Amphibians are the only group of vertebrates that undergo metamorphosis during their lifetime.The need for this change in form stems from the fact that this group of organisms has never completely given up their dependence on water. The adults are terrestrial generally requiring moist habitats but they must return to water to reproduce. The transition from an aquatic to a terrestrial habitat requires physical and physiological adaptations. Thus the life cycle of the amphibian is quite unique in the vertebrate world.
Life cycle of a frog:
Frogs lay their eggs in the shallow still water a few days after the adults begin their mating calls. Eggs found in local ponds and sloughs will come in gelatinous masses of a hundred or more eggs attached to old stems of bulrushes and other emergent water plants. If collected locally, only a few eggs should be taken for study in the classroom so as to have minimal impact on the population of frogs and to have a healthy manageable number of eggs and tadpoles in the aquarium. Separating the eggs in the center of the small mass will ensure the eggs get enough oxygen during development. In this way shallow dishes with just enough pond water to cover the eggs are all that is needed for successful hatching although the eggs should be placed in the aquarium as soon as possible.
At room temperature, the eggs will hatch in 5 to 7 days after they are deposited.The process is delayed if the eggs are kept in the refrigerator. Once in the aquarium the eggs should be kept mainly in indirect sunlight although some direct sunlight is fine, Too much sun may raise the water temperature too high, driving out oxygen and possibly increasing decomposition of organic matter at the bottom which also uses up oxygen.
Just after hatching, the embryo absorbs the last remaining nutrient from the egg yolk, so no food is needed by the young tadpoles for the next 3 to 4 days.
The newly hatched tadpole has no eyes or mouth and external feathery gills on the sides of its head. If care is exercised, it should be possible to observe the tadpole in a petri dish or other shallow container under a microscope or strong magnifying glass.
In a few days eyes will have formed, the feathery external gills will have been replaced by new ones under the skin, and a small round mouth with a parrot like beak and small teeth will be available for eating.
At first, the newly hatched tadpoles will be relatively inactive-clinging to the sides or resting on the bottom of the container. Within day or two they will become more active and will begin to feed. The natural food of a frog tadpole includes a variety of water plants and decaying vegetable material. Decaying vegetable matter could be collected from the slough with the eggs and green plant matter, particularly duck weed which can be added as it becomes available. tadpoles, however; will live very nicely on pieces of raw or boiled lettuce, small bits of boiled egg yolk, or goldfish food.
The most successful mixture for feeding the tadpoles once their mouths have developed is boiled lettuce. Boil the lettuce for 10 minutes and then cool. Gauge how much to feed your tadpoles by trying a certain amount and seeing how much they eat in 2-3 hours. You can prepare lettuce in advance and keep it in the refrigerator for several days.
It is important that the tadpoles are not overfed because the uneaten food will decay fouling up the water and reducing the oxygen content of the water. If the water becomes cloudy it should be changed immediately with new pond water, rain water or aged tap water.
An aerator should be used to add fresh oxygen to the water throughout the time of rearing the tadpoles. If this cannot be done about 1./5 to 1/4 of the water should be poured off each day and replaced with new water as would be done with a goldfish in a bowl.
Tadpoles need to grow and develop for about two to three months before they undergo the final transformation into adults. During most of this time the body remains spherical with a tail about two to three times the length of the body. Tadpoles have a sucker-like mouth with rasping teeth and they breathe through their gills. As they near the latter stages of growth and enter metamorphosis they become carnivores, consuming more and more small aquatic organisms as their need for protein increases to develop their internal organs and the legs characteristic of the adult. During this stage, fish food provides the necessary protein. Small bits if boiled egg are also a good source of protein, but uneaten bits should be removed so as not to contaminate the water.
The first noticeable change of metamorphosis is the appearance of the hind legs. Later, the forelegs appear and the tadpole switches fro gill breathing to lung breathing. Once the metamorphosis reaches this stage the tadpole or pollywogs must be able to crawl out of the water to prevent drowning. Solid surfaces such as floating leaves, or pieces of wood that just protrude above the water must be placed in the aquarium for this purpose. If the water is shallow then rocks can be used. During the final stages of metamorphosis the pollywogs will not eat because the internal organs are being transformed into their final form. During this time the tail will be reabsorbed and gradually disappear. The juvenile frogs now are carnivorous feeding solely on insects.
It is advised to release the juveniles as soon as possible in a moist location since the adults are more difficult to provide for, particularly with regards to insects that they need for food.