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Metamorphosis is a
biological process Biological processes are those processes that are vital for an organism to live, and that shape its capacities for interacting with its environment. Biological processes are made of many chemical reactions or other events that are involved in th ...
by which an animal physically develops after
birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...

birth
or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell
growth
growth
and
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
. Some
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s,
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
,
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s,
mollusk Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s,
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda, amphipods and mantis shrimp. The ...
s,
cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

cnidaria
ns,
echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of ...
s, and
tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time cal ...
s undergo metamorphosis, which is often accompanied by a change of
nutrition Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It includes ingestion, Absorption (biology), absorption, Assimilation (biology), assimilation, biosynthesis, catabolism and excretion. ...
source or
behavior Behavior (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. Cur ...
. Animals can be divided into species that undergo complete metamorphosis (" holometaboly"), incomplete metamorphosis (" hemimetaboly"), or no metamorphosis (" ametaboly"). Scientific usage of the term is technically precise, and it is not applied to general aspects of
cell growth Cell growth refers to an ''increase in the total mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or with something, whether as an attribute or as a component of said ...
, including rapid
growth spurt Human height or stature is the distance Distance is a numerical measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two c ...

growth spurt
s. References to "metamorphosis" in
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s are imprecise and only colloquial, but historically idealist ideas of transformation and
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology) In archaeology, morphology is the study of the shape of Artifact (archaeology), artefacts and ecofacts. Morphology is a major consid ...
, as in
Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of G ...

Goethe
's ''
Metamorphosis of Plants ''Versuch die Metamorphose der Pflanzen zu erklären'', known in English as ''Metamorphosis of Plants'', was published by German poet and philosopher Johann Wolfgang von Goethe in 1790. In this work, Goethe essentially discovered the (serially) hom ...
'', have influenced the development of ideas of
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
.


Etymology

The word ''metamorphosis'' derives from
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
, "transformation, transforming", from ('), "after" and ('), "form".


Hormonal control

Metamorphosis is iodothyronine-induced and an ancestral feature of all
chordate A chordate () is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...
s. In insects, growth and metamorphosis are controlled by
hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...

hormone
s synthesized by
endocrine gland Endocrine glands are ductless gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or rela ...
s near the front of the body (
anterior Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient ...
). Neurosecretory cells in an insect's brain secrete a hormone, the
prothoracicotropic hormone Prothoracicotropic hormone (PTTH) was the first insect hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, i ...
(PTTH) that activates prothoracic glands, which secrete a second hormone, usually
ecdysone Ecdysone is a steroid A steroid is a biologically active organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their ...

ecdysone
(an
ecdysteroid Ecdysteroids are arthropod Steroid hormone, steroid hormones that are mainly responsible for Moulting, molting, development and, to a lesser extent, reproduction; examples of ecdysteroids include ecdysone, ecdysterone, turkesterone and 2-deoxyecdys ...
), that induces
ecdysis Ecdysis is the moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (ofte ...
. PTTH also stimulates the corpora allata, a retrocerebral organ, to produce
juvenile hormone Juvenile hormones (JHs) are a group of acyclic sesquiterpenoid '' Phallus indusiatus'', in Cooktown, Queensland, Australia, which produces two novel sesquiterpenes">Cooktown,_Queensland.html" ;"title="Phallus indusiatus'', in Cooktown, Queensland"> ...
, which prevents the development of adult characteristics during
ecdysis Ecdysis is the moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (ofte ...
. In holometabolous insects, molts between larval
instar An instar (, from the Latin ''īnstar'', "form", "likeness") is a developmental stage of arthropods An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly ...
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 , molt has no juvenile hormone present at all. Experiments on have shown how juvenile hormone can affect the number of nymph instar stages in
hemimetabolous Hemimetabolism or hemimetaboly, also called incomplete Metamorphosis (biology), metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode ...
insects.


Insects

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 developmentMarine larval ecology is the study of the factors influencing dispersing larva A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development s ...
"), both hemimetabolous and holometabolous insects have significant morphological and behavioral differences between larval and adult forms, the most significant being the inclusion, in holometabolus organisms, of a
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communicat ...

pupa
l or resting stage between the
larval A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development such as insects, amphibians, or cnidarians typically have a larval phase of th ...
and adult forms.


Development and terminology

In
hemimetabolous Hemimetabolism or hemimetaboly, also called incomplete Metamorphosis (biology), metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode ...
insects Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

insects
, immature stages are called
nymphs A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Gree ...
. Development proceeds in repeated stages of growth and
ecdysis Ecdysis is the moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (ofte ...
(moulting); these stages are called
instar An instar (, from the Latin ''īnstar'', "form", "likeness") is a developmental stage of arthropods An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly ...
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. In
holometabolous Holometabolism, also called complete metamorphosis (biology), metamorphosis, is a form of insect development which includes four life stages: egg (biology), egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphy, synapomorphic tr ...
insects, immature stages are called
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e and differ markedly from adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communicat ...

pupa
(called a " in butterfly species), and finally emerge as adults.


