View source for Squid

{{short description|Superorder of cephalopod molluscs}}
{{about|cephalopods|other uses}}
{{good article}}
{{pp-move-indef}}
{{pp-semi-indef}}
{{Use dmy dates|date=January 2019}}
{{Paraphyletic group
| fossil_range = {{Fossil range | Early Cretaceous | Recent | refs = }}
| image = Sepioteuthis sepioidea (Caribbean Reef Squid).jpg
| image_alt = Caribbean reef squid ("Sepioteuthis sepioidea")
| image_caption = [[Caribbean reef squid]] (''Sepioteuthis sepioidea'')
| display_parents = 2
| auto = yes
| parent = Decapodiformes
<!-- | taxon = Decapodiformes | authority = [[William Elford Leach|Leach]], 1817<ref name=tolweb>Young, R. E., Vecchione, M., Mangold, K. M. (2008). [http://tolweb.org/Decapodiformes/ Decapodiformes Leach, 1817. Squids, cuttlefishes and their relatives]. in The [[Tree of Life Web Project]]</ref>
| synonyms = *Decembrachiata <small>[[Ronald Winckworth|Winckworth]], 1932</small>
| synonyms_ref =  -->
|includes =
:[[Myopsida]]
:[[Oegopsida]] <small>[[Alcide d'Orbigny|d'Orbigny]], 1845</small>
:[[Bathyteuthida]]
|includes_ref = <ref>MolluscaBase eds. (2022). MolluscaBase. Decapodiformes. Accessed through: World Register of Marine Species at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=325342 on 2022-05-09</ref>
|excludes =
:[[Sepiolida]]
:[[Sepiidae]]?
:[[Spirulida]]?
}}

A '''squid''' ({{plural form}}: squid) is a [[mollusc]] with an elongated soft body, large eyes, eight [[cephalopod limb|arms]], and two [[tentacle]]s in the orders [[Myopsida]], [[Oegopsida]], and [[Bathyteuthida]] (though many other molluscs within the broader [[Neocoleoidea]] are also called ''squid'' despite not strictly fitting these criteria). Like all other [[cephalopods]], squid have a distinct head, [[Symmetry (biology)#Bilateral symmetry|bilateral symmetry]], and a [[mantle (mollusc)|mantle]]. They are mainly soft-bodied, like [[octopus]]es, but have a small internal skeleton in the form of a rod-like [[gladius (cephalopod)|gladius]] or pen, made of [[chitin]].

Squid diverged from other cephalopods during the [[Jurassic]] and occupy a similar [[Ecological niche|role]] to [[teleost]] fish as open-water [[predator]]s of similar size and behaviour. They play an important role in the open-water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by [[Aquatic locomotion#Jet propulsion|jet propulsion]], and largely locate their prey by sight. They are among the most intelligent of [[invertebrates]], with groups of [[Humboldt squid]] having been observed [[Pack hunter|hunting cooperatively]]. They are preyed on by [[shark]]s, other fish, sea birds, [[Pinniped|seals]] and [[cetacea]]ns, particularly [[sperm whale]]s.

Squid can change colour for [[camouflage]] and [[Signalling theory|signalling]]. Some species are [[bioluminescent]], using their light for [[counter-illumination]] camouflage, while many species can eject a cloud of [[cephalopod ink|ink]] to distract predators.

Squid are used for human consumption with commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific and elsewhere. They are used in cuisines around the world, often known as "[[Squid as food|calamari]]". Squid have featured in literature since classical times, especially in tales of giant squid and [[sea monster]]s.

==Taxonomy and phylogeny==
Squid are members of the class [[Cephalopod]]a, subclass [[Coleoidea]]. The squid [[Order (taxonomy)|orders]] [[Myopsida]] and [[Oegopsida]] are in the superorder [[Decapodiformes]] (from the [[Greek language|Greek]] for "ten-legged"). Two other orders of decapodiform cephalopods are also called squid, although they are [[Taxonomy (biology)|taxonomically]] distinct from squids and differ recognizably in their gross anatomical features. They are the [[bobtail squid]] of order [[Sepiolida]] and the [[ram's horn squid]] of the monotypic order [[Spirulida]]. The [[vampire squid]] (''[[Vampyroteuthis]] infernalis''), however, is more closely related to the octopus than to any squid.<ref name="Sanchez Setiamarga 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 |date=February 2018 |doi=10.7717/peerj.4331 |pmid=29456885 |pmc=5813590 |page=e4331 |doi-access=free }}</ref>

The [[cladogram]], not fully resolved, is based on Sanchez et al., 2018.<ref name="Sanchez Setiamarga 2018" /> Their [[molecular phylogeny]] used [[mitochondria]]l and [[nuclear DNA]] marker sequences; they comment that a robust [[phylogeny]] "has proven very challenging to obtain". If it is accepted that [[Sepiidae]] cuttlefish are a kind of squid, then the squids, excluding the vampire squid, form a [[clade]] as illustrated.<ref name="Sanchez Setiamarga 2018"/> Orders are shown in boldface; all the families not included in those orders are in the [[paraphyletic]] order "Oegopsida", except Sepiadariidae and Sepiidae that are in the polyphyletic order "Sepiida", 

