We will discuss here about the life cycle of a toad.
Toad has two phases in its life cycle, the larval phase and the adult phase. It spends its larval phase in water and stays on land during adult phase. So its life starts in water.
Mating: In rainy season all adult toads come out from their hide-outs and join a party in shallow water bodies. Male toads have vocal sacs to make typical croaking sound to attract females. As the female toad approaches, the male toad mounts on the female’s back and holds it tightly with fore limbs.
Thumb pads help in making such grip tighter.
Fertilization: The female lays her eggs in water in the form of a string of jelly. The male releases its sperms over the eggs. The sperms fertilize the eggs externally in water and thus zygotes (fertilized ovum) are formed.
Larva: Soon the zygote develops into an embryo but remains as contained in egg membrane. About fourteen hours after fertilization the embryo transforms into a larva. It comes out in water by rupturing the egg membrane.
The larvae of toad look like small fishes, and are called tadpole larvae. A tadpole larva undergoes a series of changes to attain its terrestrial form. This transformation is known as metamorphosis. Young tadpole has an oval head, very short trunk and a little laterally compressed tail. It has three pairs of small external gills at two sides of its head. These are their respiratory organs. The newly hatched larvae get themselves stuck on submerged weeds with a sucker present at the ventral side of their head.
After three days, mouth develops in a newly born larva. The tail grows longer and thin membranous fin develops along the tail. At this stage larvae can swim with the help of their tail. A spiral intestine develops inside the body, which opens through a small cloaca present at the base of the tail. In this stage they feed on algae.
As they grow further, a few pores are formed at two sides of their pharynx. These are the gill slits. The skin folds between these gill slits form three pairs of internal gills. By this time the external gills disappear and the internal gills get covered by thin gill covers, called operculum. Gradually operculum fuses with the body leaving a small opening at the left back side. It is called spiracle which helps in breathing.
After some more days a pair of small pouches develops at the base of the pharynx, which finally grow into two balloon shaped lungs. But the lungs do not function till the internal gills remain active. By this time a pair of bud like structures appears at two sides of the base of the tail, which grow into hind limbs. After few days the fore limbs grow, the membranous fin of the tail and the internal gills gradually disappear. The larva then starts breathing with its lungs. Within a few weeks the tadpole looks like a tiny toad with a tail. Soon it gives up feeding and goes on fasting till its tail gets absorbed into the body. Meanwhile the long spiral intestine becomes shorter and true jaws develop in mouth. At this stage the tiny toad leaves water and starts its terrestrial life on land.