Editing Shark (section)

==Senses==
===Smell===
[[File:Hammerhead shark.jpg|thumb|left|The shape of the [[hammerhead shark]]'s head may enhance olfaction by spacing the nostrils further apart.|alt=Eyelevel photo of hammerhead from the front]]

Sharks have keen [[olfactory]] senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one [[parts per million|part per million]] of blood in seawater.<ref>{{cite web
|url=http://elasmo-research.org/education/white_shark/smell.htm
|title=Smell and Taste
|last=Martin
|first=R. Aidan
|publisher=ReefQuest Centre for Shark Research
|access-date=2009-08-21
|archive-url=https://web.archive.org/web/20091207162031/http://www.elasmo-research.org/education/white_shark/smell.htm
|archive-date=2009-12-07
|url-status=live
}}</ref> The size of the olfactory bulb varies across different shark species, with size dependent on how much a given species relies on smell or vision to find their prey.<ref name="Yopak-2015">{{Cite journal|last1=Yopak|first1=Kara E.|last2=Lisney|first2=Thomas J.|last3=Collin|first3=Shaun P.|date=2015-03-01|title=Not all sharks are "swimming noses": variation in olfactory bulb size in cartilaginous fishes|url=https://doi.org/10.1007/s00429-014-0705-0|journal=Brain Structure and Function|language=en|volume=220|issue=2|pages=1127–1143|doi=10.1007/s00429-014-0705-0|pmid=24435575|s2cid=2829434|issn=1863-2661}}</ref> In environments with low visibility, shark species generally have larger olfactory bulbs.<ref name="Yopak-2015" /> In reefs, where visibility is high, species of sharks from the family [[Requiem shark|Carcharhinidae]] have smaller olfactory bulbs.<ref name="Yopak-2015" /> Sharks found in deeper waters also have larger olfactory bulbs.<ref>{{Cite journal|last1=Yopak|first1=Kara E.|last2=Lisney|first2=Thomas J.|last3=Darlington|first3=Richard B.|last4=Collin|first4=Shaun P.|last5=Montgomery|first5=John C.|last6=Finlay|first6=Barbara L.|date=2010-07-20|title=A conserved pattern of brain scaling from sharks to primates|journal=Proceedings of the National Academy of Sciences|language=en|volume=107|issue=29|pages=12946–12951|doi=10.1073/pnas.1002195107|issn=0027-8424|pmid=20616012|pmc=2919912|bibcode=2010PNAS..10712946Y|s2cid=2151639|doi-access=free}}</ref>

Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.<ref>[http://shell.cas.usf.edu/motta/Gardiner%20and%20Atema%202010.pdf The Function of Bilateral Odor Arrival Time Differences in Olfactory Orientation of Sharks] {{webarchive|url=https://web.archive.org/web/20120308032620/http://shell.cas.usf.edu/motta/Gardiner%20and%20Atema%202010.pdf |date=2012-03-08 }}, Jayne M. Gardiner, Jelle Atema, Current Biology - 13 July 2010 (Vol. 20, Issue 13, pp. 1187–1191)</ref> This is similar to the method mammals use to determine direction of sound.

They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in [[sewage]] outfalls. Some species, such as [[nurse shark]]s, have external [[barbels]] that greatly increase their ability to sense prey.

===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>

===Hearing===
Although it is hard to test the hearing of sharks, they may have a sharp [[Hearing (sense)|sense of hearing]] and can possibly hear prey from many miles away.<ref>{{cite web
| url = http://www.elasmo-research.org/education/white_shark/hearing.htm
| title = Hearing and Vibration Detection
| first = R. Aidan
| last = Martin
| access-date = 2008-06-01
| archive-url = https://web.archive.org/web/20080501025724/http://www.elasmo-research.org/education/white_shark/hearing.htm
| archive-date = 2008-05-01
| url-status = dead
}}</ref> The hearing sensitivity for most shark species lies between 20 and 1000&nbsp;Hz.<ref name="Casper 2006">{{cite thesis |last1=Casper |first1=B. M. |title=The hearing abilities of elasmobranch fishes |type=PhD dissertation |publisher=University of South Florida |date=2006 |page=16 |url=https://digitalcommons.usf.edu/etd/2476}}</ref>
A small opening on each side of their heads (not the spiracle) leads directly into the [[inner ear]] through a thin channel. The [[lateral line]] shows a similar arrangement, and is open to the environment via a series of openings called lateral line [[Canal pore|pores]]. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and [[tetrapod]]s the external opening into the inner ear has been lost.

===Electroreception===
{{Main|Electroreception}}
[[File:Electroreceptors in a sharks head.svg|right|300px |thumb |Electromagnetic field receptors (ampullae of Lorenzini) and motion detecting canals in the head of a shark|alt=Drawing of shark head.]]

The [[ampullae of Lorenzini]] are the electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the [[electromagnetic field]]s that all living things produce.<ref>{{cite journal
|doi=10.1126/science.7134985
|author=Kalmijn AJ
|title=Electric and magnetic field detection in elasmobranch fishes
|journal=Science
|volume=218
|issue=4575
|pages=916–8
|year=1982
|pmid=7134985|bibcode=1982Sci...218..916K
}}</ref> This helps sharks (particularly the [[hammerhead shark]]) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the [[electric field]]s they produce. [[Ocean current]]s moving in the [[Earth's magnetic field|magnetic field of the Earth]] also generate electric fields that sharks can use for orientation and possibly navigation.<ref>{{cite journal
|author1=Meyer CG |author2=Holland KN |author3=Papastamatiou YP |title=Sharks can detect changes in the geomagnetic field
|journal=Journal of the Royal Society, Interface
|volume=2
|issue=2
|pages=129–30
|year=2005
|pmid=16849172
|pmc=1578252
|doi=10.1098/rsif.2004.0021}}</ref>

===Lateral line===
{{Main|Lateral line}}

This system is found in most fish, including sharks. It is a tactile sensory system which allows the organism to detect water speed and pressure changes near by.<ref>{{Cite journal|last1=Bleckmann|first1=Horst|last2=Zelick|first2=Randy|date=March 2009|title=Lateral line system of fish|journal=Integrative Zoology|volume=4|issue=1|pages=13–25|doi=10.1111/j.1749-4877.2008.00131.x|issn=1749-4877|pmid=21392273|doi-access=free}}</ref> The main component of the system is the neuromast, a cell similar to [[hair cell]]s present in the vertebrate ear that interact with the surrounding aquatic environment. This helps sharks distinguish between the currents around them, obstacles off on their periphery, and struggling prey out of visual view. The shark can sense frequencies in the range of 25 to 50&nbsp;[[Hertz|Hz]].<ref>{{cite journal
| last = Popper
| first = A. N.
| author2 = C. Platt
| title = Inner ear and lateral line
| journal = The Physiology of Fishes
| issue = 1st ed
| year = 1993}}</ref>