Editing Barnacle (section)

== Description ==

[[File:Anim1032 - Flickr - NOAA Photo Library.jpg|thumb|[[Whale barnacle]]s on a [[humpback whale]] ]]

Most barnacles are encrusters, attaching themselves to a hard substrate such as a rock, the shell of a mollusc, or a ship; or to an animal such as a whale ([[whale barnacle]]s). The most common form, [[Sessilia|acorn barnacles]], are [[sessility (motility)|sessile]], growing their shells directly onto the substrate, whereas [[goose barnacle]]s attach themselves by means of a stalk.<ref name=Doyle1997>{{cite journal |last1=Doyle |first1=Peter |last2=Mather |first2=Anne E. |last3=Bennett |first3=Matthew R. |last4=Bussell |first4=M. Andrew |title=Miocene barnacle assemblages from southern Spain and their palaeoenvironmental significance |journal=[[Lethaia]] |volume=29 |issue=3 |date=1996 |issn=0024-1164 |doi=10.1111/j.1502-3931.1996.tb01659.x |pages=267–274|bibcode=1996Letha..29..267D }}</ref>

=== Anatomy and physiology ===

[[File:Barnacles Sessile anatomy.svg|thumb|upright=1.35<!--scale to make small text labels readable-->|Anatomy of a sessile barnacle]]

Barnacles have a carapace made of six hard calcareous plates, with a lid or operculum made of four more plates. Inside the carapace, the animal lies on its stomach, projecting its limbs downwards. Segmentation is usually indistinct; the body is more or less evenly divided between the head and [[thorax]], with little or no [[abdomen]]. Adult barnacles have few appendages on their heads, with only a single, vestigial pair of antennae attached to the cement gland. The six pairs of thoracic limbs are called [[cirrus (biology)|cirri]]; these are feathery and very long. The cirri extend to filter food, such as [[plankton]], from the water and move it towards the mouth.<ref name="OceanFacts"/>

Acorn barnacles are attached to the substratum by cement glands that form the base of the first pair of [[antenna (biology)|antennae]]; in effect, the animal is fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long, muscular stalk, but in most they are part of a flat membrane or calcified plate. These glands secrete a type of natural quick [[cement]] made of complex [[protein]] bonds (polyproteins) and other trace components like [[calcium]].<ref>{{cite journal |last1=Xu |first1=Zhenzhen |last2=Liu |first2=Zhongcheng |last3=Zhang |first3=Chao |last4=Xu |first4=Donggang |title=Advance in barnacle cement with high underwater adhesion |url=https://onlinelibrary.wiley.com/doi/full/10.1002/app.52894 |date=October 2022 |journal=[[Journal of Applied Polymer Science]] |volume=139 |issue=37 |pages=1–12 |doi=10.1002/app.52894 |s2cid=251335952 }}</ref>{{rp|2–3}} This natural cement can withstand a pulling strength of {{convert|5000|lb/in2|kPa|sigfig=1|abbr=on}} and a sticking strength of {{convert|22-60|lb/in2|kPa|sigfig=1|abbr=on}}.<ref name="OceanFacts">{{cite web |url=https://oceanservice.noaa.gov/facts/barnacles.html |title=What are barnacles? |date=26 February 2021 |website=Ocean Facts |publisher=[[National Ocean Service]], [[National Oceanic and Atmospheric Administration]] |access-date=18 June 2022}}</ref>

Barnacles have no true [[heart]], although a sinus close to the [[esophagus]] performs a similar function, with blood being pumped through it by a series of muscles.<ref>{{Cite web |title=Barnacles |website=Encyclopedia of Life |url=https://eol.org/pages/45500159/articles |access-date=2023-06-07 }}</ref> The blood vascular system is minimal.<ref>{{cite book |last=Burnett |year=1987 |title<!--chapter-->=The cirripede circulatory system and its evolution}} In [[#refSouthward|A. J. Southward (ed.), 1987]].</ref> Similarly, they have no [[gill]]s, absorbing [[oxygen]] from the water through the cirri and the surface of the body.<ref name="Monterey">{{cite web |title=Acorn barnacle |url=https://www.montereybayaquarium.org/animals/animals-a-to-z/acorn-barnacle |publisher=Monterey Bay Aquarium |access-date=18 February 2024}}</ref> The excretory organs of barnacles are maxillary glands.<ref name="White Walker 1981">{{cite journal |last1=White |first1=K. N. |last2=Walker |first2=G. |title=The barnacle excretory organ |journal=[[Journal of the Marine Biological Association of the United Kingdom]] |volume=61 |issue=2 |date=1981 |doi=10.1017/S0025315400047123 |pages=529–547 |bibcode=1981JMBUK..61..529W |s2cid=83903175 }}</ref>

The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult has three photoreceptors (ocelli), one median and two lateral.  These record the stimulus for the barnacle shadow reflex, where a sudden decrease in light causes cessation of the fishing rhythm and closing of the opercular plates.<ref>{{cite journal |last1=Gwilliam |first1=G. F. |last2=Millecchia |first2=R. J. |date=January 1975 |title=Barnacle photoreceptors: Their physiology and role in the control of behavior |journal=[[Progress in Neurobiology]] |volume=4 |pages=211–239 |doi=10.1016/0301-0082(75)90002-7|s2cid=53164671 }}</ref> The photoreceptors are likely only capable of sensing the difference between light and dark.<ref name=IZ>{{cite book |last=Barnes |first=Robert D. |year=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |pages= 694–707 |isbn= 978-0-03-056747-6}}</ref> This eye is derived from the primary [[Crustacean larvae#Nauplius|naupliar eye]].<ref>{{cite journal |last1=Lacalli |first1=Thurston C. |title=Serial EM analysis of a copepod larval nervous system: Naupliar eye, optic circuitry, and prospects for full CNS reconstruction |journal=Arthropod Structure & Development |date=September 2009 |volume=38 |issue=5 |pages=361–375 |doi=10.1016/j.asd.2009.04.002 |pmid=19376268 |bibcode=2009ArtSD..38..361L }}</ref>