Evolution

The earliest insect forms showed direct development (
ametabolismAmetabolism is a type of growth or life cycle in insects in which there is slight or no metamorphosis, only a gradual increase in size. It is present only in primitive wingless insects, e.g. order: Zygentoma (Silverfish The silverfish (''Lepisma s ...
), 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 and
silverfish The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of ...

silverfish
.
Hemimetabolous Hemimetabolism or hemimetaboly, also called incomplete Metamorphosis (biology), metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode ...
insects include
cockroach Cockroaches (or roaches) are insects of the order Blattodea, which also includes termites. About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known as Pest (organism), pests. The cockroaches are ...

cockroach
es,
grasshopper Grasshoppers are a group of insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''comm ...

grasshopper
s,
dragonflies A dragonfly is a flying insect The Pterygota (Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referr ...

dragonflies
, and
true bugs Hemiptera (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Re ...

true bugs
. Phylogenetically, all insects in the
Pterygota The Pterygota (Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period (). An ...

Pterygota
undergo a marked change in form, texture and physical appearance from immature stage to adult. These insects either have
hemimetabolous Hemimetabolism or hemimetaboly, also called incomplete Metamorphosis (biology), metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode ...
development, and undergo an incomplete or partial metamorphosis, or
holometabolous Holometabolism, also called complete metamorphosis (biology), metamorphosis, is a form of insect development which includes four life stages: egg (biology), egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphy, synapomorphic tr ...
development, which undergo a complete metamorphosis, including a
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communicat ...

pupa
l or resting stage between the
larval A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development such as insects, amphibians, or cnidarians typically have a larval phase of th ...
and adult forms.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 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. More recently, scientific attention has turned to characterizing the mechanistic basis of metamorphosis in terms of its hormonal control, by characterizing spatial and temporal patterns of hormone expression relative to metamorphosis in a wide range of insects.


Temperature-Dependent Metamorphosis

According to a 2009 study, temperature plays an important role in insect development as each 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.


Recent research

According to research from 2008, adult ''
Manduca sexta ''Manduca sexta'' is a moth of the family Sphingidae present through much of the Americas, American continent. The species was Species description, first described by Carl Linnaeus in his 1763 ''Centuria Insectorum''. Commonly known as the Carol ...

Manduca sexta
'' is able to retain behavior learned as a
caterpillar Caterpillars ( ) are the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Anim ...

caterpillar
. Another 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. Many observations published in 2002, and supported in 2013 indicate that
programmed cell death Programmed cell death (PCD; sometimes referred to as cellular suicide) is the death Death is the permanent, irreversible cessation of all biological functions that sustain a living Living or The Living may refer to: Common meanings ...
plays a considerable role during physiological processes of multicellular organisms, particularly during
embryogenesis An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryogenesis
, and metamorphosis. Below is the sequence of steps in the metamorphosis of the butterfly (illustrated):
1 – The larva of a butterfly
2 – The pupa is now spewing the thread to form chrysalis
3 – The chrysalis is fully formed
4 – Adult butterfly coming out of the chrysalis Pieris rapae ''Pieris rapae'' is a small- to medium-sized butterfly Butterflies are insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arth ...

Pieris rapae
''"> File:ChristianBauer Pieris rapae caterpiller.jpg,
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
File:ChristianBauer Pieris rapae caterpiller2.jpg,
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communicat ...

pupa
File:ChristianBauer Pieris rapae cocoon.jpg, pupa ready to hatch File:ChristianBauer Pieris rapae youngadult.jpg,
adult Biologically Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

adult


Chordata


Amphioxus

In
cephalochordata A cephalochordate (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ...
, metamorphosis is
iodothyronine
iodothyronine
-induced and it could be an ancestral feature of all
chordate A chordate () is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...
s.


Fish

Some fish, both
bony fish (Osteichthyes)
bony fish (Osteichthyes)
and jawless fish (Agnatha), undergo metamorphosis. Fish metamorphosis is typically under strong control by the thyroid hormone. Examples among the non-bony fish include the
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
. Among the bony fish, mechanisms are varied. The
salmon Salmon is the common name for several species of ray-finned fish Actinopterygii ( New Latin ('having rays') + Greek ( 'wing, fins')), members of which are known as ray-finned fishes, is a clade A clade (; from grc, , ''klados'', ...

salmon
is diadromous, meaning that it changes from a
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in ...
to a
saltwater Saline water (more commonly known as salt water) is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the ...

saltwater
lifestyle. Many species of
flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish order (biology), order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the ...

