Editing Metamorphosis (section)

==Chordata==
===Amphioxus===
In [[cephalochordata]], metamorphosis is [[Iodothyronine deiodinase|iodothyronine]]-induced and it could be an ancestral feature of all [[chordate]]s.<ref name="cell.com">{{cite journal | last1 = Denser | first1 = Robert J. | year = 2008 | title = Chordate Metamorphosis: Ancient Control by Iodothyronines | url = http://www.cell.com/current-biology/pdf/S0960-9822(08)00661-1.pdf | journal = Current Biology | volume = 18 | issue = 13| pages = R567–9 | doi = 10.1016/j.cub.2008.05.024 | pmid = 18606129 | s2cid = 18587560 | doi-access = free | bibcode = 2008CBio...18.R567D }}</ref>

=== Fish ===
Some fish, both [[Osteichthyes|bony fish (Osteichthyes)]] and [[Agnatha|jawless fish (Agnatha)]], undergo metamorphosis. Fish metamorphosis is typically under strong control by the thyroid hormone.<ref name="Laudet" />

Examples among the non-bony fish include the [[lamprey]]. Among the bony fish, mechanisms are varied.

The [[salmon]] is [[diadromous]], meaning that it changes from a [[freshwater]] to a [[Seawater|saltwater]] lifestyle.

Many species of [[flatfish]] begin their life [[bilaterally symmetrical]], with an eye on either side of the body; but one eye moves to join the other side of the fish – which becomes the upper side – in the adult form.

The [[European eel]] has a number of metamorphoses, from the larval stage to the [[leptocephalus]] stage, then a quick metamorphosis to glass eel at the edge of the continental shelf (eight days for the [[Japanese eel]]), two months at the border of fresh and salt water where the glass eel undergoes a quick metamorphosis into elver, then a long stage of growth followed by a more gradual metamorphosis to the migrating phase. In the pre-adult [[Fresh water|freshwater]] stage, the eel also has [[phenotypic plasticity]] because fish-eating eels develop very wide mandibles, making the head look blunt. [[Leptocephalus|Leptocephali]] are common, occurring in all [[Elopomorpha]] ([[tarpon]]- and [[eel]]-like fish).

Most other bony fish undergo metamorphosis initially from [[fish egg|egg]] to immotile larvae known as ''sac fry'' ([[fry (biology)|fry]] with a [[yolk sac]]), then to motile larvae (often known as [[fingerling (fish)|fingerling]]s due to them roughly reaching the length of a [[human finger]]) that have to [[foraging|forage]] for themselves after the yolk sac resorbs, and then to the juvenile stage where the fish progressively start to resemble [[adult]] morphology and behaviors until finally reaching [[sexual maturity]].<ref>Mader, Sylvia, ''Biology'' 9th ed. Ch. 31</ref><ref>Peter B. Moyle and Joseph J. Cech Jr, ''Fishes: an introduction to ichthyology'' 5th ed. 9.3: "Development" pp 148ff</ref>

=== Amphibians ===
[[File:RanaTemporariaLarva2.jpg|thumb|Just before metamorphosis, only 24 hours are needed to reach the stage in the next picture.]]
[[File:Rana Temporaria - Larva Final Stage.jpg|thumb|Almost functional [[common frog]] with some remains of the gill sac and a not fully developed jaw]]

In typical amphibian development, eggs are laid in water and  larvae are adapted to an aquatic lifestyle. [[Frogs]], [[toads]], and [[newts]] all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae  start a predatory lifestyle, while [[tadpole]]s mostly scrape food off surfaces with their horny tooth ridges.

Metamorphosis in amphibians is regulated by [[thyroxin]] concentration in the blood, which stimulates metamorphosis, and [[prolactin]], which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers, and fins. They also make use of the [[lateral line]] organ. After  metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called [[apoptosis]]. The amount of adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.