{{clade
|label1=[[Cephalopoda]]
|1={{clade
   |1=''[[Nautilus]]'' [[File:Nautilus belauensis profile (white background).jpg|50px]]
   |label2=[[Coleoidea]]
   |2={{clade
      |label1=Octopuses and allies
      |1={{clade 
         |1=[[Octopoda]] [[File:Octopus_vulgaris_Merculiano.jpg|50px]]
         |2=[[Vampyroteuthidae]] (vampire squid) [[File:Vampyroteuthis_infernalis.jpg|70px]]
         }}
      |label2='''Decapodiformes'''
      |2={{clade 
         |1={{clade 
            |1=[[Cranchiinae]] (glass squid A) [[File:Cranchiidae sp (cropped).jpg|50px]]
            |2={{clade
               |1=[[Cycloteuthidae]]
               |2={{clade
                  |1=[[Psychroteuthidae]] (glacial squid) [[File:Psychroteuthis glacialis paralarva.jpg|70px]]
                  |2={{clade
                     |1={{clade
                        |1={{clade
                           |1={{clade
                              |1={{clade
                                 |1=[[Onychoteuthidae]] (hooked squid) [[File:Onychoteuthis banksii2.jpg|70px]]
                                 |2=[[Taoniinae]] (glass squid B) [[File:Sandalops melancholicus.jpg|80px]]
                                 }}
                              |2={{clade
                                 |1={{clade
                                    |1=[[Architeuthidae]] (giant squid) [[File:Architeuthis princeps image modified.PNG|80px]]
                                    |2={{clade
                                       |1=[[Lepidoteuthidae]] (Grimaldi scaled squid) [[File:Lepidoteuthis grimaldii 617 mm ML.jpg|60px]]
                                       |2=[[Octopoteuthidae]] (octopus squid) [[File:Taningia danae.gif|80px]]
                                       }}
                                    }}
                                 |2={{clade
                                    |1={{clade
                                       |1=[[Ancistrocheiridae]] (sharpear enope squid) [[File:Ancistrocheirus lesueurii.jpg|60px]]
                                       |2=[[Lycoteuthidae]] (firefly squid) [[File:Lycoteuthis lorigera male.jpg|70px]]
                                       }}
                                    |2=[[Pyroteuthidae]] (fire squid) [[File:Pyroteuthismargaritifera.jpg|60px]]
                                    }}
                                 }}
                              |3=[[Bathyteuthidae]] [[File:Bathyteuthisabyssicola (top).jpg|70px]]
                              }}
                           |2={{clade
                              |1={{clade
                                 |1={{clade
                                    |1=[[Ommastrephidae]] (flying squid) [[File:Todarodes pacificus (white background).jpg|50px]]
                                    |2={{clade
                                       |1=[[Pholidoteuthidae]] [[File:Pholidoteuthis massyae (cropped).jpg|70px]]
                                       |2=[[Gonatidae]] (armhook squid) [[File:Berryteuthis magister.jpg|50px]]
                                       }}
                                    }}
                                 |2=[[Chiroteuthidae]] (whip-lash squid{{efn|common name (whip-lash) is shared with [[Mastigoteuthidae]]}}) [[File:Chiroteuthid.png|70px]]
                                 }}
                              |2={{clade
                                 |1={{clade
                                    |1={{clade
                                       |1='''[[Sepiolida]]''' (bobtail squid) [[File:Austrorossia mastigophora (rotated).jpg|60px]]
                                       |2=[[Sepiadariidae]] (pyjama and bottletail squid) [[File:Striped pyjama squid (white background).jpg|45px]]
                                       }}
                                    |2=[[Chtenopterygidae]] [[File:Chtenopteryx sicula 1 (rotated).jpg|60px]]
                                    }}
                                 |2={{clade
                                    |1=[[Thysanoteuthidae]] [[File:Thysanoteuthis rhombus (Merculiano).jpg|55px]]
                                    |2=[[Enoploteuthidae]] [[File:Mollusques méditeranéens (Abralia veranyi).jpg|70px]]
                                    }}
                                 }}
                              }}
                           }}
                        |2={{clade
                           |1=[[Brachioteuthidae]] [[File:Brachioteuthis riisei1.jpg|70px]]
                           |2={{clade
                              |1={{clade
                                 |1=[[Neoteuthidae]]
                                 |2=[[Histioteuthidae]] (cock-eyed squid) [[File:Histioteuthidae - Histioteuthis bonnellii.JPG|60px]]
                                 }}
                              |2={{clade
                                 |1=[[Batoteuthidae]] (bush-club squid)
                                 |2={{clade
                                    |1=[[Mastigoteuthidae]] (whip-lash squid{{efn|common name (whip-lash) is shared with [[Chiroteuthidae]]}}) [[File:Mastigoteuthis agassizii1.jpg|70px]]
                                    |2={{clade
                                       |1=[[Joubiniteuthidae]] (Joubin's squid) [[File:Joubiniteuthis portieri.jpg|50px]]
                                       |2=[[Magnapinnidae]] (bigfin squid) [[File:Magnapinna talismani.jpg|50px]]
                                       }}
                                    }}
                                 }}
                              }}
                           }}
                        }}
                     |2={{clade
                        |1='''[[Spirulida]]''' (ram's horn squid) [[File:Spirula spirula illustration.jpg|30px]]
                        |2={{clade
                           |1={{clade
                              |1='''[[Myopsida]]''' (neritic squid) [[Loliginidae]] [[File:Loligo vulgaris1.jpg|70px]]
                              |2=[[Sepiidae]] (cuttlefish) [[File:Sepia officinalis1.jpg|70px]]
                              }}
                           |2=[[Idiosepiidae]] (pygmy squid) [[File:Tropical Pygmy Squid (cropped).jpg|40px]]
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

