Editing Shark (section)

==Life history==
[[File:Wobbegong claspers.jpg|thumb |right |{{center|The claspers of male [[spotted wobbegong]]}}|alt=Photo showing claspers of bottom-resting shark.]]
[[File:Scyliorhinus canicula foetus in an egg.jpg|thumb|right|{{center|Shark egg}}]]
[[File:PortJacksonShark'sEgg20050417c.JPG|thumb|{{center|The spiral [[Egg case (Chondrichthyes)|egg case]] of a [[Port Jackson shark]]}}|alt=Photo of {{convert|12|cm|in}} egg case adjacent to ruler, the egg case is a brown ovalish shape, with a spiral band running around it from top to bottom.]]

Shark lifespans vary by species. Most live 20 to 30 years. The [[spiny dogfish]] has one of the longest lifespans at more than 100 years.<ref>{{cite web |url=http://www.mote.org/index.php?src=gendocs&link=Shark+Notes&category=Aquarium |title=Mote Marine Laboratory, "Shark Notes" |publisher=Mote.org |access-date=2012-08-27 |url-status=dead |archive-url=https://web.archive.org/web/20120124203124/http://www.mote.org/index.php?src=gendocs&link=Shark+Notes&category=Aquarium |archive-date=2012-01-24 }}</ref> [[Whale sharks]] (''Rhincodon typus'') may also live over 100 years.<ref>{{cite web|url=http://www.flmnh.ufl.edu/fish/sharks/nsrc/Basics.htm#live |title=Florida Museum of Natural History Ichthyology Department, "National Shark Research Consortium–Shark Basics" |url-status=dead |archive-url=https://web.archive.org/web/20070904074724/http://www.flmnh.ufl.edu/fish/sharks/nsrc/Basics.htm |archive-date=September 4, 2007 }}</ref> Earlier estimates suggested the [[Greenland shark]] (''Somniosus microcephalus'') could reach about 200 years, but a recent study found that a {{convert|5.02|m|ft|adj=mid|-long}} specimen was 392 ± 120 years old (i.e., at least 272 years old), making it the [[List of longest-living organisms|longest-lived]] vertebrate known.<ref>{{cite journal|last1=Nielsen|first1= J.|last2=Hedeholm|first2=R. B.|last3=Heinemeier|first3=J.|last4=Bushnell|first4=P. G.|last5=Christiansen|first5=J. S.|last6=Olsen|first6=J.|last7=Ramsey|first7=C. B.|last8=Brill|first8=R. W.|last9=Simon|first9=M.|last10=Steffensen|first10=K. F.|last11=Steffensen|first11=J. F.|date= 2016-08-12|title=Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (''Somniosus microcephalus'')|journal= [[Science (journal)|Science]]|volume= 353| issue= 6300|pages= 702–704|doi= 10.1126/science.aaf1703| pmid=27516602|bibcode= 2016Sci...353..702N|s2cid= 206647043|url= https://ora.ox.ac.uk/objects/uuid:6c040460-9519-4720-9669-9911bdd03b09|hdl=2022/26597|hdl-access=free}}</ref><ref>{{cite journal | url = http://www.sciencemag.org/news/2016/08/greenland-shark-may-live-400-years-smashing-longevity-record | title = Greenland shark may live 400 years, smashing longevity record | last = Pennisi | first = Elizabeth | date = 11 August 2016 | journal = [[Science (journal)|Science]] | doi = 10.1126/science.aag0748 | access-date = 11 August 2016 | archive-url = https://web.archive.org/web/20160812013553/http://www.sciencemag.org/news/2016/08/greenland-shark-may-live-400-years-smashing-longevity-record | archive-date = 12 August 2016 | url-status = live }}</ref>

===Reproduction===
Unlike most [[bony fish]], sharks are [[r/K selection theory|K-selected]] reproducers, meaning that they produce a small number of well-developed young as opposed to a large number of poorly developed young. [[Fecundity]] in sharks ranges from 2 to over 100 young per reproductive cycle.<ref name="FAO">{{cite book
|author= Leonard J. V. Compagno
| title=Sharks of the World: An annotated and illustrated catalogue of shark species known to date
|publisher=Food and Agriculture Organization of the United Nations
|year=1984
|isbn= 978-92-5-104543-5
|oclc= 156157504}}</ref> Sharks mature slowly relative to many other fish. For example, [[lemon shark]]s reach sexual maturity at around age 13–15.<ref>{{cite web
|url=http://www.pacfish.org/sharkcon/documents/gruber.html
|first=Samuel H.
|last=Gruber
|title=LIFE STYLE OF SHARKS
|date=February 21, 2000
|access-date=June 20, 2010
|url-status=usurped
|archive-url=https://web.archive.org/web/20110727161852/http://www.pacfish.org/sharkcon/documents/gruber.html
|archive-date=July 27, 2011
}}</ref>

