Editing Hagfish (section)

==Reproduction==
[[File:Black_hagfish,_Eptatretus_deani_gonads_01.jpg|thumb|Egg development in a female black hagfish, ''Eptatretus deani'']]
[[Image:Eptatretus polytrema.jpg|thumb|right|Drawing of ''[[Eptatretus polytrema]]'']]
Very little is known about hagfish reproduction. Obtaining embryos and observing reproductive behavior are difficult due to the deep-sea habitat of many hagfish species.<ref name=":2">{{Cite journal|last1=Ota|first1=Kinya G.|last2=Kuratani|first2=Shigeru|title=The History of Scientific Endeavors Towards Understanding Hagfish Embryology|journal=Zoological Science|year=2006|volume=23|issue=5|pages=403–418|doi=10.2108/zsj.23.403|pmid=16766859|s2cid=20666604|issn=0289-0003|doi-access=free}}</ref> In the wild, females outnumber males, with the exact sex-ratio differing depending on the species. ''E. burgeri'', for example, has nearly a 1:1 ratio, while ''M. glutinosa'' females are significantly more common than males.<ref name=":2"/> Some species of hagfish are sexually undifferentiated before maturation, and possess gonadal tissue for both ovaries and testis.<ref>{{Cite journal|last1=Martini|first1=Frederic H.|last2=Beulig|first2=Alfred|date=2013-11-08|title=Morphometics and Gonadal Development of the Hagfish Eptatretus cirrhatus in New Zealand|journal=PLOS ONE|language=en|volume=8|issue=11|pages=e78740|doi=10.1371/journal.pone.0078740|issn=1932-6203|pmc=3826707|pmid=24250811|bibcode=2013PLoSO...878740M|doi-access=free}}</ref> It has been suggested that females develop earlier than males, and that this may be the reason for unequal sex ratios. Hagfish testis are relatively small.<ref name=":2"/>

Depending on species, females lay from one to 30 tough, yolky eggs. These tend to aggregate due to having [[Hook and loop fastener|Velcro]]-like tufts at either end.<ref name=":2"/> It is unclear how hagfish go about laying eggs, although researchers have proposed three hypotheses based on observations of the low percentage of males and small testis. The hypotheses are that female hagfish lay eggs in small crevices in rock formations, the eggs are laid in burrow beneath the sand, and the slime produced by the hagfish is used to hold the eggs in a small area.<ref name=":2"/> It is worth noting that no direct evidence has been found to support any of these hypotheses. Hagfish do not have a [[larva]]l stage, in contrast to [[lamprey]]s.<ref name=":2"/>

Hagfish have a [[mesonephros|mesonephric kidney]] and are often [[Neoteny|neotenic]] of their [[pronephros|pronephric kidney]]. The kidney(s) are drained via mesonephric/[[archinephric duct]]. Unlike many other vertebrates, this duct is separate from the reproductive tract, and the proximal tubule of the [[nephron]] is also connected with the [[coelom]], providing lubrication.<ref>{{Cite book|last=Kardong|first=Kenneth V. |title=Vertebrates: comparative anatomy, function, evolution|date=2019|isbn=978-1-259-70091-0|edition=Eighth |location=New York |oclc=1053847969}}</ref> The single testicle or ovary has no transportation duct. Instead, the gametes are released into the coelom until they find their way to the posterior end of the [[Caudal (anatomical term)|caudal]] region, whereby they find an opening in the digestive system.

The hagfish embryo can develop for as long as 11 months before hatching, which is shorter in comparison to other jawless vertebrates.<ref>{{cite journal| year=1997|last1=Gorbman|first1=A|title=Hagfish development|journal = Zoological Science | volume = 14|issue=3| pages=375–390 |doi=10.2108/zsj.14.375|s2cid=198158310|doi-access=free}}</ref> Not much was known about hagfish embryology until recently, when husbandry advances enabled considerable insight into the group's evolutionary development. New insights into the evolution of [[neural crest cells]], support the consensus that all vertebrates share these cells, which might be regulated by a common subset of genes.<ref>{{cite journal| year=2007 |last1=Ota|first1=K.G| last2=Kuraku | first2 = S. | last3 = Kuratani | first3 = S. | title = Hagfish embryology with reference to the evolution of the neural crest|journal =Nature | volume = 446|issue=7136|pages=672–5|doi=10.1038/nature05633|pmid=17377535|bibcode=2007Natur.446..672O|s2cid=4414164}}</ref> Their genome has a large number of microchromosomes which are lost during the animal's development, leaving only the reproductive organs with a complete genome.<ref>{{Cite web |url=https://www.bristol.ac.uk/news/2024/january/hagfish.html |title=First genome of slime eels uncovers the deep evolutionary history of our genomes and bodies |access-date=2024-01-18 |archive-date=2024-08-29 |archive-url=https://web.archive.org/web/20240829211316/https://www.bristol.ac.uk/news/2024/january/hagfish.html |url-status=live }}</ref> Hagfish possess [[gonadotropin]]s which secrete from pituitary glands to the gonads to stimulate development.<ref>{{cite journal|last=Nozaki|first=Masumi|date=2013|title=Hypothalamic-Pituitary-Gonadal Endocrine System in the Hagfish|journal=Frontiers in Endocrinology|language=English|volume=4|page=200|doi=10.3389/fendo.2013.00200|pmid=24416029|pmc=3874551|issn=1664-2392|doi-access=free}}</ref> This suggests that hagfish have an early version of the [[hypothalamic–pituitary–gonadal axis]], a system which once thought to be exclusive to the [[Gnathostomata|Gnathostomes]].[[Image:Eptatretus cirrhatus (New Zealand hagfish).gif|thumb|right|Drawing of a [[New Zealand hagfish]]]]Some species of hagfish reproduce seasonally, stimulated by hormones from their pituitary gland. ''E. burgeri'' is known to reproduce and migrate annually.<ref name=":6">{{Cite journal|last1=Powell|first1=Mickie L.|last2=Kavanaugh|first2=Scott I.|last3=Sower|first3=Stacia A.|date=2005-01-01|title=Current Knowledge of Hagfish Reproduction: Implications for Fisheries Management|url=https://academic.oup.com/icb/article/45/1/158/604629|journal=Integrative and Comparative Biology|language=en|volume=45|issue=1|pages=158–165|doi=10.1093/icb/45.1.158|pmid=21676757|issn=1540-7063|doi-access=free|citeseerx=10.1.1.491.7210|access-date=2021-05-11|archive-date=2021-05-12|archive-url=https://web.archive.org/web/20210512090157/https://academic.oup.com/icb/article/45/1/158/604629|url-status=live}}</ref>