carbon dioxide]], and waste [[solute]]s occurs through outpockets (called [[nephridiopore|nephridial appendages]]) in the venae cavae walls that enable gas exchange and excretion via the mantle cavity seawater.<ref name=hanlon/>

===Buoyancy===
[[File:Cranchiidae sp.jpg|thumb|The body of glass squids ([[Cranchiidae]]) is mainly filled by a transparent [[coelom]] containing [[ammonium]] ions for buoyancy.]]

Unlike nautiloids and cuttlefish which have gas-filled chambers inside their shells which provide buoyancy, and octopuses which live near and rest on the seabed and do not require to be buoyant, many squid have a fluid-filled receptacle, equivalent to the [[swim bladder]] of a fish, in the [[coelom]] or [[connective tissue]]. This reservoir acts as a chemical buoyancy chamber, with the heavy metallic cations typical of seawater replaced by low [[Molecular mass|molecular-weight]] [[ammonium]] ions, a product of excretion. The small difference in density provides a small contribution to buoyancy per unit volume, so the mechanism requires a large buoyancy chamber to be effective. Since the chamber is filled with liquid, it has the advantage over a swim bladder of not changing significantly in volume with pressure. Glass squids in the family [[Cranchiidae]] for example, have an enormous transparent coelom containing ammonium ions and occupying about two-thirds the volume of the animal, allowing it to float at the required depth. About half of the 28 families of squid use this mechanism to solve their buoyancy issues.<ref name=Ruppert/> The family [[Bathyteuthis|Bathyteuthidae]] get their buoyancy from an oily substance found in their liver and around their mantle and head.<ref>{{Cite web|url=https://books.google.com/books?id=_9sBEAAAQBAJ&dq=Bathyteuthidae+oily+substance+liver&pg=PA67|title=Squid|first=Martin|last=Wallen|date=14 October 2020|publisher=Reaktion Books|via=Google Books}}</ref>

===Largest and smallest===
{{see also|Giant squid|Colossal squid|Cephalopod size}}
[[File:Giant Squid NASA.jpg|thumb|left|upright=1.25<!--format for low image-->|A [[giant squid]]. The bars are a metre (3 feet) apart.|alt=Photo of squid with prominent eye]]

The majority of squid are no more than {{convert|60|cm|in|abbr=on}} long, although the giant squid may reach {{convert|13|m|ft|abbr=on}}.<ref name=OShea>{{cite web |last=O'Shea |first=S. |year=2003 |url=http://www.tonmo.com/science/public/giantsquidfacts.php |title=Giant Squid and Colossal Squid Fact Sheet |publisher=The Octopus News Magazine Online}}</ref> The smallest species are probably the benthic pygmy squids ''[[Idiosepius]]'', which grow to a mantle length of {{convert|10|to|18|mm|in|1|abbr=on}}, and have short bodies and stubby arms.<ref>{{cite web |url=https://reefs.com/2017/10/06/meet-worlds-smallest-weirdest-squid-idiosepius/ |title=Meet The World's Smallest & Weirdest Squid, ''Idiosepius'' |author=Rowlett, Joe |date=6 October 2017 |publisher=Reefs.com |access-date=19 January 2019}}</ref>

In 1978, sharp, curved claws on the suction cups of squid tentacles cut up the rubber coating on the hull of the [[USS Stein (FF-1065)|USS ''Stein'']]. The size suggested the largest squid known at the time.<ref name="proceedings">{{cite journal |author=Johnson, C. Scott |title=Sea Creatures and the Problem of Equipment Damage |journal=United States Naval Institute Proceedings |issue=599 |date=August 1978 |pages=106–107}}</ref>

In 2003, a large specimen of an abundant<ref>{{cite journal |last1=Xavier |first1=J. C. |first2=P. G. |last2=Rodhouse |first3=P. N. |last3=Trathan |first4=A. G. |last4=Wood |year=1999 |url= http://www.nerc-bas.ac.uk/public/mlsd/squid-atlas/ |title=A Geographical Information System (GIS) Atlas of cephalopod distribution in the Southern Ocean |journal=Antarctic Science |volume=11 |issue=1 |pages=61–62 |doi=10.1017/S0954102099000097|bibcode=1999AntSc..11...61X |s2cid=140591721 |url-access=subscription }}</ref> but poorly understood species, ''Mesonychoteuthis hamiltoni'' (the [[colossal squid]]), was discovered. This species may grow to {{convert|10|m|ft|abbr=on}} in length, making it the largest invertebrate.<ref>{{cite web |last=Anderton | first=Jim |date=21 March 2007 |url=https://www.beehive.govt.nz/speech/colossal-squid-gifted-te-papa |title=Colossal squid gifted to Te Papa |publisher=New Zealand Government}}</ref> In February&nbsp;2007, a New Zealand fishing vessel caught the largest squid ever documented, weighing {{convert|495|kg|lb|abbr=on}} and measuring around {{convert|10|m|ft|abbr=on}} off the coast of Antarctica.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/asia-pacific/6478801.stm |title=Microwave plan for colossal squid |publisher=BBC |date=22 March 2007 |access-date=25 January 2018}}</ref> Dissection showed that the eyes, used to detect prey in the deep Southern Ocean, exceeded the size of footballs; these may be among the largest eyes ever to exist in the animal kingdom.<ref>{{cite news |title=Colossal squid's big eye revealed  |author=Black, Richard |url=http://news.bbc.co.uk/1/hi/sci/tech/7374297.stm |newspaper=BBC News |date=30 April 2008 |access-date=19 January 2019}}</ref>