View source for Cnidaria

==Reproduction==
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===Sexual===
Cnidarian [[sexual reproduction]] often involves a complex life cycle with both [[polyp (zoology)|polyp]] and [[Medusa (biology)|medusa]] stages. For example, in [[Scyphozoa]] (jellyfish) and [[Cubozoa]] (box jellies), a [[Planula|larva]] swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called [[strobilation]]. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adult medusae have [[gonad]]s in the [[gastrodermis|gastroderm]], and these release [[ovum|ova]] and [[sperm]] into the water in the breeding season.<ref name="Hinde2001" /><ref name="Ruppert" />

This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by a [[free-swimming]] medusa or a sessile polyp that reproduces sexually)<ref>{{cite book|author=Vernon A. Harris |year=1990 |chapter=Hydroids |title=Sessile animals of the sea shore |publisher=Springer |page=223 |isbn=978-0-412-33760-4 |chapter-url=https://books.google.com/books?id=DbaBQm_ZQkIC&q=alternation+of+generation+&pg=PA223}}</ref> is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with the [[alternation of generations]] as found in plants.

Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the polyp stage completely, and cubozoan polyps produce only one medusa. [[Hydrozoa]] have a variety of life cycles. Some have no polyp stages and some (e.g. ''[[hydra (genus)|hydra]]'') have no medusae. In some species, the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae. Meanwhile, life cycle reversal, in which polyps are formed directly from medusae without the involvement of sexual reproduction process, was observed in both Hydrozoa (''Turritopsis dohrnii''<ref>{{Cite journal|url = http://www.icm.csic.es/scimar/pdf/56/sm56n2137.pdf|title = Bi-directional conversion in Turritopsis nutricula (Hydrozoa)|last = Bavestrello|date = 1992|journal = Scientia Marina|access-date = 2015-12-31|display-authors = etal|archive-date = 2014-12-14|archive-url = https://web.archive.org/web/20141214014457/http://www.icm.csic.es/scimar/pdf/56/sm56n2137.pdf}}</ref> and ''Laodicea undulata''<ref>{{Cite journal|title = Evidence of reverse development in Leptomedusae (Cnidaria, Hydrozoa): the case of Laodicea undulata (Forbes and Goodsir 1851)|last = De Vito|date = 2006|journal = Marine Biology|doi = 10.1007/s00227-005-0182-3|display-authors=etal|volume=149|issue = 2|pages=339–346| bibcode=2006MarBi.149..339D |s2cid = 84325535}}</ref>) and Scyphozoa (''Aurelia'' sp.1<ref>{{Cite journal|title = Life Cycle Reversal in Aurelia sp.1 (Cnidaria, Scyphozoa)|last = He|date = 21 December 2015|journal = PLOS ONE|doi = 10.1371/journal.pone.0145314|pmid = 26690755|display-authors=etal|pmc=4687044|volume=10|issue = 12|article-number=e0145314|bibcode=2015PLoSO..1045314H|doi-access = free}}</ref>). [[Anthozoa]] have no medusa stage at all and the polyps are responsible for sexual reproduction.<ref name="Hinde2001" />

Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the [[Lunar phase|phase of the moon]]. Many species of Cnidaria may spawn simultaneously in the same location, so that there are too many ova and sperm for predators to eat more than a tiny percentage&nbsp;— one famous example is the [[Great Barrier Reef]], where at least 110 [[coral]]s and a few non-cnidarian [[invertebrate]]s produce enough gametes to turn the water cloudy. These mass spawnings may produce [[Hybrid (biology)|hybrids]], some of which can settle and form polyps, but it is not known how long these can survive. In some species the ova release chemicals that attract sperm of the same species.<ref name="Hinde2001" />

The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere ([[blastula]]) and then a depression forms at one end ([[gastrulation]]) and eventually becomes the digestive cavity. However, in cnidarians the depression forms at the end further from the yolk (at the [[animal pole]]), while in [[bilaterian]]s it forms at the other end ([[vegetal pole]]).<ref name="Ruppert" /> The larvae, called [[planula]]e, swim or crawl by means of [[cilia]].<ref name="Hinde2001" /> They are cigar-shaped but slightly broader at the "front" end, which is the aboral, vegetal-pole end and eventually attaches to a substrate if the species has a polyp stage.<ref name="Ruppert" />

Anthozoan larvae either have large [[yolk]]s or are capable of feeding on [[plankton]], and some already have [[endosymbiotic]] [[algae]] that help to feed them. Since the parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and [[metamorphose]] into polyps. Instead, these species rely on their medusae to extend their ranges.<ref name="Ruppert" />

===Asexual===
All known cnidarians can reproduce [[asexual reproduction|asexually]] by various means, in addition to regenerating after being fragmented. [[Hydrozoa]]n polyps only bud, while the medusae of some hydrozoans can divide down the middle. [[Scyphozoa]]n polyps can both bud and split down the middle. In addition to both of these methods, [[Anthozoa]] can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian a clone of the adult. The ability of cnidarians to asexually reproduce ensures a greater number of mature medusa that can mature to reproduce sexually.<ref name="Hinde2001" /><ref name="Ruppert" />

===DNA repair===

Two classical [[DNA repair]] pathways, [[nucleotide excision repair]] and [[base excision repair]], are present in [[hydra (genus)|hydra]],<ref name = Barve2021>{{cite journal|pmid=33995496 |date=2021 |last1=Barve |first1=A. |last2=Galande |first2=A. A. |last3=Ghaskadbi |first3=S. S. |last4=Ghaskadbi |first4=S. |title=DNA Repair Repertoire of the Enigmatic Hydra |journal=Frontiers in Genetics |volume=12 |article-number=670695 |doi=10.3389/fgene.2021.670695 |pmc=8117345 |doi-access=free }}</ref> and these repair pathways facilitate unhindered reproduction.  The identification of these pathways in hydra is based, in part, on the presence in the hydra [[genome]] of genes homologous to genes in other genetically well studied species that have been demonstrated to play key roles in these DNA repair pathways.<ref name = Barve2021/>