Editing Ctenophora (section)

===Reproduction and development===

[[File:Juvenile Bolinopsis ctenophore.jpg|thumb|Cydippid larva of ''Bolinopsis'' sp., a few millimetres long]]

Adults of most species can regenerate tissues that are damaged or removed,<ref>{{cite journal
|last=Martindale |first=M. Q. |title=The ontogeny and maintenance of adult symmetry properties in the ctenophore, ''Mnemiopsis mccradyi''|journal=Developmental Biology |date=December 1986|volume=118 |issue=2 |pages=556–576 |pmid=2878844|doi=10.1016/0012-1606(86)90026-6}}</ref> although only platyctenids reproduce by [[cloning]], splitting off from the edges of their flat bodies fragments that develop into new individuals.<ref name="RuppertBarnes2004Ctenophora" /> Lab research on ''Mnemiopsis leidyi'' also show that when two individuals have parts of their bodies removed, they are able to fuse together, including their nervous and digestive systems, even when the two individuals are genetically different; a phenomenon so far only found in comb jellies.<ref>[https://www.cell.com/current-biology/abstract/S0960-9822(24)01023-6 Rapid physiological integration of fused ctenophores]</ref>

The [[Most recent common ancestor|last common ancestor (LCA)]] of the ctenophores was [[hermaphrodite|hermaphroditic]].<ref name="Sasson">{{cite journal |last1=Sasson |first1=Daniel A. |last2=Ryan |first2=Joseph F. |title=A reconstruction of sexual modes throughout animal evolution |journal=BMC Evolutionary Biology |date=December 2017 |volume=17 |issue=1 |page=242 |doi=10.1186/s12862-017-1071-3 |pmid=29207942 |pmc=5717846 |doi-access=free |bibcode=2017BMCEE..17..242S }}</ref> Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. There is no [[metamorphosis]].<ref>{{cite journal |doi=10.1073/pnas.2122052119 |title=Ctenophores are direct developers that reproduce continuously beginning very early after hatching |year=2022 |last1=Edgar |first1=Allison |last2=Ponciano |first2=José Miguel |last3=Martindale |first3=Mark Q. |journal=Proceedings of the National Academy of Sciences of the USA |volume=119 |issue=18 |pages=e2122052119 |doi-access=free |pmid=35476523 |pmc=9170174 |bibcode=2022PNAS..11922052E }}</ref> At least three species are known to have evolved separate sexes ([[dioecy]]); ''Ocyropsis crystallina'' and ''Ocyropsis maculata'' in the genus [[Ocyropsis]] and ''Bathocyroe fosteri'' in the genus [[Bathocyroe]].<ref name="Harbison">{{cite journal |last1=Harbison |first1=G. R. |last2=Miller |first2=R. L. |title=Not all ctenophores are hermaphrodites. Studies on the systematics, distribution, sexuality and development of two species of Ocyropsis |journal=[[Marine Biology (journal)|Marine Biology]] |date=February 1986 |volume=90 |issue=3 |pages=413–424 |doi=10.1007/bf00428565 |bibcode=1986MarBi..90..413H |s2cid=83954780 }}</ref> The [[gonad]]s are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is generally [[external fertilization|external]], but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus ''[[Mnemiopsis]]'',<ref name="RuppertBarnes2004Ctenophora"/> and most hermaphroditic species are presumed to be self-fertile.<ref name="MillsNotesFromExpert"/>

Development of the fertilized eggs is direct; there is no distinctive larval form. Juveniles of all groups are generally [[plankton]]ic, and most species resemble miniature adult cydippids, gradually developing their adult body forms as they grow. In the genus ''Beroe'', however, the juveniles have large mouths and, like the adults, lack both tentacles and tentacle sheaths. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae. They live among the plankton and thus occupy a different [[ecological niche]] from their parents, only attaining the adult form by a more radical [[ontogeny]]<ref name="RuppertBarnes2004Ctenophora"/> after dropping to the sea-floor.<ref name="MillsNotesFromExpert"/>

At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features enable ctenophores to increase their populations very quickly.<ref name="MillsNotesFromExpert" /> Members of the Lobata and Cydippida have a reproduction form called dissogeny; two sexually mature stages, first as larva and later as juveniles and adults. During their time as larvae they release gametes periodically. After their first reproductive period is over they do not produce more gametes until later. A population of ''Mertensia ovum'' in the central [[Baltic Sea]] have become [[Neoteny|paedogenetic]], and consist solely of sexually mature larvae less than 1.6&nbsp;mm.<ref>{{cite journal|pmc=4971632 |pmid=27489613 |doi=10.1186/s13227-016-0051-9 |volume=7 |title=Developmental expression of 'germline'- and 'sex determination'-related genes in the ctenophore ''Mnemiopsis leidyi'' |year=2016 |journal=[[EvoDevo]] |page=17 |last1=Reitzel |first1=A. M. |last2=Pang |first2=K. |last3=Martindale |first3=M. Q. |doi-access=free }}</ref><ref>{{cite journal|pmc=3440961 |pmid=22535640 |doi=10.1098/rsbl.2012.0163 |volume=8 |issue=5 |title=Ctenophore population recruits entirely through larval reproduction in the central Baltic Sea |year=2012 |journal=Biology Letters |pages=809–12 |last1=Jaspers |first1=C. |last2=Haraldsson |first2=M. |last3=Bolte |first3=S. |last4=Reusch |first4=T. B. |last5=Thygesen |first5=U. H. |last6=Kiørboe |first6=T.}}</ref>

In ''Mnemiopsis leidyi'', nitric oxide (NO) signaling is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms.<ref>{{cite journal|doi=10.3389/fnins.2023.1125433 |volume=17 |title=Nitric oxide signaling in ctenophores |year=2023 |journal=Front. Neurosci. |page=1125433 |last1=Moroz |first1=Leonid |last2=Mukherjee |first2=Krishanu |last3=Romanova |first3=Daria |pmid=37034176 |pmc=10073611 |doi-access=free }}</ref> The mature form of the same species is also able to revert back to the cydippid stage when triggered by environmental stressors.<ref>[https://www.zmescience.com/science/news-science/this-benjamin-button-like-jellyfish-can-age-in-reverse-from-adult-to-juvenile/ This Benjamin Button-like Jellyfish Can Age in Reverse, From Adult to Juvenile]</ref>