Editing Shark (section)

===Sight===
[[File:Hexanchus nakamurai JNC2615 Eye.JPG|thumb|Eye of a [[bigeyed sixgill shark]] (''Hexanchus nakamurai'')]]

Shark [[eye]]s are similar to the eyes of other [[vertebrates]], including similar [[lens (anatomy)|lenses]], [[cornea]]s and [[retina]]s, though their eyesight is well adapted to the [[ocean|marine]] environment with the help of a tissue called [[tapetum lucidum]]. This tissue is behind the [[retina]] and reflects light back to it, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger [[nocturnal]] adaptations. Many sharks can contract and dilate their [[pupil]]s, like humans, something no [[teleost fish]] can do. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have [[nictitating membrane]]s. This membrane covers the eyes while hunting and when the shark is being attacked. However, some species, including the [[great white shark]] (''Carcharodon carcharias''), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey. The importance of sight in shark hunting behavior is debated. Some believe that [[electroreception|electro-]] and [[chemoreception]] are more significant, while others point to the nictating membrane as evidence that sight is important, since presumably the shark would not protect its eyes were they unimportant. The use of sight probably varies with species and water conditions. The shark's field of vision can swap between [[Monocular vision|monocular]] and [[Stereopsis|stereoscopic]] at any time.<ref>{{cite web
|url=http://elasmo-research.org/education/white_shark/vision.htm
|title=Vision and a Carpet of Light
|last=Martin
|first=R. Aidan
|publisher=ReefQuest Centre for Shark Research
|access-date=2009-08-22
|archive-url=https://web.archive.org/web/20090429051419/http://www.elasmo-research.org/education/white_shark/vision.htm
|archive-date=2009-04-29
|url-status=dead
}}</ref> A [[Spectrophotometry|micro-spectrophotometry]] study of 17 species of sharks found 10 had only [[Rod cell|rod photoreceptors]] and no cone cells in their [[retina]]s giving them good night vision while making them [[colorblind]]. The remaining seven species had in addition to rods a single type of [[Cone cell|cone photoreceptor]] sensitive to green and, seeing only in shades of grey and green, are believed to be effectively colorblind. The study indicates that an object's contrast against the background, rather than colour, may be more important for object detection.<ref>{{cite web|url=http://www.australiangeographic.com.au/journal/sharks-are-colour-blind-new-study-finds.htm|title=Sharks are colour-blind, new study finds|access-date=2011-02-03|url-status=dead|archive-url=https://web.archive.org/web/20110124150604/http://www.australiangeographic.com.au/journal/sharks-are-colour-blind-new-study-finds.htm|archive-date=2011-01-24}}</ref>
<ref>{{cite news
|url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_9365000/9365750.stm
|title=Sharks are probably colour-blind
|last=Gill
|first=Victoria
|access-date=2011-01-19
|work=BBC News
|date=2011-01-18
|archive-url=https://web.archive.org/web/20110119050351/http://news.bbc.co.uk/earth/hi/earth_news/newsid_9365000/9365750.stm
|archive-date=2011-01-19
|url-status=live
}}</ref><ref>{{cite journal
|title=Microspectrophotometric evidence for cone monochromacy in sharks
|author1=Nathan Scott Hart |author2=Susan Michelle Theiss |author3=Blake Kristin Harahush |author4=Shaun Patrick Collin
|doi=10.1007/s00114-010-0758-8
|pmid=21212930
|volume=98
|issue=3
|journal=Naturwissenschaften
|pages=193–201
|year=2011|bibcode=2011NW.....98..193H
|s2cid=30148811
}}</ref>