Editing Shark (section)

===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.