Editing Jellyfish (section)

== Life history and behavior ==
{{See also|Biological life cycle|Developmental biology}}

[[File:Schleiden-meduse-2.jpg|thumb|upright=1.35|The developmental stages of [[scyphozoan]] jellyfish's life cycle:<br />'''1–3''' Larva searches for site<br />'''4–8''' Polyp grows<br />'''9–11''' Polyp [[strobilation|strobilates]]<br />'''12–14''' Medusa grows|alt=Illustration of two life stages of seven jelly species]]

=== Life cycle ===
Jellyfish have a complex life cycle which includes both sexual and asexual phases, with the medusa being the sexual stage in most instances. Sperm fertilize eggs, which develop into larval planulae, become polyps, bud into ephyrae and then transform into adult medusae. In some species certain stages may be skipped.<ref>{{cite magazine |url=http://www.scientificamerican.com/article.cfm?id=how-do-jellyfish-reproduc |title=How do jellyfish reproduce? What effect does their sting have on humans? What's the difference between red and translucent jellyfish? |magazine=Scientific American |date=15 October 2013 |access-date=22 October 2013 |url-status=live |archive-url=https://web.archive.org/web/20131023060403/http://www.scientificamerican.com/article.cfm?id=how-do-jellyfish-reproduc |archive-date=23 October 2013}}</ref>

Upon reaching adult size, jellyfish [[Spawn (biology)|spawn]] regularly if there is a sufficient supply of food. In most species, spawning is controlled by light, with all individuals spawning at about the same time of day; in many instances this is at dawn or dusk.<ref>{{Cite journal |last=Mills|first=Claudia|s2cid=13914997|year=1983|title=Vertical migration and diel activity patterns of hydromedusae: studies in a large tank|journal=Journal of Plankton Research |volume=5 |pages=619–635 |doi=10.1093/plankt/5.5.619|issue=5}}</ref> Jellyfish are usually either male or female (with occasional [[hermaphrodite]]s). In most cases, adults release [[spermatozoon|sperm]] and eggs into the surrounding water, where the unprotected eggs are fertilized and develop into larvae. In a few species, the sperm swim into the female's mouth, fertilizing the eggs within her body, where they remain during early development stages. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing [[planula]] larvae.<ref>{{cite web |url=http://www.thecephalopodpage.org/MarineInvertebrateZoology/Aureliaaurita.html |title=Moon Jelly (''Aurelia aurita'') |author=Bishop, Andrew |website=Marine Invertebrates of Bermuda |access-date=11 June 2018 |archive-date=5 October 2017 |archive-url=https://web.archive.org/web/20171005112929/http://www.thecephalopodpage.org/MarineInvertebrateZoology/Aureliaaurita.html |url-status=dead }}</ref>

The planula is a small [[larva]] covered with [[cilium|cilia]]. When sufficiently developed, it settles onto a firm surface and develops into a [[polyp (zoology)|polyp]]. The polyp generally consists of a small stalk topped by a mouth that is ringed by upward-facing tentacles. The polyps resemble those of closely related [[anthozoa]]ns, such as [[sea anemone]]s and [[coral]]s. The jellyfish polyp may be [[Sessility (zoology)|sessile]], living on the bottom of boat hulls or other substrates, or it may be free-floating or attached to tiny bits of free-living plankton<ref name="Mills 1987">{{Cite book |last=Mills |first=C. E. |year=1987 |title=In situ and shipboard studies of living hydromedusae and hydroids: preliminary observations of life-cycle adaptations to the open ocean |work=Modern Trends in the Systematics, Ecology, and Evolution of Hydroids and Hydromedusae |editor=J. Bouillon |editor2=F. Boero |editor3=F. Cicogna |editor4=P. F. S. Cornelius |publisher=Clarendon Press |isbn=978-0-19-857190-2}}</ref> or rarely, fish<ref>{{Cite journal|last=Fewkes|first=J. Walter|year=1887|title=A hydroid parasitic on a fish|journal=Nature|volume=36|pages=604–605|doi=10.1038/036604b0 | issue=939|bibcode=1887Natur..36..604F|s2cid=4078889|url=https://zenodo.org/record/1429303}}</ref><ref>{{cite web|url=http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa-directory.htm|title=The Hydrozoa|last=Schuchert|first=Peter|access-date=24 January 2010|url-status=live|archive-url=https://web.archive.org/web/20100204070833/http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa-directory.htm|archive-date=4 February 2010}}</ref> or other invertebrates. Polyps may be solitary or colonial.<ref>{{Cite web|title=How Jellyfish Grow, From Eggs to Polyps to Medusas|url=https://www.thoughtco.com/life-cycle-of-a-jellyfish-4112280|access-date=2023-02-10|website=ThoughtCo|language=en}}</ref> Most polyps are only millimetres in diameter and feed continuously. The polyp stage may last for years.<ref name=Ruppert />