==== Frogs and toads ====
With frogs and toads, the  external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral‐shaped gut to digest that diet. Recent studies suggest tadpoles do not have a balanced homeostatic feedback control system until the beginning stages of metamorphosis. At this point, their long gut shortens and begins favoring the diet of insects.<ref>{{Cite journal|last=Bender|first=Melissa|date=March 28, 2018|title=To eat or not to eat: ontogeny of hypothalamic feeding controls and a role for leptin in modulating life-history transition in amphibian tadpoles|journal= Proceedings of the Royal Society B: Biological Sciences|volume=285|issue=1875 |doi=10.1098/rspb.2017.2784 |pmid=29593109 |pmc=5897637 |s2cid=4853293 }}</ref>

Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day. It is not until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.<ref>{{Cite web |date=2021-01-13 |title=15.2: The Life Cycle of Amphibians |url=https://bio.libretexts.org/Courses/Lumen_Learning/Fundamentals_of_Biology_I_(Lumen)/15:_Module_12-_Vertebrates/15.02:_The_Life_Cycle_of_Amphibians |access-date=2025-01-24 |website=Biology LibreTexts |language=en}}</ref>
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==== Salamanders ====
Salamander development is highly diverse; some species go through a dramatic reorganization when transitioning from aquatic larvae to terrestrial adults, while others, such as the [[axolotl]], display [[Neoteny|pedomorphosis]] and never develop into terrestrial adults. Within the genus ''[[Ambystoma]]'', species have evolved to be pedomorphic several times, and pedomorphosis and complete development can both occur in some species.<ref name="Laudet">{{cite journal | last1 = Laudet | first1 = Vincent | date = September 27, 2011 | title = The Origins and Evolution of Vertebrate Metamorphosis | journal = Current Biology | volume = 21 | issue = 18| pages = R726–R737 | doi=10.1016/j.cub.2011.07.030| pmid = 21959163 | doi-access = free | bibcode = 2011CBio...21.R726L }}</ref>

==== Newts ====
[[File:LarveKamsalamander.JPG|thumb|The large external gills of the [[Northern crested newt|crested newt]]]]

In newts, metamorphosis occurs due to the change in habitat, not a change in diet, because newt larvae already feed as predators and continue doing so as adults. Newts' gills are never covered by a gill sac and will be resorbed only just before the animal leaves the water. Adults can move faster on land than in water.<ref>{{cite journal |last1=Wilson |first1=Robbie |title=Consequences of Metamorphosis for the Locomotor Performance and Thermal Physiology of the Newt Triturus cristatus |journal=Physiological and Biochemical Zoology |date=November–December 2005 |volume=78 |issue=6 |pages=967–975 |doi=10.1086/432923 |jstor=10.1086/432923 |pmid=16228936 |s2cid=34285867 |url=https://www.jstor.org/stable/10.1086/432923 |access-date=28 December 2020}}</ref>  Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, [[prolactin]] is the required hormone, and for adaptation to the land phase, [[thyroxin]]. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.

==== Caecilians ====
Basal caecilians such as ''[[Ichthyophis]]'' go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the [[lateral line]].<ref>{{cite journal | last1 = Dunker | first1 = Nicole | last2 = Wake | first2 = Marvalee H. | last3 = Olson | first3 = Wendy M. | date = January 2000 | title = Embryonic and Larval Development in the Caecilian Ichthyophis kohtaoensis (Amphibia, Gymnophiona): A Staging Table | journal = Journal of Morphology | volume = 243 | issue = 1| pages = 3–34 | doi=10.1002/(sici)1097-4687(200001)243:1<3::aid-jmor2>3.3.co;2-4| pmid = 10629095 }}</ref> More recently diverged caecilians (the [[Teresomata]]) do not undergo an [[ontogenetic niche shift]] of this sort and are in general [[fossorial]] throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis.<ref>{{cite journal | last1 = San Mauro | first1 = D. | last2 = Gower | first2 = D. J. | last3 = Oommen | first3 = O. V. | last4 = Wilkinson | first4 = M. | last5 = Zardoya | first5 = R. | date = November 2004 | title = Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1 | journal = Molecular Phylogenetics and Evolution | volume = 33 | issue = 2| pages = 413–427 | doi=10.1016/j.ympev.2004.05.014 | pmid=15336675| bibcode = 2004MolPE..33..413S }}</ref>