===Evolution===
[[Crown group|Crown]] coleoids (the common ancestor of octopuses and squid) diverged in the late [[Paleozoic]] ([[Mississippian (geology)|Mississippian]]), according to fossils of ''[[Syllipsimopodi]]'', an early relative of vampire squids and octopuses.<ref>{{Cite journal |last1=Whalen |first1=Christopher D. |last2=Landman |first2=Neil H. |date=2022-03-08 |title=Fossil coleoid cephalopod from the Mississippian Bear Gulch Lagerstätte sheds light on early vampyropod evolution |journal=Nature Communications |language=en |volume=13 |issue=1 |page=1107 |doi=10.1038/s41467-022-28333-5 |pmid=35260548 |pmc=8904582 |bibcode=2022NatCo..13.1107W |issn=2041-1723}}</ref> True squid diverged during the Jurassic, but many squid families appeared in or after the [[Cretaceous]].<ref name="Tanner Fuchs Winkelmann Gilbert p=20162818">{{cite journal | last1=Tanner | first1=Alastair R. | last2=Fuchs | first2=Dirk | last3=Winkelmann | first3=Inger E. | last4=Gilbert | first4=M. Thomas P. | last5=Pankey | first5=M. Sabrina | last6=Ribeiro | first6=Ângela M. | last7=Kocot | first7=Kevin M. | last8=Halanych | first8=Kenneth M. | last9=Oakley | first9=Todd H. | last10=da Fonseca | first10=Rute R. | last11=Pisani | first11=Davide | last12=Vinther | first12=Jakob | title=Molecular clocks indicate turnover and diversification of modern coleoid cephalopods during the Mesozoic Marine Revolution | journal=Proceedings of the Royal Society B: Biological Sciences | volume=284 | issue=1850 | date=March 2017 | doi=10.1098/rspb.2016.2818 | pmid=28250188 | pmc=5360930 | page=20162818}}</ref> Both the coleoids and the teleost fish were involved in much adaptive radiation at this time, and the two modern groups resemble each other in size, ecology, habitat, morphology and behaviour, however some fish moved into fresh water while the coleoids remained in marine environments.<ref name=Ruppert/>

The ancestral coleoid was probably nautiloid-like with a strait septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the [[cuttlefish]]es, the squids and the [[octopus]]es. Squid have differentiated from the ancestral [[mollusc]] such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been the [[Foot (mollusc)|foot]] of the ancestor is modified into a complex set of appendages around the mouth. The sense organs are highly developed and include advanced eyes similar to those of [[vertebrate]]s.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology |edition=7th |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=[[CEngage Learning]] |isbn=978-81-315-0104-7 |pages=343–367 |url=https://books.google.com/books?id=A3opAQAAMAAJ}}</ref>

The ancestral shell has been lost, with only an internal [[gladius (cephalopod)|gladius]], or pen, remaining. The pen, made of a chitin-like material,<ref name=Ruppert/><ref name="ifuku">{{cite journal |last=Ifuku |first=S. |title=Chitin and chitosan nanofibers: preparation and chemical modifications |pmid=25393598 |pmc=6271128 |journal=Molecules |year=2014 |volume=19 |issue=11 |pages=18367–80 |doi=10.3390/molecules191118367 |doi-access=free }}</ref> is a feather-shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment. The [[cuttlebone]] or sepion of the Sepiidae is [[calcareous]] and appears to have evolved afresh in the [[Tertiary]].<ref name="Bonnaud 2006">{{cite journal | last1=Bonnaud | first1=Laure | last2=Lu | first2=C. C. | last3=Boucher-Rodoni | first3=Renata | title=Morphological character evolution and molecular trees in sepiids (Mollusca: Cephalopoda): is the cuttlebone a robust phylogenetic marker? | journal=Biological Journal of the Linnean Society | volume=89 | issue=1 | date=2006 | doi=10.1111/j.1095-8312.2006.00664.x | pages=139–150| doi-access=free }}</ref>

<gallery mode="packed">
File:Rhomboteuthis lehmani (cropped).jpg|Fossil ''[[Rhomboteuthis]]'' from the [[Callovian|Lower Callovian]] ({{circa| 164 [[Million years ago|Mya]]}}, middle Jurassic) of [[La Voulte-sur-Rhône (lagerstätte)|La Voulte-sur-Rhône]], France
File:Plesioteuthis prisca 01.jpg|Fossil ''[[Plesioteuthis]]'' from the [[Tithonian]] ({{circa|150 Mya}}, upper Jurassic), [[Solnhofen limestone|Solnhofen]], Germany
</gallery>

