Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.
Generally organisms with a stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics.
Etymology
The word
metamorphosis derives from Ancient Greek μεταμόρφωσις, "transformation, transforming",
from μετα- (
meta-), "after" and μορφή (
morphe), "form".
Hormonal control
In insects, growth and metamorphosis are controlled by
synthesized by
near the front of the body (anterior). Neurosecretory cells in an
insect 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).
[Davies, 1998. Chapter 3.]
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
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, molt has no juvenile hormone present at all.
[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.] Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in
hemimetabolous insects.
In chordates, metamorphosis is iodothyronine-induced and an ancestral feature of all .
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"), 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
or resting stage between the
larval and adult forms.
Development and terminology
In
Hemimetabolism insects, immature stages are called nymphs. Development proceeds in repeated stages of growth and
ecdysis (moulting); these stages are called
. 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 insects, immature stages are called and differ markedly from adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called pupa (called a "chrysalis" in butterfly species), and finally emerge as adults.
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 and
silverfish.
Hemimetabolism insects include
,
,
dragonfly, and
Hemiptera. 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 development, and undergo an incomplete or partial metamorphosis, or
Holometabolism development, which undergo a complete metamorphosis, including a
or resting stage between the
larval 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.
[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.]
Recent research
According to research from 2008, adult
Manduca sexta is able to retain behavior learned as a
caterpillar.
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 plays a considerable role during physiological processes of multicellular organisms, particularly during 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">
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
Chordata
Amphioxus
In
cephalochordata, metamorphosis is iodothyronine-induced and it could be an ancestral feature of all
.
Fish
Some fish, both
Osteichthyes and
Agnatha, undergo metamorphosis. Fish metamorphosis is typically under strong control by the thyroid hormone.
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 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 stage, the eel also has phenotypic plasticity because fish-eating eels develop very wide mandibles, making the head look blunt. Leptocephalus are common, occurring in all Elopomorpha (tarpon- and eel-like fish).
Most other bony fish undergo metamorphosis initially from fish egg to immotile larvae known as sac fry (fry with a yolk sac), then to motile larvae (often known as fingerlings due to them roughly reaching the length of a human finger) that have to foraging 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.[Mader, Sylvia, Biology 9th ed. Ch. 31][Peter B. Moyle and Joseph J. Cech Jr, Fishes: an introduction to ichthyology 5th ed. 9.3: "Development" pp 148ff]
Amphibians
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
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.
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, display
Neoteny 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 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.
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.
See also
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
