Metamorphosis
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Metamorphosis is a
biological process Biological processes are those processes that are necessary 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 ...
by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell
growth Growth may refer to: Biology *Auxology, the study of all aspects of human physical growth *Bacterial growth *Cell growth *Growth hormone, a peptide hormone that stimulates growth *Human development (biology) *Plant growth *Secondary growth, growt ...
and differentiation. Some
insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s,
jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...
,
fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
,
amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
s,
mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000  extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...
s,
crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
s,
cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
ns,
echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s, and
tunicate Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping 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 ...
s undergo metamorphosis, which is often accompanied by a change of
nutrition Nutrition is the biochemistry, biochemical and physiology, physiological process by which an organism uses food and water to support its life. The intake of these substances provides organisms with nutrients (divided into Macronutrient, macro- ...
source or
behavior Behavior (American English) or behaviour (British English) is the range of actions of Individual, individuals, organisms, systems or Artificial intelligence, artificial entities in some environment. These systems can include other systems or or ...
. Animals can be divided into species that undergo complete metamorphosis (" holometaboly"), incomplete metamorphosis (" hemimetaboly"), or no metamorphosis (" ametaboly"). Generally organisms with a
larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
l stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics.


Etymology

The word ''metamorphosis'' derives from
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
, "transformation, transforming", from ('), "after" and ('), "form".


Hormonal control

In insects, growth and metamorphosis are controlled by
hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...
s synthesized by
endocrine gland The endocrine system is a network of glands and organs located throughout the body. Along with the nervous system, it makes the neuroendocrine system, which controls and regulates many of the body's functions. Endocrine glands are ductless gland ...
s near the front of the body (
anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
). Neurosecretory cells in an insect's brain secrete a hormone, the
prothoracicotropic hormone Prothoracicotropic hormone (PTTH) was the first insect hormone to be discovered. It was originally described simply as "brain hormone" by early workers such as Stefan Kopeć (1922) and Vincent Wigglesworth (1934), who realized that ligation of th ...
(PTTH) that activates prothoracic glands, which secrete a second hormone, usually
ecdysone Ecdysone is a prohormone of the major insect molting hormone 20-hydroxyecdysone, secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. Ecdy ...
(an
ecdysteroid Ecdysteroids are arthropod steroid hormones that are mainly responsible for molting (ecdysis), development and, to a lesser extent, reproduction; examples of ecdysteroids include ecdysone, 20-hydroxyecdysone (ecdysterone), turkesterone and 2- ...
), that induces
ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnant ...
(shedding of the exoskeleton). PTTH also stimulates the
corpora allata In insect physiology and anatomy, the corpus allatum (plural: corpora allata) is an endocrine gland that generates juvenile hormone; as such, it plays a crucial role in metamorphosis. Surgical removal of the corpora allata (an allatectomy) can cau ...
, a retrocerebral organ, to produce
juvenile hormone Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. The first discovery of a JH was by Vincent Wigglesworth. JHs regulate development, reproduction, diapause, and polyphenisms. In ...
, which prevents the development of adult characteristics during
ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnant ...
. In holometabolous insects, molts between larval
instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...
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 imaginal, molt has no juvenile hormone present at all. Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in
hemimetabolous Hemimetabolism or hemimetaboly, also called partial metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode of development of certain ...
insects. In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s.


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 development Direct development is a concept in biology. It refers to forms of growth to adulthood that do not involve metamorphosis. An animal undergoes direct development if the immature organism resembles a small adult rather than having a distinct larval ...
"), 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 A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
l or resting stage between the
larval A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
and adult forms.


Development and terminology

In
hemimetabolous Hemimetabolism or hemimetaboly, also called partial metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode of development of certain ...
insects Insects (from Latin ') are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed ...
, immature stages are called
nymphs A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...
. Development proceeds in repeated stages of growth and
ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnant ...
(moulting); these stages are called
instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...
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. In
holometabolous Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphic trait of all insects in the clade Holometabola. Immatur ...
insects, immature stages are called
larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
e and differ markedly from adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called
pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
(called a "chrysalis" in butterfly species), and finally emerge as adults.