==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>

==Development==
The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as it [[Direct development|develops directly]], without an intervening [[veliger]] larval stage. The embryo grows as a [[germinal disc|disc of cells]] on [[animal pole|top of the yolk]]. During the [[gastrulation]] stage, the margins of the disc grow to surround the yolk, forming a yolk sac, which eventually forms part of the animal's gut. The dorsal side of the disc grows upwards 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 upwards, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo grows. Some juvenile squid live higher in the water column than do adults. Squids tend to be short-lived; ''[[Loligo]]'' for example lives from one to three years according to species, typically dying soon after spawning.<ref name=Ruppert/>

[[File:Bobtail Squid Light Organ.svg|thumb|upright=1.2|[[Sagittal section]] of the large eye-like light-producing organ of [[Euprymna scolopes|Hawaiian bobtail squid]], ''Euprymna scolopes''. The organ houses symbiotic ''[[Aliivibrio fischeri]]'' bacteria.]]

In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized with ''[[Aliivibrio fischeri]]'' bacteria within hours of hatching. This light-organ colonization requires this particular bacterial species for a symbiotic relationship; no colonization occurs in the absence of ''A.&nbsp;fischeri''.<ref name="Nyholm2004"/> Colonization occurs in a horizontal manner, such that the hosts acquires its bacterial partners from the environment. The symbiosis is [[Symbiosis#Obligate versus facultative|obligate]] for the squid, but facultative for the bacteria. Once the bacteria enter the squid, they colonize interior [[epithelium|epithelial cells]] in the light organ, living in [[Crypt (anatomy)|crypts]] with complex [[Microvillus|microvilli]] protrusions. The bacteria also interact with [[hemocyte]]s, macrophage-like blood cells that migrate between epithelial cells, but the mechanism and function of this process is not well understood. Bioluminescence reaches its highest levels during the early evening hours and bottoms out before dawn; this occurs because at the end of each day, the contents of the squid's crypts are expelled into the surrounding environment.<ref name="McFall-Ngai 2014">{{cite journal|last=McFall-Ngai|first=M. J.|title=The importance of microbes in animal development: lessons from the squid-''Vibrio'' symbiosis|journal=Annual Review of Microbiology|date=2014|volume=68|pages=177–194|doi=10.1146/annurev-micro-091313-103654|pmid=24995875|pmc=6281398}}</ref> Approximately 95% of the bacteria are voided each morning before the bacterial population builds up again by nightfall.<ref name="Jones 2004"/>

==Behaviour==

===Locomotion===
[[File:Euprymna scolopes - image.pbio.v12.i02.g001.png|thumb|left|upright=1.2|Hawaiian bobtail squid swimming slowly by undulating its fins]]

Squid can move about in several different ways. Slow movement is achieved by a gentle undulation of the muscular lateral fins on either side of the trunk which drives the animal forward. A more common means of locomotion providing sustained movement is achieved using jetting, during which contraction of the muscular wall of the mantle cavity provides jet propulsion.<ref name=Ruppert/>

Slow jetting is used for ordinary locomotion, and ventilation of the gills is achieved at the same time. The circular muscles in the mantle wall contract; this causes the inhalant valve to close, the exhalant valve to open and the mantle edge to lock tightly around the head. Water is forced out through the funnel which is pointed in the opposite direction to the required direction of travel. The inhalant phase is initiated by the relaxation of the circular muscles causes them to stretch, the connective tissue in the mantle wall recoils elastically, the mantle cavity expands causing the inhalant valve to open, the exhalant valve to close and water to flow into the cavity. This cycle of exhalation and inhalation is repeated to provide continuous locomotion.<ref name=Ruppert/>

Fast jetting is an escape response. In this form of locomotion, radial muscles in the mantle wall are involved as well as circular ones, making it possible to hyper-inflate the mantle cavity with a larger volume of water than during slow jetting. On contraction, water flows out with great force, the funnel always being pointed anteriorly, and travel is backwards. During this means of locomotion, some squid exit the water in a similar way to [[flying fish]], gliding through the air for up to {{convert|50|m|ft|-1|abbr=on}}, and occasionally ending up on the decks of ships.<ref name=Ruppert/>

===Feeding===
Squid are [[carnivore]]s, and, with their strong arms and suckers, can overwhelm relatively large animals efficiently. Prey is identified by sight or by touch, grabbed by the tentacles which can be shot out with great rapidity, brought back to within reach of the arms, and held by the hooks and suckers on their surface.<ref name=Messenger/> In some species, the squid's saliva contains [[toxin]]s which act to subdue the prey. These are injected into its bloodstream when the prey is bitten, along with vasodilators and chemicals to stimulate the heart, and quickly circulate to all parts of its body.<ref name=Ruppert/> The deep sea squid ''[[Taningia danae]]'' has been filmed releasing blinding flashes of light from large photophores on its arms to illuminate and disorientate potential prey.<ref>{{cite journal |author1=Kubodera, T. |author2=Koyama, Y. |author3=Mori, K. |year=2006 |title=Observations of wild hunting behaviour and bioluminescence of a large deep-sea, eight-armed squid, ''Taningia danae'' |journal=Proceedings of the Royal Society B: Biological Sciences |volume=274 |issue=1613 |pages=1029–1034 |doi=10.1098/rspb.2006.0236 |pmid=17301020 |pmc=2124471 |url=http://www.pubs.royalsoc.ac.uk/media/proceedings_b/papers/RSPB20060236.pdf |access-date=13 January 2019 |archive-url=https://web.archive.org/web/20070216024900/http://www.pubs.royalsoc.ac.uk/media/proceedings_b/papers/RSPB20060236.pdf |archive-date=16 February 2007 |df=dmy-all }}</ref>

[[File:Ventouse calmar.jpg|thumb|The whip-like tentacles of ''[[Mastigoteuthis]]'' are covered with tiny suckers to catch small organisms like [[flypaper]].]]