flatfish
begin their life
bilaterally symmetrical Symmetry in biology refers to the symmetry observed in organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
, 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 The European eel (''Anguilla anguilla'') is a species of eel Eels are ray-finned fish Actinopterygii ( New Latin ('having rays') + Greek ( 'wing, fins')), members of which are known as ray-finned fishes, is a clade A clade (; fr ...
has a number of metamorphoses, from the larval stage to the
leptocephalus Leptocephalus (meaning "slim head") is the flat and Transparency (optics), transparent larva of the eel, marine eels, and other members of the superorder Elopomorpha. This is one of the most diverse groups of teleosts, containing 801 species in 4 ...

leptocephalus
stage, then a quick metamorphosis to glass eel at the edge of the continental shelf (eight days for the
Japanese eel The Japanese eel (''Anguilla japonica''; Japanese: 日本鰻 ''nihon'unagi'') is a species of anguillid eel Eels are ray-finned fish Actinopterygii ( New Latin ('having rays') + Greek ( 'wing, fins')), members of which are known as ray- ...
), 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
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in ...

freshwater
stage, the eel also has
phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Mol ...
because fish-eating eels develop very wide mandibles, making the head look blunt.
Leptocephali Leptocephalus (meaning "slim head") is the flat and transparent larva A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development such a ...

Leptocephali
are common, occurring in all
Elopomorpha The superorder Elopomorpha contains a variety of types of fishes that range from typical silvery-colored species, such as the tarpons and ladyfishes of the Elopiformes and the bonefishes of the Albuliformes, to the long and slender, smooth-bodi ...
(
tarpon Tarpons are fish of the genus ''Megalops''. They are the only members of the family Megalopidae. Of the two species, one (''M. atlanticus'') is native to the Atlantic, and the other (''M. cyprinoides'') to the Indo-Pacific Oceans. Species and ...

tarpon
- and
eel Eels are ray-finned fish Actinopterygii ( New Latin ('having rays') + Greek ( 'wing, fins')), members of which are known as ray-finned fishes, is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic gro ...
-like fish). Most other bony fish undergo metamorphosis from embryo to larva ( fry) and then to the juvenile stage during absorption of the yolk sac, because after that phase the individual needs to be able to feed for itself.


Amphibians

In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle.
Frogs A frog is any member of a diverse and largely carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization all ...
,
toads Toad is a common name for certain frog A frog is any member of a diverse and largely carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since preh ...
, and
newts A newt is a salamander Salamanders are a group of amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variet ...
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 A tadpole is the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
s mostly scrape food off surfaces with their horny tooth ridges. Metamorphosis in amphibians is regulated by
thyroxin
thyroxin
concentration in the blood, which stimulates metamorphosis, and
prolactin Prolactin, also known as lactotropin, is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir Jo ...
, 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 The lateral line, also called lateral line system (LLS) or lateral line organ (LLO), is a system of sensory organs found in aquatic jawed vertebrates, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensor ...
organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called
apoptosis Apoptosis (from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ) ...

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. 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, so it is truly a metamorphosis. It is not until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.


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 The axolotl (; from nci, āxōlōtl ), ''Ambystoma mexicanum'', also known as the Mexican walking fish, is a neotenic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing o ...

axolotl
, display
pedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the Physiology, physiological (or Somatic (biology), somatic) development of an organism, typically an animal. Neoteny is f ...

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.


Newts

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. Just as in tadpoles, their lungs are functional early, but newts use them less frequently than tadpoles. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase,
prolactin Prolactin, also known as lactotropin, is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir Jo ...
is the required hormone, and for adaptation to the land phase,
thyroxin
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 ''Ichthyophis'' is a genus of caecilians (limbless amphibians, sometimes called the Asian caecilians) found in Southeast Asia Southeast Asia or Southeastern Asia is the United Nations geoscheme for Asia#South-eastern Asia, southeastern subr ...
'' go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the
lateral line The lateral line, also called lateral line system (LLS) or lateral line organ (LLO), is a system of sensory organs found in aquatic jawed vertebrates, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensor ...
. More recently diverged caecilians (the Teresomata) do not undergo an ontogenetic niche shift of this sort and are in general
fossorial 230px, Cape ground squirrel A fossorial (from Latin ''fossor'', meaning "digger") animal is one adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, naked mole-rats, clams, meerkats, and mole salamande ...
throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis.


See also

*
Developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that fu ...
*
Direct developmentMarine larval ecology is the study of the factors influencing dispersing larva A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development s ...
* Gosner stage *
Hypermetamorphosis Hypermetamorphosis, or heteromorphosis,P.J. Gullan & P.S. Cranston. 2010. The Insects: An Outline of Entomology, 4th Edition. Wiley-Blackwell. is a term used in entomology upright=1.2, A Phyllium sp., mimicking a leaf Entomology () is the scie ...
*
Morphogenesis Morphogenesis (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...


References


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

{{Authority control Animal developmental biology Animal anatomy