Evolution

The earliest insect forms showed direct development (
ametabolism Ametabolism 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: the orders Archaeognatha and Zygentoma.McGavin, George C. ''E ...
), 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 bristletails and
silverfish The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect in the order Zygentoma (formerly Thysanura). Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearanc ...
.
Hemimetabolous Hemimetabolism or hemimetaboly, also called partial metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode of development of certain ...
insects include
cockroach Cockroaches (or roaches) are insects belonging to the Order (biology), order Blattodea (Blattaria). About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known Pest (organism), pests. Modern cockro ...
es,
grasshopper Grasshoppers are a group of insects belonging to the suborder Caelifera. They are amongst what are possibly the most ancient living groups of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago. Grassh ...
s,
dragonflies A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of dragonflies are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threate ...
, and
true bugs Hemiptera (; ) is an order (biology), order of insects, commonly called true bugs, comprising more than 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, assassin bugs, Cimex, bed bugs, and shield bugs. They ...
. Phylogenetically, all insects in the
Pterygota Pterygota ( ) is a subclass of insects that includes all winged insects and groups who lost them secondarily. Pterygota group comprises 99.9% of all insects. The orders not included are the Archaeognatha (jumping bristletails) and the Zygent ...
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 partial metamorphosis and paurometabolism,McGavin, George C. ''Essential Entomology: An Order-by-Order Introduction''. Oxford: Oxford University Press, 2001. pp. 20. is the mode of development of certain ...
development, and undergo an incomplete or partial metamorphosis, or
holometabolous Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphic trait of all insects in the clade Holometabola. Immatur ...
development, which undergo a complete metamorphosis, including a
pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
l or resting stage between the
larval A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
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.


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.


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. The species was first described by Carl Linnaeus in his 1763 ''Centuria Insectorum''. Commonly known as the Carolina sphinx moth and the tobacco hawk mo ...
'' is able to retain behavior learned as a
caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder ...
. Another caterpillar, 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. Many observations published in 2002, and supported in 2013 indicate that
programmed cell death Programmed cell death (PCD) sometimes referred to as cell, or cellular suicide is the death of a cell (biology), cell as a result of events inside of a cell, such as apoptosis or autophagy. PCD is carried out in a biological process, which usual ...
plays a considerable role during physiological processes of multicellular organisms, particularly during
embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...
, and metamorphosis. Additional research in 2019 found that both
autophagy Autophagy (or autophagocytosis; from the Greek language, Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-depe ...
and
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
, the two ways programmed cell death occur, are processes undergone during insect 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 species of the whites-and-yellows family Pieridae. It is known in Europe as the small white, in North America as the cabbage white or cabbage butterfly, on several continents as the small ca ...
''"> File:ChristianBauer Pieris rapae caterpiller.jpg,
larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
File:ChristianBauer Pieris rapae caterpiller2.jpg,
pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
File:ChristianBauer Pieris rapae cocoon.jpg, pupa ready for eclosion File:ChristianBauer Pieris rapae youngadult.jpg,
adult An adult is an animal that has reached full growth. The biological definition of the word means an animal reaching sexual maturity and thus capable of reproduction. In the human context, the term ''adult'' has meanings associated with social an ...


Chordata


Amphioxus

In
cephalochordata The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets div ...
, metamorphosis is iodothyronine-induced and it could be an ancestral feature of all
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s.


