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Metamorphosis
METAMORPHOSIS
Metamorphosis is the transition during the life history of some animals from birth
or hatching to the adult stage. Metamorphosis is considered an indirect form of
development, in that a metamorphic animal passes through morphologically distinct
stages before reaching the adult form. In contrast, higher animals undergo direct
development in which young and old resemble one another except in size and sexual
maturity.
Metamorphosis occurs in at least 17 phyla of the animal kingdom, including
Porifera (sponges), Cnidaria (jellyfish), Platyhelminthes (flat worms), Mollusca (snails),
Annelida (worms), Arthropoda (insects), Echinodermata (sea urchins), and Chordata
(vertebrates).
Metamorphosis is associated with adaptive changes in the way an organism
interacts with its environment. For example, adult amphibians often eat very different
foods than their larvae. Thus, adults and larvae do not compete for food. A second
example of the adaptive significance of metamorphosis is in barnacles in which adults
are sessile but the larvae are free-swimming. Thus, the dispersal of larvae gives adults
the opportunity to colonize new habitats where the local environment might be more
favorable.
Cnidarians have varying types of metamorphosis. Some species have three
distinct life history stages: the planula larva, medusa, and polyp. The former two
stages are free swimming and the polyp stage is sessile and may involve a single
individual or a colony of individuals.
METAMORPHOSIS IN AMPHIBIA
Metamorphosis differs in many amphibian species. In frog development the
eggs hatch into tadpoles that have external gills and are vegetarian. Several significant
changes occur during metamorphosis into the adult, including growth of a large mouth
and tongue, loss of gills, formation of lungs, growth of the front legs and loss of tail.
Numerous biochemical changes accompany these morphological changes, such as
synthesis of a new visual pigment in the eyes and a new oxygen-binding hemoglobin
protein in the blood. The adult is mainly insectivorous and partly terrestrial. During
metamorphosis bones begin to ossify, the tail is reabsorbed, limbs form, and larval
respiratory and feeding structures are replaced by adult structures. The digestive system
is remodeled to accommodate strictly carnivorous diet.
METAMORPHOSIS IN INSECTS
Some of the best known cases of metamorphosis occur in insects.
The
transformation from larva to adult butterfly undergoes during pupation and is an example
of metamorphosis. The Ametabola such as silverfish do not undergo metamorphosis.
During development, these insects increase in size, but do not undergo distinct changes.
This is called direct metamorphosis or anamorphosis.
The Hemimetabola undergo gradual metamorphosis. This is exemplified by
insects such as the dragonflies, termites, roaches, and grasshoppers. In these, the
nymph hatches from the egg and resembles the adult, except that they are smaller. The
wings of the Hemimetabola grow gradually during a series of molts in which the cuticular
exoskeleton is shed, allowing for growth. This process is also known as simple, gradual
or incomplete metamorphosis. The differences between juveniles in different instars are
small, often just differences in body proportions and the number of segments.
The Holometabola, which includes moths, butterflies, beetles etc. undergo
complete metamorphosis. A worm-like larva with short legs, no wings, and simple eyes,
hatches from the egg and increases in size through a series of molts, eventually
developing into a pupa inside a cocoon. The pupa is often considered a resting stage
and can often survive in unfavorable environments. Eventually, the pupa
metamorphoses into an adult. In this process, the pupa resorbs larval organs and an
imaginal discs is formed and reshape the insect. The adult typically has wings,
compound eyes, legs, antennae, and sexual organs. Holometabolism is also known as
complete and complex or indirect metamorphosis. Whilst inside the pupa, the insect
will excrete digestive juices, to destroy much of the larva's body, leaving a few cells
intact. The remaining cells will begin the growth of the adult, using the nutrients from the
broken down larva. This process of cell death is called histolysis. Histolysis is the decay
and dissolution of organic tissues or of blood and cell regrowth histogenesis.
Histogenesis is the formation of different tissues from undifferentiated cells.
HORMONAL CONTROL
Two major classes of hormones act together to control amphibian
metamorphosis: the thyroxin from thyroid gland and prolactin from pituitary gland.
Thyroid hormones function somewhat like the molting hormones of insects, in that an
increase of their concentration relative to prolactin leads to metamorphosis of the larva
into the adult. Prolactin functions somewhat like the juvenile hormones of insects, in
that it tempers the action of the thyroid hormones. In most species, thyroid hormones
increase dramatically in concentration during metamorphosis and this stimulates
resorption of certain larval organs and differentiation of new adult organs.
The molting process is initiated in the brain, where neurosecretory cells release
prothoracicotropic hormone (PTTH) in response to neural, hormonal or environmental
factors. PTTH is a family of peptide hormones with a molecular weight of approximately
40,000, and it stimulates the production of ecdysone by the prothoracic glands.
Ecdysone, however, is not an active hormone, but a prohormone that must be converted
into an active form. This conversion is accomplished by a heme-containing oxidase in
the mitochondria and microsomes of peripheral tissues such as the fat body. Here the
ecdysone is changed to the active hormone 20-hydroxyecdysone.
The second major effector hormone in insect development is juvenile hormone
(JH). JH is secreted by the corpora allata. This hormone is responsible for preventing
metamorphosis. As long as JH is present, the hydroxyecdysone-stimulated molts result
in a new larval instar. In the last larval instar, the medial nerve from the brain to the
corpora allata inhibits the gland from producing juvenile hormone, and there is a
simultaneous increase in the body's ability to degrade existing JH (Safranek and
Williams 1989). Both these mechanisms cause JH levels to drop below a critical
threshold value. This triggers the release of PTTH from the brain. PTTH, in turn,
stimulates the prothoracic glands to secrete a small amount of ecdysone. The resulting
hydroxyecdysone, in the absence of JH, commits the cells to pupal development. Larvalspecific mRNAs are not replaced, and new mRNAs are synthesized whose protein
products inhibit the transcription of the larval messages. After the second ecysone pulse,
new pupal-specific gene products are synthesized and the subsequent molt shifts the
organism from larva to pupa. It appears, then, that the first ecdysone pulse during the
last larval instar triggers the processes that inactivate the larva-specific genes and
prepare the pupa-specific genes to be transcribed. The second ecdysone pulse
transcribes the pupa-specific genes and initiates the molt.
RETROGRESSIVE METAMORPHOSIS
Majority of metamorphoses are progressive in which larvae possess primitive
characters and as they attain adulthood they develop advanced characters, as seen in
amphibians and majority of insects. But sometimes larvae possess advanced characters
such as locomotory organs and sense organs but as they metamorphose into adult,
these advanced features degenerate and adult develops primitive features. This type of
metamorphosis is called retrogressive and is generally found in sedentary and parasitic
animals, which do not require advanced features for their mode of life.
Herdmania is a urochordate sedentary animal whose larva is tadpole-like, with a
notochord in tail, a dorsal hollow nervous system, pharyngeal gill slits, sense organs in
the form of statocyst and ocellus and muscle bundles, which are all advanced chordate
characters. As the larva attaches to a rock and undergoes metamorphosis, all chordate
characters are lost except for the pharyngeal gill slits. However, the adult attains
advancement in digestive system, circulatory system, reproductive and excretory
systems, which are useful in adult life.
Parasitic animals such as helminthes, crustancean parasites such as Sacculina,
and sedentary animals such as scale insects and lac insects, barnacles have an active
larval stage that is necessary for dispersal of the species, but as they metamorphose
into adult, advanced characters are lost as they are not needed in sedentary way of life.
Retrogressive Metamorphosis
RETROGRESSIVE METAMORPHOSIS
in Urochordates
Metamorphosis is a change from the juvenile to adult stage in which larval stage is quite
different from the adult stage. In retrogressive metamorphosis the larva possesses
advanced characters which are lost during the development and the adult is either
sedentary or degenerated with primitive characters. Urochordate adults, being sedentary
show degenerative characters while the free swimming tadpole larva shows advanced
chordate characters which are lost during metamorphosis. Parasitic crustaceans, like
Sacculina and copepod parasites and stylopids and scale insects (Insecta) also show
retrogressive metamorphosis.
Retrogressive Metamorphosis in Herdmania
The tadpole larva of Herdmania is only 1-2 mm long when it hatches out of the
egg. It does not feed and hence has only 3 hours of survival during which it has to swim
about in search of a suitable substratum for attachment. The larva needs advanced
features for its free swimming existence, which is so necessary for dispersal of the
population to distant places which the sedentary adult has no means to do.
The advanced Chordate characters of the larva

