Editing Barnacle (section)

=== Sexual reproduction ===

[[File:Balanus reproduction.svg|thumb|upright=1.35|Pseudocopulation:<ref name="Barazandeh Davis Neufeld Coltman 2013"/> the acorn barnacle uses its long penis to reach across to transfer sperm to another individual nearby.<ref name="Callaway 2009"/>]]

Most barnacles are [[Hermaphrodite|hermaphroditic]], producing both eggs and sperms. A few species [[gonochorism|have separate sexes]], or have [[Androdioecy|both males and hermaphrodites]]. The ovaries are located in the base or stalk, and may extend into the mantle, while the testes are towards the back of the head, often extending into the thorax. Typically, recently moulted hermaphroditic individuals are receptive as females. Self-fertilization, although theoretically possible, has been experimentally shown to be rare in barnacles.<ref name="Museum Victoria">{{cite web |url=http://www.museum.vic.gov.au/crust/barnbiol.html |publisher=[[Museum Victoria]] |title=Biology of Barnacles |year=1996 |access-date=April 20, 2012 |archive-url=https://web.archive.org/web/20070217160059/http://www.museum.vic.gov.au/crust/barnbiol.html |archive-date=February 17, 2007}}</ref><ref>{{cite book |last=Charnov |first=E. L. |year=1987 |title=Sexuality and hermaphroditism in barnacles: A natural selection approach |pages=89–104}} In [[#refSouthward|A. J. Southward (ed.), 1987]].</ref>

The sessile lifestyle of acorn barnacles makes [[sexual reproduction]] difficult, as they cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have developed extraordinarily long [[penis]]es⁠. Barnacles possess the largest penis-to-body size ratio of any known animal,<ref name="Museum Victoria"/> up to eight times their body length, though on exposed coasts the penis is shorter and thicker.<ref name="Callaway 2009">{{cite magazine |last=Callaway |first=Ewen |title=Penis length isn't everything … for barnacle males |date=17 April 2009 |url=https://www.newscientist.com/article/dn16971-penis-length-isnt-everything-for-barnacle-males/ |magazine=[[New Scientist]] |access-date=19 February 2024}}</ref> The mating of acorn barnacles is described as pseudocopulation.<ref name="Barazandeh Davis Neufeld Coltman 2013"/><ref name="Bishop Pemberton 2005">{{Cite journal |last1=Bishop |first1=J. D. D. |last2=Pemberton |first2=A. J. |date=2005 |title=The third way: spermcast mating in sessile marine invertebrates |journal=[[Integrative and Comparative Biology]] |volume=46 |issue=4 |pages=398–406 |doi=10.1093/icb/icj037 |pmid=21672752 |doi-access=free}}</ref>

The goose barnacle ''[[Pollicipes polymerus]]'' can alternatively reproduce by spermcasting, in which the male barnacle releases his sperm into the water, to be taken up by females. Isolated individuals always made use of spermcasting and sperm capture, as did a quarter of individuals with a close neighbour. This 2013 discovery overturned the long-held belief that barnacles were limited to pseudocopulation or hermaphroditism.<ref name="Barazandeh Davis Neufeld Coltman 2013">{{cite journal |last1=Barazandeh |first1=Marjan |last2=Davis |first2=Corey S. |last3=Neufeld |first3=Christopher J. |last4=Coltman |first4=David W. |last5=Palmer |first5=A. Richard |title=Something Darwin didn't know about barnacles: spermcast mating in a common stalked species |journal=Proceedings of the Royal Society B: Biological Sciences |volume=280 |issue=1754 |date=2013-03-07 |pmid=23325777 |pmc=3574338 |doi=10.1098/rspb.2012.2919 |page=20122919}}</ref><!--<ref name="Yong 2013">{{cite web |last=Yong |first=Ed |title=Poorly-Endowed Barnacles Overthrow 150-Year-Old Belief |website=[[National Geographic]] |date=15 January 2013 |url=https://www.nationalgeographic.com/science/article/poorly-endowed-barnacles-spermcasting |access-date=19 February 2024}}</ref>-->

Rhizocephalan barnacles had been considered hermaphroditic, but their males inject themselves into females' bodies, degrading to little more than sperm-producing cells.<ref name="Dale 2013">{{cite book |last=Dale |first=Brian |title=Mechanism of Fertilization: Plants to Humans |publisher=Springer |year=2013 |isbn=978-3-642-83965-8 |page=[https://books.google.com/books?id=fVjxCAAAQBAJ&pg=PA702 702]}}</ref>