Fish

Some fish, both 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 a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...
. Among the bony fish, mechanisms are varied. The
salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...
is
diadromous Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousa ...
, meaning that it changes from a
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
to a
saltwater Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish wat ...
lifestyle. Many species of
flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish Order (biology), suborder Pleuronectoidei, also called the Heterosomata. In many species, both eyes lie on one side of the head, one or the other migrating through or around ...
begin their life
bilaterally symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symme ...
, 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. Their life history was a mystery for thousands of years, and mating in the wild has not yet been observed. The five stages of their development were originally thought to be differe ...
has a number of metamorphoses, from the larval stage to the
leptocephalus A leptocephalus (meaning "slim head") is the flat and Transparency (optics), transparent larva of eels and other members of the superorder Elopomorpha. This is one of the most diverse groups of teleosts, containing 801 species in 4 orders, 24 f ...
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''; ) is a species of anguillid eel found in Japan, Korea, Taiwan, China, and Vietnam, as well as the northern Philippines. Like all the eels of the genus ''Anguilla'' and the family Anguillidae, it is catadrom ...
), 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 containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
stage, the eel also has
phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...
because fish-eating eels develop very wide mandibles, making the head look blunt.
Leptocephali A leptocephalus (meaning "slim head") is the flat and transparent larva of eels and other members of the superorder Elopomorpha. This is one of the most diverse groups of teleosts, containing 801 species in 4 orders, 24 families, and 156 gener ...
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-bodie ...
(
tarpon Tarpon 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 ...
- and
eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
-like fish). Most other bony fish undergo metamorphosis initially from
egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...
to immotile larvae known as ''sac fry'' ( fry with a
yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
), then to motile larvae (often known as fingerlings due to them roughly reaching the length of a
human finger A finger is a prominent digit (anatomy), digit on the forelimbs of most tetrapod vertebrate animals, especially those with prehensile extremities (i.e. hands) such as humans and other primates. Most tetrapods have five digits (dactyly, pentadact ...
) that have to
forage Forage is a plant material (mainly plant leaves and stems) eaten by grazing livestock. Historically, the term ''forage'' has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used m ...
for themselves after the yolk sac resorbs, and then to the juvenile stage where the fish progressively start to resemble
adult An adult is an animal that has reached full growth. The biological definition of the word means an animal reaching sexual maturity and thus capable of reproduction. In the human context, the term ''adult'' has meanings associated with social an ...
morphology and behaviors until finally reaching
sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans, it is related to both puberty and adulthood. ''Puberty'' is the biological process of sexual maturation, while ''adulthood'', the condition of being socially recognized ...
.


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 semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to ...
,
toads Toad (also known as a hoptoad) is a common name for certain frogs, especially of the family Bufonidae, that are characterized by dry, leathery skin, short legs, and large bumps covering the parotoid glands. In popular culture (folk taxonomy), ...
, and
newts A newt is a salamander in the subfamily Pleurodelinae. The terrestrial juvenile phase is called an eft. Unlike other members of the family Salamandridae, newts are semiaquatic, alternating between aquatic and terrestrial habitats. Not all aqua ...
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 or polliwog (also spelled pollywog) is the Larva, larval stage in the biological life cycle of an amphibian. Most tadpoles are fully Aquatic animal, aquatic, though some species of amphibians have tadpoles that are terrestrial animal, ...
s mostly scrape food off surfaces with their horny tooth ridges. Metamorphosis in amphibians is regulated by
thyroxin File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 2 ...
concentration in the blood, which stimulates metamorphosis, and
prolactin Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secr ...
, 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 the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelia ...
organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
. 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. 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.


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 ) (''Ambystoma mexicanum'') is a neoteny, paedomorphic salamander, one that Sexual maturity, matures without undergoing metamorphosis into the terrestrial adult form; adults remain Aquatic animal, fully aquatic with obvio ...
, display
pedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism ...
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. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase,
prolactin Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secr ...
is the required hormone, and for adaptation to the land phase,
thyroxin File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 2 ...
. 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, the southern Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, ...
'' 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 the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelia ...
. More recently diverged caecilians (the Teresomata) do not undergo an
ontogenetic niche shift Ontogenetic niche shift (abbreviated ONS) is an ecological phenomenon where an organism (usually an animal) changes its diet or habitat during its ontogeny (development). During the ontogenetic niche shifting an ecological niche of an individual c ...
of this sort and are in general
fossorial A fossorial animal () is one that is adapted to digging and which lives primarily (but not solely) underground. Examples of fossorial vertebrates are Mole (animal), moles, badgers, naked mole-rats, meerkats, armadillos, wombats, and mole salamand ...
throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis.


See also

* * * * * *


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