====Sexual====
Sharks practice [[internal fertilization]].<ref name="Adams">{{cite journal|last1=Adams|first1=Kye R.|last2=Fetterplace|first2=Lachlan C.|last3=Davis|first3=Andrew R.|last4=Taylor|first4=Matthew D.|last5=Knott|first5=Nathan A.|date=January 2018|title=Sharks, rays and abortion: The prevalence of capture-induced parturition in elasmobranchs|journal=Biological Conservation|volume=217|pages=11–27|doi=10.1016/j.biocon.2017.10.010|bibcode=2018BCons.217...11A |s2cid=90834034 |url=http://marxiv.org/k2qvy/|access-date=2018-11-24|archive-url=https://web.archive.org/web/20190223020619/https://marxiv.org/k2qvy/|archive-date=2019-02-23|url-status=dead}}</ref> The posterior part of a male shark's pelvic fins are modified into a pair of [[intromittent organ]]s called [[clasper]]s, analogous to a [[mammalian penis]], of which one is used to deliver [[sperm]] into the female.<ref name="Mating">{{cite web
 | url =http://elasmo-research.org/education/topics/lh_2penises.htm
 | title =Why Do Sharks Have Two Penises?
 | last =Martin
 | first =R. Aidan
 | publisher =ReefQuest Centre for Shark Research
 | archive-url =https://web.archive.org/web/20090828095620/http://www.elasmo-research.org/education/topics/lh_2penises.htm
 | archive-date =2009-08-28
 | url-status =dead
 | access-date =2009-08-22}}</ref>

[[Mating]] has rarely been observed in sharks.<ref>{{Cite news|url=http://www.andersoncabotcenterforoceanlife.org/blog/how-do-sharks-mate/|title=How Do Sharks Mate? - Center For Ocean Life|work=Center For Ocean Life|access-date=2018-09-09|archive-url=https://web.archive.org/web/20180906025855/http://www.andersoncabotcenterforoceanlife.org/blog/how-do-sharks-mate|archive-date=2018-09-06|url-status=dead}}</ref> The smaller [[Scyliorhinidae|catshark]]s often mate with the male curling around the female. In less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the female's [[oviduct]]. Females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during [[mating]]. The bite marks may also come from courtship behavior: the male may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these bites.<ref name="Mating"/>

====Asexual====
There have been a number of documented cases in which a female shark who has not been in contact with a male has conceived a pup on her own through [[parthenogenesis]].<ref>{{cite journal
|author1=Chapman DD |author2=Shivji MS |author3=Louis E |author4=Sommer J |author5=Fletcher H |author6=Prodöhl PA |title=Virgin birth in a hammerhead shark
|journal=Biology Letters
|volume=3
|issue=4
|pages=425–7
|year=2007
|pmid=17519185
|pmc=2390672
|doi=10.1098/rsbl.2007.0189}}</ref><ref>''[https://www.boston.com/news/nation/articles/2008/10/10/in_shark_tank_an_asexual_birth/ In shark tank, an asexual birth] {{Webarchive|url=https://web.archive.org/web/20090709165903/http://www.boston.com/news/nation/articles/2008/10/10/in_shark_tank_an_asexual_birth/ |date=2009-07-09 }}'', Boston Globe, 10 Oct. 2008</ref> The details of this process are not well understood, but [[genetic fingerprinting]] showed that the pups had no paternal genetic contribution, ruling out [[female sperm storage|sperm storage]]. The extent of this behavior in the wild is unknown. Mammals are now the only major [[vertebrate]] group in which [[asexual reproduction]] has not been observed.