After an interval and stimulated by seasonal or hormonal changes, the polyp may begin reproducing asexually by [[budding]] and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. Budding produces more scyphistomae and also ephyrae.<ref name=Ruppert /> Budding sites vary by species; from the [[tentacle]] bulbs, the manubrium (above the mouth), or the [[gonad]]s of hydromedusae.<ref name="Mills 1987" /> In a process known as [[strobilation]], the polyp's tentacles are reabsorbed and the body starts to narrow, forming transverse constrictions, in several places near the upper extremity of the polyp. These deepen as the constriction sites migrate down the body, and separate segments known as ephyra detach. These are free-swimming precursors of the adult medusa stage, which is the life stage that is typically identified as a jellyfish.<ref name=Ruppert /><ref>{{cite web |url=http://www.actforlibraries.org/lifecycle-of-the-box-jellyfish/ |title=Lifecycle of the Box Jellyfish |last=Hughes |first=Clare |website=Artforlibraries.org |access-date=2 January 2016 |url-status=live |archive-url=https://web.archive.org/web/20160304065648/http://www.actforlibraries.org/lifecycle-of-the-box-jellyfish/ |archive-date=4 March 2016}}</ref>
The ephyrae, usually only a millimeter or two across initially, swim away from the polyp and grow. [[Limnomedusae]] polyps can asexually produce a creeping ''frustule'' larval form, which crawls away before developing into another polyp.<ref name=Ruppert /> A few species can produce new medusae by budding directly from the medusan stage. Some hydromedusae reproduce by fission.<ref name="Mills 1987" />

=== Lifespan ===
Little is known of the life histories of many jellyfish as the places on the seabed where the benthic forms of those species live have not been found. However, an asexually reproducing strobila form can sometimes live for several years, producing new medusae (ephyra larvae) each year.<ref name="Brusca 2016">{{cite book | last=Brusca | first=Richard | title=Invertebrates | publisher=Sinauer Associates | year=2016 | isbn=978-1-60535-375-3 | page=310}}</ref>

An unusual species, ''[[Turritopsis dohrnii]]'', formerly classified as ''[[Turritopsis nutricula]]'',<ref>{{cite journal |last=Miglietta |first=M. P. |author2= Piraino, S. |author3=Kubota, S. |author4=Schuchert, P. |title=Species in the genus ''Turritopsis'' (Cnidaria, Hydrozoa): a molecular evaluation |journal=Journal of Zoological Systematics and Evolutionary Research |volume=45 |issue=1 |pages=11–19 |year=2007 |doi=10.1111/j.1439-0469.2006.00379.x|doi-access=free }}</ref> might be effectively immortal because of its ability under certain circumstances to transform from medusa back to the polyp stage, thereby escaping the death that typically awaits medusae post-reproduction if they have not otherwise been eaten by some other organism. So far this reversal has been observed only in the laboratory.<ref>{{cite journal | doi=10.2307/1543022 | pmid=29227703 | last1=Piraino | first1=S. | last2=Boero | year=1996 | first2=F. | last3=Aeschbach | first3=B. | last4=Schmid | first4=V. | s2cid=3956265 | title=Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in ''Turritopsis nutricula'' (Cnidaria, Hydrozoa) | jstor=1543022| journal=Biological Bulletin | volume=190 | issue=3| pages=302–312}}</ref>

=== Locomotion ===
[[File:Jellyfish locomotion.svg|thumb|upright=1.3|Jellyfish [[animal locomotion|locomotion]] is highly efficient. [[Muscle]]s in the jellylike bell contract, setting up a start [[vortex]] and propelling the animal. When the contraction ends, the bell recoils elastically, creating a stop vortex with no extra energy input.]]

Using the moon jelly ''[[Aurelia aurita]]'' as an example, jellyfish have been shown to be the most energy-efficient swimmers of all animals.<ref>{{cite web |url=https://arstechnica.com/science/2014/05/jellyfish-are-the-most-energy-efficient-swimmers-new-metric-confirms/ |title=Jellyfish are the most energy-efficient swimmers, new metric confirms |author=Rathi, Akshat |website=Ars Technica |access-date=3 December 2014 |date=15 May 2014 |url-status=live |archive-url=https://web.archive.org/web/20141103061826/http://arstechnica.com/science/2014/05/jellyfish-are-the-most-energy-efficient-swimmers-new-metric-confirms/|archive-date=3 November 2014}}</ref> They move through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion phases to create two [[vortex]] rings. Muscles are used for the contraction of the body, which creates the first vortex and pushes the animal forward, but the mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. Meanwhile, the second vortex ring starts to spin faster, sucking water into the bell and pushing against the centre of the body, giving a secondary and "free" boost forward. The mechanism, called passive energy recapture, only works in relatively small jellyfish moving at low speeds, allowing the animal to travel 30 percent farther on each swimming cycle. Jellyfish achieved a 48 percent lower cost of transport (food and oxygen intake versus energy spent in movement) than other animals in similar studies. One reason for this is that most of the gelatinous tissue of the bell is inactive, using no energy during swimming.<ref>{{cite journal | last1=Gemmell | first1=B. J. | last2=Costello | first2=J. H. | last3=Colin | first3=S. P. | last4=Stewart | first4=C. J. | last5=Dabiri | first5=J. O. | last6=Tafti | first6=D. | last7=Priya | first7=S. |display-authors=3 | doi=10.1073/pnas.1306983110 | title=Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans | journal=Proceedings of the National Academy of Sciences | year=2013 | pmid= 24101461| pmc=3816424| volume=110 | issue=44 | pages=17904–17909| bibcode=2013PNAS..11017904G | doi-access=free }}</ref>