Although squid can catch large prey, the mouth is relatively small, and the food must be cut into pieces by the chitinous beak with its powerful muscles before being swallowed. The radula is located in the buccal cavity and has multiple rows of tiny teeth that draw the food backwards and grind it in pieces.<ref name=Ruppert/> The deep sea squid ''[[Mastigoteuthis]]'' has the whole length of its whip-like tentacles covered with tiny suckers; it probably catches small organisms in the same way that [[flypaper]] traps flies. The tentacles of some [[bathypelagic]] squids bear [[photophore]]s which may bring food within its reach by attracting prey.<ref name=Messenger>{{cite book |author1=Hanlon, Roger T. |author2=Messenger, John B. |title=Cephalopod Behaviour |url=https://books.google.com/books?id=Nxfv6xZZ6WYC&pg=PA47 |year=1998 |publisher=Cambridge University Press |isbn=978-0-521-64583-6 |pages=47–49}}</ref>

Squid are among the most intelligent invertebrates. For example, groups of [[Humboldt squid]] hunt cooperatively, spiralling up through the water at night and coordinating their vertical and horizontal movements while foraging.<ref>{{cite web|url=http://deepseanews.com/2012/06/coordinated-hunting-in-red-devils/ |title=Coordinated Hunting in Red Devils |publisher=Deep Sea News|author=Smith, Helena|date= 5 June 2012 |url-status=live |archive-url=https://web.archive.org/web/20120611054534/http://deepseanews.com/2012/06/coordinated-hunting-in-red-devils/|archive-date=11 June 2012|access-date=9 December 2018}}</ref>

===Reproduction===
[[File:Squid colors 2.jpg|thumb|left|The [[Caribbean reef squid]] (''Sepioteuthis sepioidea'') employs a complex array of colour changes during courtship and social interactions.]]

Courtship in squid takes place in the open water and involves the male selecting a female, the female responding, and the transfer by the male of spermatophores to the female. In many instances, the male may display to identify himself to the female and drive off any potential competitors.<ref>{{cite journal |author=Arnold, John M. |year=1965 |title=Observations on the Mating Behavior of the Squid ''Sepioteuthis sepioidea'' |journal=Bulletin of Marine Science |volume=15 |issue=1 |pages=216–222 |url=https://www.ingentaconnect.com/content/umrsmas/bullmar/1965/00000015/00000001/art00008 }}</ref> Elaborate changes in body patterning take place in some species in both agonistic and courtship behaviour. The [[Caribbean reef squid]] (''Sepioteuthis sepioidea''), for example, employs a complex array of colour changes during courtship and social interactions and has a range of about 16 body patterns in its repertoire.<ref>{{cite book |author1=Hanlon, Roger T. |author2=Messenger, John B. |title=Cephalopod Behaviour|url=https://books.google.com/books?id=Nxfv6xZZ6WYC&pg=PA42 |year=1998 |publisher=[[Cambridge University Press]] |isbn=978-0-521-64583-6 |page=42}}</ref>

The pair adopt a head-to-head position, and "jaw locking" may take place, in a similar manner to that adopted by some [[cichlid]] fish.<ref>{{cite journal |author1=Jackson, George D. |author2=Jackson, Christine H. |year=2004 |title=Mating and spermatophore placement in the onychoteuthid squid ''Moroteuthis ingens'' |journal=Journal of the Marine Biological Association of the United Kingdom |volume=84 |issue=4 |pages=783–784 |doi=10.1017/S0025315404009932 |bibcode=2004JMBUK..84..783J |s2cid=86725399 }}</ref> The heterodactylus of the male is used to transfer the spermatophore and deposit it in the female's mantle cavity in the position appropriate for the species; this may be adjacent to the gonopore or in a seminal receptacle.<ref name=Ruppert/>

[[File:Squid Eggs (6997601241).jpg|thumb|Squid eggs<!--in the Philippines-->]]

The sperm may be used immediately or may be stored. As the eggs pass down the oviduct, they are wrapped in a gelatinous coating, before continuing to the mantle cavity, where they are fertilised. In ''[[Loligo]]'', further coatings are added by the nidimental glands in the walls of the cavity and the eggs leave through a funnel formed by the arms. The female attaches them to the substrate in strings or groups, the coating layers swelling and hardening after contact with sea water. ''Loligo'' sometimes forms breeding aggregations which may create a "community pile" of egg strings. Some pelagic and deep sea squid do not attach their egg masses, which float freely.<ref name=Ruppert/>

