Editing Metamorphosis

{{Short description|Profound change in body structure during the postembryonic development of an organism}}
{{About|the biological process|the Franz Kafka novella|The Metamorphosis{{!}}''The Metamorphosis''|other uses}}
[[File:Dragonfly metamorphosis.jpg|thumb|A [[dragonfly]] in its final [[ecdysis|moult]], undergoing metamorphosis, it begins transforming from its [[nymph (biology)|nymph]] form to an adult]]

'''Metamorphosis''' is a [[biological process]] by which an animal physically [[developmental biology|develops]]  including  [[birth|birth transformation]] or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell [[cell growth#Cell division|growth]] and [[cellular differentiation|differentiation]].<ref>{{Cite web |title=metamorphosis {{!}} biology {{!}} Britannica |url=https://www.britannica.com/science/metamorphosis |access-date=2022-04-01 |website=www.britannica.com |language=en}}</ref> Some [[insect]]s, [[jellyfish]], [[fish]], [[amphibian]]s, [[mollusk]]s, [[crustacean]]s, [[cnidaria]]ns, [[echinoderm]]s, and [[tunicate]]s undergo metamorphosis, which is often accompanied by a change of [[nutrition]] source or [[behavior]].<ref>{{Cite web |title=What animals undergo incomplete metamorphosis? – Easierwithpractice.com |url=https://easierwithpractice.com/what-animals-undergo-incomplete-metamorphosis/ |access-date=2022-04-01 |website=easierwithpractice.com}}</ref> Animals can be divided into species that undergo complete metamorphosis ("[[Holometabolism|holometaboly]]"), incomplete metamorphosis ("[[Hemimetabolism|hemimetaboly]]"), or no metamorphosis ("[[Ametabolism|ametaboly]]").<ref>{{Cite journal|last=Truman|first=James W.|date=2019-12-02|title=The Evolution of Insect Metamorphosis|journal=Current Biology|language=English|volume=29|issue=23|pages=R1252–R1268|doi=10.1016/j.cub.2019.10.009|issn=0960-9822|pmid=31794762|s2cid=208541817 |doi-access=free|bibcode=2019CBio...29R1252T }}</ref>

Generally organisms with a [[larva]]l stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics.<ref>{{cite journal |last1=Hadfield |first1=Michael G. |title=Why and how marine-invertebrate larvae metamorphose so fast |journal=Seminars in Cell & Developmental Biology |date=1 December 2000 |volume=11 |issue=6 |pages=437–443 |doi=10.1006/scdb.2000.0197 |pmid=11145872 |url=https://www.sciencedirect.com/science/article/pii/S1084952100901976 |access-date=7 March 2022 |language=en |issn=1084-9521}}</ref>

== Etymology ==
The word ''metamorphosis'' derives from [[Ancient Greek language|Ancient Greek]] {{lang|el|μεταμόρφωσις}}, "transformation, transforming",<ref>{{cite dictionary|url=https://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2366805 |title=Metamorphosis|first1=Henry George|last1=Liddell|first2=Robert|last2=Scott|date=1940|location=Oxford|publisher=Clarendon Press |dictionary=A Greek-English Lexicon|access-date=2012-08-26|via=perseus.tufts.edu}}</ref> from {{lang|el|μετα-}} (''{{lang|el|[[Meta (prefix)|meta-]]}}''), "after" and {{lang|el|μορφή}} (''{{lang|el|morphe}}''), "form".<ref>{{cite web|url=http://www.etymonline.com/index.php?search=Metamorphosis&searchmode=none |title=Online Etymology Dictionary |publisher=Etymonline.com |access-date=2012-08-26}}</ref>

== Hormonal control ==
In insects, growth and metamorphosis are controlled by [[hormone]]s synthesized by [[endocrine gland]]s near the front of the body ([[Anatomical terms of location|anterior]]). Neurosecretory cells in an [[insect brain|insect's brain]] secrete a hormone, the [[prothoracicotropic hormone]] (PTTH) that activates prothoracic glands, which secrete a second hormone, usually [[ecdysone]] (an [[ecdysteroid]]), that induces [[ecdysis]] (shedding of the exoskeleton).<ref>Davies, 1998. Chapter 3.</ref>
PTTH also stimulates the [[corpora allata]], a retrocerebral organ, to produce [[juvenile hormone]], which prevents the development of adult characteristics during [[ecdysis]].  In holometabolous insects, molts between larval [[instar]]s have a high level of juvenile hormone, the moult to the pupal stage has a low level of juvenile hormone, and the final, or [[imago|imaginal]], molt has no juvenile hormone present at all.<ref>Gullan, P.J. & Cranston, P.S.  6.3 Process and Control of Moulting in ''The Insects: An Outline of Entomology''. Blackwell Publishing,  2005. pp. 153–156.</ref> Experiments on [[Pyrrhocoris apterus|firebugs]] have shown how juvenile hormone can affect the number of nymph instar stages in [[hemimetabolous]] insects.<ref name="SlamaWilliams1965">{{cite journal|last1=Slama|last2=Williams|title=Juvenile hormone activity for the bug Pyrrhocoris apterus|journal=Proceedings of the National Academy of Sciences|volume=54 |issue=2 |year=1965 |pages=411–414 |doi=10.1073/pnas.54.2.411|pmid=5217430|pmc=219680|bibcode=1965PNAS...54..411S|doi-access=free}}</ref><ref name="SinghKonopova2011">{{cite journal |last1=Singh|first1=Amit |last2=Konopova |first2=Barbora |last3=Smykal |first3=Vlastimil |last4=Jindra |first4=Marek |title=Common and Distinct Roles of Juvenile Hormone Signaling Genes in Metamorphosis of Holometabolous and Hemimetabolous Insects |journal=PLOS ONE |volume=6 |issue=12 |year=2011 |pages=e28728 |issn=1932-6203 |doi=10.1371/journal.pone.0028728 |doi-access=free |pmid=22174880 |pmc=3234286|bibcode=2011PLoSO...628728K }}</ref>

