Metamorphosis is nothing but the changes that occur during a life cycle of an animal. Hence, during the metamorphosis period of a frog (tadpole), the egg first hatches into a tadpole which then develops legs, and at last, it will develop into a full-grown frog!
Anurans (a tailless amphibian of the order Anura) include frogs, toad, and tree frogs.
The fertilized egg of a frog develops into an immature young called embryo. The embryo gradually elongates and develops a postanal tail (as shown in the figure below).
The head becomes prominent. The nasal pits deeper. A shallow median depression called stomodaeum is formed at the anterior end of the head.
A pair of adhesive glands or oral suckers develop on the ventral side of the head. The myotomes or muscle bands appear as > shaped bands on each side of the trunk and tail. The tail becomes prominent with dorsal and ventral fins.
Finally, the embryo hatches by wriggling out of the jelly-like capsule around it.
At hatching, the embryo appears as a little blind creature and is nearly 5.6 mm. in length. The time taken for development from fertilization up to hatching depends upon temperature.
At 26-28°C the embryos of Indian bullfrog (Rana tigrina) hatches in about 24 hours but at a lower temperature, the period may be longer.
Tadpole on hatching
On hatching, the embryo attaches itself to the vegetation of the pond or tank with its oral sucker. It derives its nourishment from the yolk present in its body.
In this condition, it remains for a couple of days.
During this period, the following developments occur:
- The body becomes fusiform with ciliated ectoderm.
- The mouth opens on the ventral side of the head.
- Horny jaws and horny teeth develop around the mouth.
- The alimentary canal is in the form of a straight tube in the beginning but later on, it becomes long and much coiled.
- Three pairs of external gills develop in the anterior region.
- The internal gills develop by the perforation of the pharynx and open into the opercular chamber. The opercular chamber opens outside by an aperture called spiracle on the left side.
- The heart is recognized as a simple s-shaped tube.
- The rudimentary eyes now become functional.
- The tail is short and compressed and becomes a powerful organ of locomotion with dorsal and ventral fin.
- The color of the body at this stage is whitish with dark pigment granules.
The embryo gradually becomes a free-swimming, actively feeding tadpole larva. It is herbivorous feeding on algae, moss, and floating vegetation.
The alimentary canal has a long coiled intestine for the complete digestion of vegetable matters. Feeding becomes intensive and growth takes place rapidly.
The tail becomes the powerful organ of locomotion. The oral suckers disappear. The larva appears like a fish in its body form, mode of swimming, breathing, and possessing the lateral line system.
Towards the later part of the larval life, the hind limbs appear as small buds on either side of the cloaca, and then appear the forelimb buds on either side behind the region of the head.
The internal gills start degenerating and the lungs appear as buds from the floor of the pharynx.
The larva comes to the surface of water frequently to gulp in the air for the process of respiration. The vascular system tends to become modified accordingly.
The larval life lasts for about 2 to 3 weeks depending on the temperature and availability of the food materials. The fish-like tadpole gradually changes into a miniature frog through the process of metamorphosis.
Metamorphosis of Tadpole Larva
What is metamorphosis?
Metamorphosis (GK, metamorphoum – to transform) means the radical changes which occur during the transformation of an organism from its larval stage to the adult stage with corresponding changes in form and structure of the former to the later stage.
During such changes some of the larval characters are lost while other characters are modified, giving rise to the adult characters.
The tadpole larva is met within the life cycle of a frog. This larva undergoes metamorphosis so as to attain adulthood through a series of changes.
How metamorphosis is caused?
The onset of metamorphosis is marked by the functionally developed thyroid glands which pour their hormone secretion called thyroxine into the bloodstream.
This hormone initiates and controls metamorphosis in the tadpole larva of the frog. The thyroid gland is activated by the action hypothalamus and anterior pituitary of the brain.
When the thyroid gland is removed from a tadpole metamorphosis does not occur.
Necessity of metamorphosis
The tadpole belongs to the aquatic phase in the life cycle of a frog. They are fish-like creatures adapted for life in water.
The special features are (i) a fish-like body (ii) presence of internal gills for respiration, (iii) fish-like heart and circulatory system, (iv) a powerful tail with dorsal and ventral fins for swimming (v) presence of lateral line sense organ and (vi) excretion of nitrogenous wastes in the form of ammonia.
The larva, thus, differs structurally and functionally from the adult.
The tadpole gradually changes into a miniature frog, which grows to maturity and during which many changes take place in the form and structure accompanied by physiological changes.
Here the aquatic fish like free swimming, water breathing, and the herbivorous tadpole are to be converted into a semiterrestrial, air-breathing, leaping, and carnivorous frog.
Therefore, metamorphosis is greatly necessary.
Changes during metamorphosis
The early metamorphic changes are slow and gradual and occur towards the latter part of the tadpole’s life.
These changes are:
- Increase in the length of hindlimb
- Thickening of skin
- Formation of mucous glands in the skin
- Development of belly muscles
- Formation of the pelvic girdle.
The tadpole ultimately stops growing in length, ceases feeding, and enters the phase of metamorphic climax.