==Ecology==
Squid mostly have an annual life cycle, growing fast and dying soon after spawning. The diet changes as they grow but mostly consists of large [[zooplankton]] and small [[nekton]]. In Antarctica for example, [[krill]] is the main constituent of the diet, with other food items being [[Amphipoda|amphipods]], other small [[crustacean]]s, and large [[Chaetognatha|arrow worms]]. Fish are also eaten, and some squid are [[Cannibalism|cannibalistic]].<ref>{{cite book |author1=Nemoto. T. |author2=Okiyama M. |author3=Takahashi, M. |year=1985 |chapter=Aspects of the Roles of Squid in Food Chains of Marine Antarctic Ecosystems |title=Antarctic Nutrient Cycles and Food Webs |pages=415–420 |doi=10.1007/978-3-642-82275-9_58 |isbn=978-3-642-82277-3 }}</ref>

Different species of squid vary wildly in size, and even giant squid young are rice-grain-sized at hatching.<ref>{{cite book|author=Staaf, Danna |title=Squid Empire: The Rise and Fall of the Cephalopods|url=https://books.google.com/books?id=udIwDwAAQBAJ |year=2017 |publisher=University Press of New England |isbn=978-1-5126-0128-2 |page=172}}</ref> Throughout their life cycle, they can serve as a food source for many sizes of predator.<ref name="Staaf">{{cite book|author=Staaf, Danna |title=Squid Empire: The Rise and Fall of the Cephalopods|url=https://books.google.com/books?id=udIwDwAAQBAJ |year=2017 |publisher=University Press of New England |isbn=978-1-5126-0128-2 |page=2}}</ref> Juvenile squid provide part of the diet for [[Polychaete|worms]] and small fish. Larger squid are food for larger predators, including sharks, sea birds, seals and whales.

When researchers studied the contents of the stomachs of [[elephant seal]]s in South Georgia, they found 96% squid by weight.<ref name=Staaf/> In a single day, a sperm whale can eat 700 to 800 squid,<ref name="Staaf"/> and a [[Risso's dolphin]] entangled in a net in the Mediterranean was found to have eaten [[Ancistroteuthis|angel clubhook squid]], [[Histioteuthis bonnellii|umbrella squid]], [[Histioteuthis reversa|reverse jewel squid]] and [[European flying squid]], all identifiable from their indigestible beaks.<ref name=Blanco>{{cite journal |author1=Würtz, M. |author2=Poggi, R. |author3=Clarke, Malcolm R. |year=1992 |title=Cephalopods from the stomachs of a Risso's dolphin (''Grampus griseus'') from the Mediterranean |journal=Journal of the Marine Biological Association of the United Kingdom  |volume=72 |issue=4 |pages=861–867 |doi=10.1017/S0025315400060094 |bibcode=1992JMBUK..72..861W |s2cid=83587961 }}</ref> 

''[[Ornithoteuthis volatilis]]'', a common squid from the tropical Indo-Pacific, is predated by [[yellowfin tuna]], [[Alepisaurus ferox|longnose lancetfish]], [[Mahi-mahi|common dolphinfish]] and [[swordfish]], the [[tiger shark]], the [[scalloped hammerhead|scalloped hammerhead shark]] and the [[Smooth hammerhead|smooth hammerhead shark]]. Sperm whales also hunt this species extensively as does the [[brown fur seal]].<ref>{{cite book | editor1 = Jereb, P. | editor2 = Roper, C.F.E. | year = 2010 | title = Cephalopods of the World an Annotated and Illustrated Catalogue of Cephalopod Species Known to Date Volume 2 Myopsid and Oegopsid Squids | publisher = [[Food and Agriculture Organization]] Rome | isbn =  978-92-5-106720-8 | pages = 309–310 | url = http://www.fao.org/docrep/014/i1920e/i1920e.pdf}}</ref> In the [[Southern Ocean]], [[penguin]]s and [[wandering albatross]]es are major predators of ''[[Gonatus antarcticus]]''.<ref>{{cite journal | last1 = Guerreiro | first1 = Miguel | last2 = Phillips | first2 = Richard A. | last3 = Cherel | first3 = Yves | last4 = Ceia | first4 = Filipe R. | last5 = Alvito | first5 = Pedro | last6 = Rosa | first6 = Rui | last7 = Xavier | first7 = José C. | year = 2015 | title = Habitat and trophic ecology of Southern Ocean cephalopods from stable isotope analyses | journal = Marine Ecology Progress Series | volume =  530| pages =  119–134| doi = 10.3354/meps11266 | bibcode = 2015MEPS..530..119G | doi-access = free }}</ref>

==Human uses==
{{further|Molluscs in culture}}

===In literature and art===
[[File:20000 squid holding sailor.jpg|thumb|upright|Giant squid-like [[sea monster]], by [[Alphonse-Marie-Adolphe de Neuville|Alphonse de Neuville]] to illustrate [[Jules Verne]]'s ''[[Twenty Thousand Leagues Under the Seas]]'', 1870]]