In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all [[chordate]]s.<ref name="cell.com"/>

== Insects ==
[[File:Locust instars NMNS.jpg|thumb|Incomplete metamorphosis in the grasshopper with different instar nymphs. The largest specimen is adult.]]

All three categories of metamorphosis can be found in the diversity of insects, including no metamorphosis ("ametaboly"), incomplete or partial metamorphosis ("hemimetaboly"), and complete metamorphosis ("holometaboly"). While ametabolous insects show very little difference between larval and adult forms (also known as "[[direct development]]"), both hemimetabolous and holometabolous insects have significant morphological and behavioral differences between larval and adult forms, the most significant being the inclusion, in holometabolous organisms, of a [[pupa]]l or resting stage between the [[larval]] and adult forms.

=== Development and terminology ===
[[File:Holometabolous vs. Hemimetabolous.svg|thumb|Two types of metamorphosis are shown. In a complete (holometabolous) metamorphosis the insect passes through four distinct phases, which produce an adult that does not resemble the larva. In an incomplete (hemimetabolous) metamorphosis an insect does not go through a full transformation, but instead transitions from a nymph to an adult by molting its exoskeleton as it grows.]]
In [[Hemimetabolism|hemimetabolous]] [[insects]], immature stages are called [[Nymph (biology)|nymphs]].  Development proceeds in repeated stages of growth and [[ecdysis]] (moulting); these stages are called [[instar]]s. The juvenile forms closely resemble adults, but are smaller and lack adult features such as wings and genitalia. The size and morphological differences between nymphs in different instars are small, often just differences in body proportions and the number of segments; in later instars, external wing buds form. The period from one molt to the next is called a stadium.<ref>{{cite journal |last1=Schaefer |first1=C.W. |title=Instar, Stadium, and Stage: A New Look at Old Questions |journal=Bulletin of the Entomological Society of America |date=March 1971 |volume=17 |issue=1 |page=17 |doi=10.1093/besa/17.1.17c }}</ref>

In [[holometabolous]] insects, immature stages are called [[larva]]e and differ markedly from  adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called [[pupa]] (called a [[pupa#Chrysalis|"chrysalis]]" in butterfly species), and finally emerge as adults.<ref name="Lowe">{{cite journal |first1=Tristan |last1=Lowe |first2=Russell P. |last2=Garwood |first3=Thomas |last3=Simonsen |first4=Robert S. |last4=Bradley |first5=Philip J. |last5=Withers |date=July 6, 2013 |volume=10 |issue=84 |at=20130304 |doi=10.1098/rsif.2013.0304|pmid=23676900 |pmc=3673169 |title=Metamorphosis revealed: Time-lapse three-dimensional imaging inside a living chrysalis |journal=Journal of the Royal Society Interface }}</ref>

=== Evolution ===
The earliest insect forms showed direct development ([[ametabolism]]), and the evolution of metamorphosis in insects is thought to have fuelled their dramatic radiation (1,2). Some early ametabolous "true insects" are still present today, such as [[Archaeognatha|bristletails]] and [[silverfish]]. [[Hemimetabolism|Hemimetabolous]] insects include [[cockroach]]es, [[grasshopper]]s, [[dragonfly|dragonflies]], and [[Hemiptera|true bugs]]. Phylogenetically, all insects in the [[Pterygota]] undergo a marked change in form, texture and physical appearance from immature stage to adult. These insects either have [[Hemimetabolism|hemimetabolous]] development, and undergo an incomplete or partial metamorphosis, or [[Holometabolism|holometabolous]] development, which undergo a complete metamorphosis, including a [[pupa]]l or resting stage between the [[larval]] and adult forms.<ref name="autogenerated143">Gullan, P.J. & Cranston, P.S. 6.2 ''Life History Patterns and Phases in The Insects: An Outline of Entomology''.  pp. 143–153. 2005 by Blackwell Publishing</ref>

A number of hypotheses have been proposed to explain the evolution of holometaboly from hemimetaboly, mostly centering on whether or not the intermediate stages of hemimetabolous forms are homologous in origin to the pupal stage of holometabolous forms.