A. Morphological and Structural Changes
The morphological and structural changes which occur during metamorphic climax can be classified into three categories.
1. Regressive or destructive changes
These include the reduction or complete disappearance of structures that are useless for adults. These changes are as follows:
- The tail begins to be absorbed gradually by the wandering amoeboid cells or phagocytes by the process of phagocytosis. The phagocytes possess a large number of lysosomes containing digestive enzymes called cathepsin. The tissues of the tail are dissolved into liquid and are absorbed. Thus the tail is gradually eaten up. The process is also known as histolysis.
- The larval horny jaws and horny teeth also vanish.
- The lateral line sense organ disappears.
- The internal gills become completely closed.
- Opercular chamber disappears.
- The external gills fall off.
2. Progressive or constructive changes
These include the development of certain structures needed for adult life. They are as follows:
- The hind limbs increase in length and strength.
- The forelimbs emerge.
- The lungs do enlarge.
- Formation of the middle ear and tympanum take place.
- Tongue gradually increases in size and vomerine teeth develop.
- The mouth opening becomes wide.
- Eyes enlarge and bulge out on the head.
- Nictitating membranes are formed in the eyes.
3. Modification or remodeling of structures
Some larval structures which are useful for adult life are modified or remodeled. Changes of this kind are as follows:
- The heart becomes three-chambered.
- The blood vascular system which was fish-like becomes frog-like.
- Some aortic arches disappear.
- Blood supply to the lungs and skin is established.
- The intestine which was much coiled for herbivorous diet becomes shortened for carnivorous feeding.
- The color pattern of the body changes. The pigmented larval skin is cast off and the skin becomes lighter with dark patches on the dorsal side.
- The cartilaginous skeleton becomes bony.
- The larval pronephric kidney changes into the mesonephric kidney.
- The brain becomes more highly differentiated.
B. Physiological Changes
The Physiological changes occurring during metamorphosis are many.
Some of the important changes are as follows:
- The liver takes an active part in the metabolism of carbohydrates.
- The pancreas produces the hormone insulin from its endocrine glands.
- The excretory product changes from ammonia to urea.
- The larval hemoglobin changes to the adult type hemoglobin.
- The peptic activity starts in the stomach to enable the froglets to digest the animal diet.
C. Changes in the habit and habitat
- With the beginning of metamorphosis, the larva stops feeding till it can pray upon its animal food. Herbivorous diet changes into the carnivorous diet.
- It frequently rises to the surface to gulp in air and inflate the lungs. Glandular skin makes the availability of oxygen requirements.
- The miniature frog begins to take short excursions on land but not far from the water.
The changes convert the tadpole larva into a miniature frog that comes out of water and hops on land. The life cycle of the frog is depicted in the figure.
Factors affecting Metamorphosis
In Rana tigrina the entire process of metamorphosis takes about one week, provided the environmental conditions are normal.
Besides hormonal control, certain environmental factors affect the process of metamorphosis. These factors include temperature, crowding, starvation, illumination, chemicals, and supplementary diet.
For normal metamorphosis, 20-25°C temperature is necessary. By altering the temperature the time span of metamorphosis can be altered.
Summary on Metamorphosis of Tadpole
A larval stage occurs in the life cycle of a frog. Its developmental pattern is egg-fertilized egg-embryo-larva-adult.
The fertilized egg develops into an embryo which through further differentiation becomes a larva. The larva of a frog is called a tadpole larva.
The tadpole larva differs structurally and functionally from an adult frog. Its habit, habitat, and mode of feeding are quite different.
The larva is fish-like in its body form and swims utilizing a powerful tail with a dorsal and ventral fin.
Respiration takes place initially through external gills, followed by internal gills. The larva is herbivorous in nature. There is a lateral line system just like the fish.
Towards the later part of larval life, the hind limbs appear as buds. The larval life lasts for about 2 to 3 weeks depending on temperature and availability of food.
The tadpole is gradually transformed into a miniature frog. For achieving this, many changes do take place.
Here, the aquatic fish-like, free swimming, water breathing, and the herbivorous tadpole is to be converted into a semi-terrestrial air-breathing, leaping, and carnivorous frog.
Therefore, metamorphosis becomes greatly necessary.
The morphological and structural changes associated with metamorphosis are (a) regressive or destructive and (b) progressive or constructive.
The regressive changes include the reduction or complete disappearance of structures that are useless for adults.
These include the disappearance of a tail, lateral line, opercular chamber, external and internal gills.
The progressive changes include the development of certain structures necessary for adult life.
These include strengthening of hind limbs, the appearance of forelimbs, formation of middle ear and tympanum, widening of mouth, the appearance of vomerine teeth, and development of nictitating membrane.
Some larval structures useful for adult life undergo modification. These include the heart, blood vessels, blood supply, intestine, kidney, and color pattern of the skin.
The physiological changes associated with metamorphosis are many. They are mostly centered around the liver, pancreas, excretory products, hemoglobin, diet, and peptic activity.
These changes convert the tadpole larva into a miniature frog that comes out of water and hops on the ground. The aquatic life now becomes amphibious.