Giant squid have featured as [[sea monster|monsters of the deep]] since classical times. Giant squid were [[Aristotle's biology|described by Aristotle]] (4th century BC) in his ''[[History of Animals]]''<ref>[[Aristotle]]. N.d. [http://classics.mit.edu/Aristotle/history_anim.mb.txt ''Historia animalium''].</ref> and [[Pliny the Elder]] (1st century AD) in his ''[[Natural History (Pliny)|Natural History]]''.<ref name=Ellis>{{cite book |author-link=Richard Ellis (biologist) |author=Ellis, Richard |year=1999 |title=The Search for the Giant Squid |publisher=Penguin |isbn=978-0-14-028676-2 |url=https://www.nytimes.com/books/first/e/ellis-squid.html<!--Chapter 1 'The Big Calamari' online-->}}</ref><ref>[[Pliny the Elder]]. n.d. ''[[Natural History (Pliny)|Naturalis historia]]''.</ref> The [[Gorgon]] of [[Greek mythology]] may have been inspired by squid or octopus, the animal 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. |year=2000 |publisher=Oxford University Press |isbn=978-0-19-988773-6}}</ref> The six-headed sea monster of the ''[[Odyssey]]'', [[Scylla]], may have had a similar origin. The Nordic legend of the kraken may also have derived from sightings of large cephalopods.<ref name=hogenboom>{{cite web |last1=Hogenboom |first1=Melissa |title=Are massive squid really the sea monsters of legend? |url=https://www.bbc.co.uk/earth/story/20141212-quest-for-the-real-life-kraken |publisher=BBC |access-date=27 July 2016 |date=12 December 2014}}</ref>

In literature, [[H. G. Wells]]' short story "[[The Sea Raiders]]" featured a man-eating squid species ''Haploteuthis ferox''.<ref>{{cite web |last1=Wells |first1=H. G. |title=The Sea Raiders |url=http://www.online-literature.com/wellshg/2872/ |publisher=The Literature Network |access-date=12 December 2018 |date=1896}}</ref> The [[science fiction]] writer [[Jules Verne]] told a tale of a [[kraken]]-like monster in his 1870 novel ''[[Twenty Thousand Leagues Under the Seas]]''.<ref name=hogenboom/>

===As food===
{{main|Squid as food}}
[[File:Fried calamari.jpg|thumb|left|Fried [[calamari]]: breaded, [[deep frying|deep-fried]] squid|alt=Photo of rings of breaded, fried squid]]

Squid form a major food resource and are used in cuisines around the world, notably in Japan where it is eaten as [[ika sōmen]], sliced into vermicelli-like strips; as [[sashimi]]; and as [[tempura]].<ref name="Davidson 2014">{{cite book| author=Alan Davidson | author-link=Alan Davidson (food writer) | editor=Tom Jaine | title=The Oxford Companion to Food | edition=3rd | location= Oxford | publisher= [[Oxford University Press]] | year=2014 | isbn=978-0-19-967733-7 |pages=773–774| title-link=The Oxford Companion to Food }}</ref> Three species of ''Loligo'' are used in large quantities: ''[[Loligo vulgaris|L.&nbsp;vulgaris]]'' in the Mediterranean (known as {{Lang|es|Calamar}} in Spanish, {{Lang|it|Calamaro}} in Italian); ''[[Loligo forbesii|L.&nbsp;forbesii]]'' in the Northeast Atlantic; and ''[[Loligo pealei|L.&nbsp;pealei]]'' on the American East Coast.<ref name="Davidson 2014"/> Among the Ommastrephidae, ''[[Todarodes pacificus]]'' is the main commercial species, harvested in large quantities across the North Pacific in Canada, Japan and China.<ref name="Davidson 2014"/>

In English-speaking countries, squid as food is often called ''[[calamari]]'', adopted from Italian into English in the 17th century.<ref>{{cite dictionary |title=Calamari |url=https://www.merriam-webster.com/dictionary/calamari |dictionary=[[Merriam-Webster]] |access-date=12 December 2018 |quote=Definition of calamari: squid used as food}}</ref> Squid are found abundantly in certain areas, and provide large catches for [[fishery|fisheries]]. The body can be stuffed whole, cut into flat pieces, or sliced into rings. The arms, tentacles, and ink are also edible; the only parts not eaten are the beak and gladius (pen). Squid is a good food source for [[zinc]] and [[manganese]], and high in copper,<ref>{{cite web |url=http://www.foodmarketexchange.com/datacenter/product/seafood/squid/detail/dc_pi_sf_squid_0204.htm |title=Squid – Overview: Food Market Exchange – B2B e-marketplace for the food industry |archive-url=https://web.archive.org/web/20100327225259/http://www.foodmarketexchange.com/datacenter/product/seafood/squid/detail/dc_pi_sf_squid_0204.htm |archive-date=27 March 2010 |date=August 2002}}</ref> [[selenium]], [[vitamin B12|vitamin B<sub>12</sub>]], and [[riboflavin]].<ref>{{cite web |url=http://www.fishwatch.gov/profiles/california-market-squid |website=FishWatch |title=California Market Squid |access-date=27 March 2017}}</ref>

===Commercial fishing===
According to the [[FAO]], the cephalopod catch for 2002 was {{convert|3173272|t|lb|sigfig=7}}. Of this, 2,189,206 tonnes, or 75.8 percent, was squid.<ref name="Rodhouse">{{cite journal |last=Rodhouse |first=Paul G. |year=2005 |title=Review of the state of world marine fishery resources: Fisheries technical paper |url=http://www.fao.org/docrep/009/y5852e/Y5852E08.htm#ch3.2 |journal=World Squid Resources }}</ref> The following table lists squid species fishery catches that exceeded {{convert|10000|t|lb}} in 2002.