=== Temperature-dependent metamorphosis ===
According to a 2009 study, temperature plays an important role in insect development as individual species are found to have specific thermal windows that allow them to progress through their developmental stages. These windows are not significantly affected by ecological traits, rather, the windows are phylogenetically adapted to the ecological circumstances insects are living in.<ref>Dixon, A. F. G., A. Honěk, P. Keil, M. A. A. Kotela, A. L. Šizling, and V. Jarošík. 2009. Relationship between the minimum and maximum temperature thresholds for development in insects. Funct. Ecol. 23: 257–264.</ref>

=== Recent research ===

According to research from 2008, adult ''[[Manduca sexta]]'' is able to retain behavior learned as a [[caterpillar]].<ref>{{cite journal |author=Douglas J. Blackiston, Elena Silva Casey & Martha R. Weiss |year=2008 |title=Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar? |journal=[[PLoS ONE]] |volume=3 |issue=3 |page=e1736 |doi=10.1371/journal.pone.0001736 |doi-access=free |pmc=2248710 |pmid=18320055 |bibcode=2008PLoSO...3.1736B }}</ref> Another caterpillar, [[Utetheisa ornatrix|the ornate moth caterpillar]], is able to carry toxins that it acquires from its diet through metamorphosis and into adulthood, where the toxins still serve for protection against predators.<ref>{{cite book|last=Conner|first=W.E.|title=Tiger Moths and Woolly Bears—behaviour, ecology, and evolution of the Arctiidae|year=2009|publisher=[[Oxford University Press]]|location=New York|pages=1–10}}</ref>

Many observations published in 2002, and supported in 2013 indicate that [[programmed cell death]] plays a considerable role during physiological processes of multicellular organisms, particularly during [[embryogenesis]], and metamorphosis.<ref>{{Cite journal|last1=Lee|first1=Gyunghee|last2=Sehgal|first2=Ritika|last3=Wang|first3=Zixing|last4=Nair|first4=Sudershana|last5=Kikuno|first5=Keiko|last6=Chen|first6=Chun-Hong|last7=Hay|first7=Bruce|last8=Park|first8=Jae H.|date=2013-03-15|title=Essential role of grim-led programmed cell death for the establishment of corazonin-producing peptidergic nervous system during embryogenesis and metamorphosis in Drosophila melanogaster|journal=Biology Open|volume=2|issue=3|pages=283–294|doi=10.1242/bio.20133384|issn=2046-6390|pmc=3603410|pmid=23519152}}</ref><ref>{{Cite journal|last1=Zakeri|first1=Zahra|last2=Lockshin|first2=Richard A.|date=2002-07-01|title=Cell death during development|journal=Journal of Immunological Methods|volume=265|issue=1–2|pages=3–20|issn=0022-1759|pmid=12072175|doi=10.1016/s0022-1759(02)00067-4}}</ref> Additional research in 2019 found that both [[autophagy]] and [[apoptosis]], the two ways programmed cell death occur, are processes undergone during insect metamorphosis. <ref name="Rolff Johnston Reynolds p=20190063">{{cite journal | last1=Rolff | first1=Jens | last2=Johnston | first2=Paul R. | last3=Reynolds | first3=Stuart | title=Complete metamorphosis of insects | journal=Philosophical Transactions of the Royal Society B: Biological Sciences | publisher=The Royal Society | volume=374 | issue=1783 | date=2019-08-26 | issn=0962-8436 | doi=10.1098/rstb.2019.0063 | page=20190063| pmid=31438816 | pmc=6711294 }}</ref>

Below is the sequence of steps in the metamorphosis of the butterfly (illustrated):<br/>
[[File:Metamorphosis of butterfly (PSF).png|left|thumb|Metamorphosis of butterfly (PSF)]]

1 – The larva of a butterfly <br/>
2 – The pupa is now spewing the thread to form chrysalis <br/>
3 – The chrysalis is fully formed <br/>
4 – Adult butterfly coming out of the chrysalis
{{-}}
<gallery class="center" caption="Sequence illustrating complete metamorphosis in the cabbage white butterfly, ''[[Pieris rapae]]''">
File:ChristianBauer Pieris rapae caterpiller.jpg|[[larva]]
File:ChristianBauer Pieris rapae caterpiller2.jpg|[[pupa]]
File:ChristianBauer Pieris rapae cocoon.jpg|pupa ready for eclosion
File:ChristianBauer Pieris rapae youngadult.jpg|[[adult]]
</gallery>

==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>
{{clear}}

==== 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>

== See also ==
* {{annotated link|Developmental biology}}
* {{annotated link|Direct development}}
* {{annotated link|Gosner stage}}
* {{annotated link|Hypermetamorphosis}}
* {{annotated link|Morphogenesis}}

== References ==
{{Reflist}}

==Bibliography==
* Davies, R.G. (1998). ''Outlines of Entomology''. Chapman and Hall. Second Edition. Chapter 3.
* Williamson D.I. (2003). ''The Origins of Larvae''. Kluwer.

== External links ==
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[[Category:Animal developmental biology]]
[[Category:Animal anatomy]]