Scientists say that asexual reproduction in the wild is rare, and probably a last-ditch effort to reproduce when a mate is not present. Asexual reproduction diminishes [[genetic diversity]], which helps build defenses against threats to the species. Species that rely solely on it risk extinction. Asexual reproduction may have contributed to the [[blue shark]]'s decline off the [[Ireland|Irish]] coast.<ref>{{cite news
|url=https://www.nytimes.com/2007/05/23/science/23shark.html
|title=Female sharks reproduce without male DNA, scientists say
|newspaper=The New York Times
|access-date=2007-11-13
|first=Henry
|last=Fountain
|date=2007-05-23
|archive-url=https://web.archive.org/web/20090417112821/http://www.nytimes.com/2007/05/23/science/23shark.html
|archive-date=2009-04-17
|url-status=live
}}</ref>

===Brooding===
Sharks display three ways to bear their young, varying by species, [[oviparity]], [[Viviparous|viviparity]] and [[ovoviviparity]].<ref name="Birth">{{cite web|url=http://www.seaworld.org/animal-info/info-books/sharks-&-rays/birth-&-care.htm |title=SHARKS & RAYS, SeaWorld/Busch Gardens ANIMALS, BIRTH & CARE OF YOUNG |publisher=Busch Entertainment Corporation |access-date=2009-09-03 |url-status=dead |archive-url=https://web.archive.org/web/20130803103514/http://www.seaworld.org/animal-info/info-books/sharks-%26-rays/birth-%26-care.htm |archive-date=2013-08-03 }}</ref><ref>{{cite journal|doi=10.1016/j.biocon.2017.10.010|title=Sharks, rays and abortion: The prevalence of capture-induced parturition in elasmobranchs|journal=Biological Conservation|volume=217|pages=11–27|year=2018|last1=Adams|first1=Kye R|last2=Fetterplace|first2=Lachlan C|last3=Davis|first3=Andrew R|last4=Taylor|first4=Matthew D|last5=Knott|first5=Nathan A|bibcode=2018BCons.217...11A |s2cid=90834034 |url=http://marxiv.org/k2qvy/|access-date=2018-11-24|archive-url=https://web.archive.org/web/20190223020619/https://marxiv.org/k2qvy/|archive-date=2019-02-23|url-status=dead}}</ref>

====Ovoviviparity====
Most sharks are [[ovoviviparous]], meaning that the eggs hatch in the [[oviduct]] within the mother's body and that the egg's [[yolk]] and fluids secreted by glands in the walls of the oviduct nourishes the embryos. The young continue to be nourished by the remnants of the yolk and the oviduct's fluids. As in viviparity, the young are born alive and fully functional. [[Lamniforme]] sharks practice ''[[oophagy]]'', where the first embryos to hatch eat the remaining eggs. Taking this a step further, [[sand tiger shark]] pups cannibalistically consume neighboring embryos. The survival strategy for ovoviviparous species is to [[Broodiness|brood]] the young to a comparatively large size before birth. The [[whale shark]] is now classified as ovoviviparous rather than oviparous, because extrauterine eggs are now thought to have been aborted. Most ovoviviparous sharks give birth in sheltered areas, including bays, river mouths and shallow reefs. They choose such areas for protection from predators (mainly other sharks) and the abundance of food. [[Squalidae|Dogfish]] have the longest known [[gestation period]] of any shark, at 18 to 24 months. [[Basking shark]]s and [[frilled shark]]s appear to have even longer gestation periods, but accurate data are lacking.<ref name="Birth"/>

====Oviparity====
Some species are [[oviparous]], laying their fertilized eggs in the water. In most oviparous shark species, an [[Egg case (Chondrichthyes)|egg case]] with the consistency of [[leather]] protects the developing embryo(s). These cases may be corkscrewed into crevices for protection. The egg case is commonly called a ''[[mermaid's purse]]''. Oviparous sharks include the [[horn shark]], [[Scyliorhinidae|catshark]], [[Port Jackson shark]], and [[swellshark]].<ref name="Birth"/><ref>{{cite web
|url=http://www.lifesciences.napier.ac.uk/teaching/MB/Fish02.html
|archive-url=https://web.archive.org/web/20030823085910/http://www.lifesciences.napier.ac.uk/teaching/MB/Fish02.html
|url-status=dead
|archive-date=2003-08-23
|title=Marine Biology notes
|publisher=School of Life Sciences, [[Napier University]]
|access-date=2006-09-12
}}</ref>

====Viviparity====
Viviparity is the gestation of young without the use of a traditional egg, and results in live birth.<ref name="Carrier-2012">{{Cite book|title=Biology of Sharks and Their Relatives: Second Edition|last1=Carrier|first1=J.C|last2=Musick|first2=J.A.|last3=Heithaus|first3=M.R.|publisher=Taylor & Francis Group|year=2012}}</ref> Viviparity in sharks can be placental or aplacental.<ref name="Carrier-2012" /> Young are born fully formed and self-sufficient.<ref name="Carrier-2012" /> Hammerheads, the [[requiem sharks]] (such as the [[bull shark|bull]] and [[blue shark]]s), and [[smoothhound]]s are viviparous.<ref name="FAO" /><ref name="Birth"/>