Editing Squid (section)

===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"/>

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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>