{| class="wikitable sortable"
|+ '''World squid catch in 2002'''<ref name="Rodhouse"/>
|-
! Species
! Family
! Common name
! Catch<br />tonnes
! Percent
|-
| ''Loligo gahi'' or ''[[Doryteuthis gahi]]''
| [[Loliginidae]]
| [[Patagonian squid]]
| 24,976
| 1.1
|-
| ''Loligo pealei''
| [[Loliginidae]]
| [[Longfin inshore squid]]
| 16,684
| 0.8
|-
| Common squid nei{{efn|Nei: not elsewhere included|name=nei}}
| [[Loliginidae]]
|
| 225,958
| 10.3
|-
| ''Ommastrephes bartramii''
| [[Ommastrephidae]]
| [[Neon flying squid]]
| 22,483
| 1.0
|-
| ''[[Illex argentinus]]''
| [[Ommastrephidae]]
| Argentine shortfin squid
| 511,087
| 23.3
|-
| ''Dosidicus gigas''
| [[Ommastrephidae]]
| [[Humboldt squid]]
| 406,356
| 18.6
|-
| ''Todarodes pacificus''
| [[Ommastrephidae]]
| [[Japanese flying squid]]
| 504,438
| 23.0
|-
| ''[[Nototodarus sloanii]]''
| [[Ommastrephidae]]
| Wellington flying squid
| 62,234
| 2.8
|-
| Squid nei{{efn|name=nei}}
| Various
|
| 414,990
| 18.6
|-
| '''Total squid'''
|
|
| 2,189,206
| 100.0
|}

===In biomimicry===
[[File:Smitt hysteresis graph.svg|thumb|Schmitt trigger (B) [[Biomimicry|mimicking]] the [[squid giant axon]] removes noise from noisy analog input (U), where ordinary [[comparator]] (A) does not. Green dashed lines are thresholds.]]

Prototype chromatophores that [[Biomimicry|mimic]] the squid's adaptive camouflage have been made by [[Bristol University]] researchers using an electroactive [[dielectric]] [[elastomer]], a flexible "smart" material that changes its colour and texture in response to electrical signals. The researchers state that their goal is to create an artificial skin that provides rapid active camouflage.<ref>{{cite magazine |last1=Culpan |first1=Daniel |title=Squid-inspired 'skin' could lead to smart camouflage |url=https://www.wired.co.uk/article/artificial-squid-skin-camouflage |magazine=[[Wired UK|Wired]] |access-date=16 December 2018 |date=16 June 2015}}</ref>

The squid giant axon inspired [[Otto Schmitt]] to develop a [[comparator]] circuit with [[hysteresis]] now called the [[Schmitt trigger]], replicating the axon's propagation of [[nerve impulse]]s.<ref>{{cite web |title=Otto H. Schmitt, Como People of the Past |url=https://sites.google.com/a/comogreenvillage.info/como-history/home/people-of-the-past-documents/como-people-of-the-past/otto-h-schmitt |first=Connie |last=Sullivan |website=Como History |access-date=13 February 2019 |archive-date=7 October 2013 |archive-url=https://web.archive.org/web/20131007174931/https://sites.google.com/a/comogreenvillage.info/como-history/home/people-of-the-past-documents/como-people-of-the-past/otto-h-schmitt }}</ref>

==See also==
* [[Cephalopod]]
* [[Paralarva]]

==Notes==
{{notelist}}
{{-}}

==References==
{{reflist|30em}}

==Sources==
* {{cite book |last=Cott |first=Hugh B. |author-link=Hugh B. Cott |year=1940 |title=Adaptive Coloration in Animals |publisher=Methuen |title-link=Adaptive Coloration in Animals |oclc=222479116 }}

==External links==
{{Commons|Teuthida|Squid}}
{{cookbook}}
* [https://web.archive.org/web/20141230164932/http://www.cephbase.utmb.edu/spdb/squid.cfm CephBase: Teuthida]
* [https://www.tepapa.govt.nz/discover-collections/read-watch-play/colossal-squid Colossal Squid at the Museum of New Zealand Te Papa Tongarewa]
* [https://web.archive.org/web/20110715195736/http://diving.rogerbly.com/video/squid/ Market squid mating, laying eggs (video)]
* [https://www.scientificamerican.com/article/live-giant-squid-photogra/ Scientific American – Giant Squid]
* [http://www.thecephalopodpage.org/ The Cephalopod Page]
* [https://tonmo.com/ The Octopus News Magazine Online]

{{commercial fish topics}}
{{Edible molluscs}}
{{taxonbar|from=Q81900|from2=Q843338}}
{{Authority control}}

[[Category:Squid| ]]
[[Category:Commercial molluscs]]
[[Category:Cenozoic cephalopods]]
[[Category:Cretaceous cephalopods]]
[[Category:Extant Devonian first appearances]]
[[Category:Mollusc common names]]