(Dr. Girish Chandra)
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.
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. Larval-specific 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.
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.