Metamorphosis in Amphibians

Metamorphosis in Amphibians "During the development of Anurans (tail-less amphibians), a free swimming larval stage present called tadpole which is herbivorous." "As the result of metamorphosis, it gets converted into a miniature adult which is amphibious and carnivorous." "During the metamorphosis of larva, following changes occur which are as follows:" "1. Ecological changes" "2. Morphological changes" "3. Physiological and biochemical changes" Ecological Changes "According to the change of environment, the fish-like aquatic tadpole larva changes its habitat to become the terrestrial frog." "Its feeding habit changes from herbivorous to insectivorous. But in urodela amphibians (salamanders & newts), there is no substantial change of diet as the larva and adult both carnivorous." Morphological Changes "The changes in the morphology of the animal during metamorphosis are in part progressive and in part regressive." Regressive Metamorphosis Changes of Anura "True it may be grouped into categories, but most organs necessary during larval life but most organs are reduced in adult." "Some organs develop and become functional only during and after metamorphosis." "Some organs present and functional in both larva and adult." Regressive Metamorphic Changes "Retain larval characters such as external gills, tail with finfold are not needed in the adult. Thus this character gets lost in adult." "Gill clefts are closed. The shape of mouth change. Lateral lines organs of skin disappear." Progressive Metamorphic Changes "The progressive metamorphic changes involves the progress development of limb, which increase in size and differentiation." "The forelimb develops and three gill arches become modified into the hyoid apparatus." "The eye protrudes on the dorsal surface of the head and develops eyelids." Regressive Metamorphic Changes (Continued) "Some organs function in both larva and adult but change their differentiation during metamorphosis." "The single layer skin becomes stratified (many layers)." "Intestine is very long in tadpole which becomes shorter in adult." "Brain becomes more highly differentiated." "In urodela amphibians, undergo less striking ecological and morphological metamorphic changes." "The tailed is retained in larva & adult." Physiological Changes (Biochemical Metamorphic Changes) "The activity of pepsin and trypsin increases as the result show high protein diet due to carnivorous habitat." "Insulin secretion begins from the pancreas, it increases the utilization of glycogen." "Urea synthesis begins in the liver, thus they become ureotelic adults from ammoniotelic larva." "Kidney tubules show marked re-absorption of H2O due to adaptation for terrestrial life." "In the retina of larva, the rod contains porphyrosin (derivative of vitamin A2), but during metamorphosis, it is converted into rhodopsin (vitamin A1)." Oxygen Loading and Unloading Capacity "Oxygen loading capacity of blood decreases but unloading capacity increases in the adult because adult become terrestrial and gets oxygen from air." Hormonal Control of Amphibian Metamorphosis "Initiation of metamorphosis begins the hypothalamus region of brain." "It is influenced by temperature, feeding stage, iodine concentration of surrounding water." "Proper environmental stimuli activate the neurosecretory cell of hypothalamus to release thyrotropin-releasing factor hormone." "Thyroid-releasing factor stimulates the anterior pituitary glands, which release thyroid-stimulating hormone (thyrotropin)." "Thyrotropin stimulates the thyroid gland to release thyroxine." "Thyroxine responsible for control of amphibian metamorphosis, it can be proved by following experiment:" "(1) If the thyroid gland of tadpole is removed, the tadpole larva fails to undergo metamorphic. Thus the larva becomes a giant tadpole." "(2) If the tadpole is treated with thyroxine, it shows rapid metamorphosis, producing a small frog which should dies." Hormonal Control of Amphibian Metamorphosis (Continued) "Long exposure of very low doses of thyroid promoter, the differentiation of 8-digit stage in the limb." "Higher level of concentration is needed for formation of 5-digit stage." Necturus, Siren & Amphiuma "In urodela, three conditions are same:" "1. Necturus - No metamorphosis" "2. Siren & Amphiuma - Portal metamorphosis" "3. Axolotel larva - Delayed metamorphosis" Metamorphosis (Post-embryonic Development) "Meta = Change" "Morphe = form" "The metamorphosis is a process by which the larva is transformed into an adult." "Metamorphosis occurs in metazoan phyla, which begins from the phylum Porifera (Amphiblastula) and ends with the amphib (tadpole)." "The animal which show indirect development includes a variety of larva that undergo metamorphosis." "Basically metamorphosis are of two types:" Retrogressive Metamorphosis "The developmental process that transforms the advanced larva into the primitive adult is called retrogressive metamorphosis." "It occurs in Ascidians (Hermania)." Progressive Metamorphosis "The developmental process that the simple larval form is transformed into the more complex adult is called progressive metamorphosis." "It occurs in Insect, Amphibia." Insect Metamorphosis "Insect metamorphosis is a special form of moulting." "In the moulting process, the old cuticle casting (shedding off) and the new cuticle are gained." Moulting/Ecdysis "As we know that an exoskeleton of insect is dead (non-living) and can not grow." "Thus the old exoskeleton is shedding periodically and a new one are secreted from the underline hypodermis. This process is called Moulting or Ecdysis." Internal Changes During Metamorphosis "Moulting of the cuticle occurs at a fixed time, and the interval between two moults is called stadium." "The form that takes during a particular stadium is called instar." "The final or last instar is a fully developed form called an adult or imago." "Thus the overall changes in form between the first larval instar and the adult is called metamorphosis." Type of Insect Metamorphosis "On the basis of variation in metamorphosis process, the insects can be classified into two main groups." Ametabolous (Ametamorphosis / Epimorphosis / Mon metamorphosis) "The wingless insects such as silverfish, springtail do not undergo the process of metamorphosis." "After egg-laying, soon after the egg hatched into a juvenile (nymph) stage like adult except in size, color, and sexual maturity." Metamorphosis (Continued) "As they grow, they molt several times and finally reach the sexual maturity and performed adult junction." "Egg -> Juvenile -> Adult" Metabola (Metamorphosis) "In the more advanced insect Pterygota, during growth and development, the some distinct changes in form and size are observed." "It is again divided into two parts:" "(i) Hemimetabola (incomplete metamorphosis)" "(ii) Holometabola (complete metamorphosis)" Hemimetabola (Incomplete Metamorphosis) "Some insects like fly, stone fly, and dragon fly show incomplete metamorphosis." "They have aquatic young called nymphs." "The aquatic nymphs contain tracheal gills for respiration and some other modifications are also occur for adaptive aquatic life." "When as adults, they are terrestrial or aerial and respire by tracheal." "The nymphs are similar to adult except for size, body proportion, and lack of wings that gradually develop during the several molts." Hemimetabola (Incomplete Metamorphosis) (Continued) "These wings are developed externally as bud-like outgrowths in the early instar and increase in size as the animal grows by successive molts, become into adult." "Thus the nymphs ready to leave the water." "In body becomes dry, the exoskeleton splits and wings are developed. So the nymphs emerge to aerial habitat with the tracheal respiration." "So the developmental stages are:" "Egg -> Series of Nymphs -> Adult" Holometabola (Complete Metamorphosis) "The large group of insects show complete metamorphosis; their life cycle passes through the complex process of development." "Thus those insects which show complete metamorphosis consist of four stages in their life cycle:" "Egg -> Larva -> Pupa -> Adult" "Complete metamorphosis in butterfly"