Editing Barnacle (section)

== Life cycle ==

Barnacles pass through two distinct larval stages, the [[nauplius (larva)|nauplius]] and the cyprid, before developing into a mature adult.

=== Nauplius larva ===

A [[fertilisation|fertilised egg]] hatches into a nauplius: a one-eyed larva comprising a head and a [[telson]] with three pairs of limbs, lacking a thorax or abdomen. This undergoes six moults, passing through five [[instar]]s, before transforming into the cyprid stage. Nauplii are typically initially brooded by the parent, and released after the first moult as larvae that swim freely using [[seta]]e.<ref name="Newman">{{cite book |last=Newman |first=William A. |chapter=Cirripedia |editor1=Sol Felty Light |editor2=James T. Carlton |pages=475–484 |title=The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon |edition=4th |publisher=[[University of California Press]] |year=2007 |isbn=978-0-520-23939-5}}</ref><ref>{{cite book |title=Invertebrate Zoology |edition=7th |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=683}}</ref> All but the first instars are filter feeders.<ref name=PooreSyme>{{cite book |last1=Poore |first1=Gary C. B. |last2=Syme |first2=Anna |year=2009 |title=Barnacles |publisher=[[Museums Victoria]] |isbn=9780980381351 |page=11}}</ref>

<gallery mode="packed" heights=160>
File:Elminius modestus nauplius.jpg|Nauplius larva of ''[[Elminius modestus]]''
File:Cirripedia nauplius.png|Nauplius with fronto-lateral horns<ref>{{cite journal |last1=Pérez-Losada |first1=Marcos |last2=Høeg |first2=Jens T. |last3=Crandall |first3=Keith A. |author-link3=Keith A. Crandall |title=Remarkable convergent evolution in specialized parasitic Thecostraca (Crustacea) |journal=[[BMC Biology]] |date=17 April 2009 |volume=7 |issue=1 |pages=15 |doi=10.1186/1741-7007-7-15 |pmid=19374762 |pmc=2678073 |doi-access=free }}</ref>
</gallery>

{{anchor|Cyprid}}

=== Cypris larva ===

The cypris larva is the second and final larval stage before adulthood. In Rhizocephala and Thoracica an abdomen is absent in this stage, but the y-cyprids (post-naupliar instar) has three distinct abdominal segments.<ref>{{cite journal |last1=Hoeg |first1=Jens |last2=Pérez-Losada |first2=Marcos |last3=Glenner |first3=Henrik |last4=Kolbasov |first4=Gregory |last5=Crandall |first5=Keith |title=Evolution of Morphology, Ontogeny and Life Cycles within the Crustacea Thecostraca |journal=Arthropod Systematics & Phylogeny |date=25 August 2009 |volume=67 |issue=2 |pages=199–217 |doi=10.3897/asp.67.e31697 |doi-access=free}}</ref> It is not a feeding stage; its role is to find a suitable place to settle, since the adults are [[Sessility (zoology)|sessile]].<ref name="Newman"/> The cyprid stage lasts from days to weeks. It explores potential surfaces with modified [[Antenna (biology)#Crustaceans|antennules]]; once it has found a suitable spot, it attaches head-first using its antennules and a secreted [[Glycoprotein|glycoproteinous]] cement. Larvae assess surfaces based upon their surface texture, chemistry, relative wettability, color, and the presence or absence and composition of a surface [[biofilm]]; swarming species are more likely to attach near other barnacles.<ref name="Anderson">{{cite book |last=Anderson |first=Donald Thomas |year=1994 |title=Barnacles: Structure, Function, Development and Evolution |publisher=[[Springer Science+Business Media|Springer]] |isbn=978-0-412-44420-3 |chapter=Larval development and metamorphosis |pages=197–246 |chapter-url=https://books.google.com/books?id=2MDH9IRDkdkC&pg=PA219}}</ref> As the larva exhausts its energy reserves, it becomes less selective in the sites it selects. It cements itself permanently to the substrate with another proteinaceous compound, and then undergoes [[Metamorphosis (biology)|metamorphosis]] into a juvenile barnacle.<ref name="Anderson"/>

<gallery class="center" mode="nolines" widths=220 heights=220>
Balanus improvisus, cypris (rotated).jpg|Cypris larva of ''[[Amphibalanus improvisus]]''
File:Barnacles Cypris anatomy diagram vecorized.svg|Anatomy of cypris larva
</gallery>

=== Adult ===

Typical [[acorn barnacle]]s develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives, they are cemented to the substrate, using their feathery legs (cirri) to capture plankton. Once metamorphosis is over and they have reached their adult form, barnacles continue to grow by adding new material to their heavily calcified plates. These plates are not [[ecdysis|moulted]]; however, like all [[ecdysozoa]]ns, the barnacle moults its [[Cuticle#Invertebrate zoology|cuticle]].<ref>{{cite book |author=E. Bourget |year=1987 |title=Barnacle shells: composition, structure, and growth |pages=267–285}} In [[#refSouthward|A. J. Southward (ed.), 1987]].</ref>

=== 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>