There is a rod-like notochord in the tail to which are attached muscle bands for
swimming.

There is a dorsal hollow nerve cord which is enlarged to form brain at the anterior
end. A photoreceptor ocellus and a balancing organ, the statocyst are attached to
the brain.

There are only two pairs of gill slits in pharynx but the mouth is closed by a
membrane and intestine is rudimentary.

Endostyle on the ventral side of pharynx is very well developed which functions
like thyroid gland and helps in metamorphosis.

Heart is on the ventral side of gut but is non-functional.

The larva possesses on the anterior end three ectodermal adhesive papillae which
help in firm attachment on the substratum.
Changes during metamorphosis

Larva attaches to the substratum with the help of chin warts, head downward and
tail up.

Rapid growth takes place between the chin warts (adhesive papillae) and mouth
and almost no growth on the opposite side of body.

Due to rapid growth on one side, body starts rotating in such a way that mouth
gradually migrates to the upper side.

Meanwhile pharynx enlarges and stigmata increase in numbers.

Intestine becomes functional and atrial opening is formed on the opposite side of
oral aperture.

Both tail and notochord are gradually absorbed in the body during
metamorphosis.

The hollow nerve cord is reduced into a solid nerve ganglion on the dorsal side.

Sense organs, namely ocellus and statocyst are lost.
When the metamorphosis is over, Herdmania is transformed into a bag-like sedentary
animal attached to the rock by a foot and having branchial and atrial openings for inlet
and outlet of water respectively. Pharynx becomes enormously enlarged with a large
number of stigmata for filter feeding and digestive system becomes well developed.
However, other advanced chordate characters of the larva are degenerated into simple
structures, due to which it is called retrogressive metamorphosis