Frogs are a very common type of amphibian and widely distributed throughout the planet. They depend to a great extent on water, since they need it to breathe, and their life cycle includes several different phases, well distinguishable and that can vary from one species to another. Like all species, they are a fundamental factor in maintaining the balance of their ecosystems.
Stages and images of the life cycle of a frog
As a summary, we can indicate that the life cycle of a frog has the following stages or phases , starting from the moment of reproduction and the laying of the eggs:
- Egg laying and embryonic stage.
- Hatching of the eggs or birth of the tadpoles.
- Metamorphosis of tadpoles to adult frogs.
- Reproduction of adult frogs.
From this last point, the life cycle of frogs begins again with the first point. An adult frog reproduces several times throughout its life until it dies.
Embryonic stage of frogs
Frogs, like all amphibians , are oviparous animals , that is, they have sexual reproduction through eggs. Frog eggs are characterized by having a large amount of yolk, which is a reserve of nutrients that the embryo will use to complete its development. This yolk has bilateral symmetry.
Frogs also use a reproductive strategy consisting of laying many eggs and then not investing much effort in caring for and protecting the young, in the hope that, although many die due to lack of protection, at least several will reach adulthood. In ecology this is known as “R strategists” or “R reproductive strategy. Eggs, moreover, are usually submerged in rafts of water and protected by some type of mucilage or gelatinous substance that holds them together.
The embryonic development of the frog is common with that of other amphibians. Some of the most important stages are:
- Blastulation or blastula formation: the fertilized oocyte begins to divide into a “ball” of cells (morula). This ball then remains hollow in the center. This structure is known as a “blastula”.
- Gastrulation: A cavity is formed that is reintroduced into the cavity of the blastula. The axis that follows is determined by the point where fertilization has occurred. The inner layer is called endoderm, and from it will derive the internal organs, such as the digestive system; and the upper ectoderm layer, which will give rise to the skin and other superficial organs.
- Formation of the notochord: the frogs what we know as “chordate” animals, among which we also include ourselves and all other vertebrates. The notochord or chord is an elongated embryonic structure that determines the formation of the nervous system.
- Formation of the mesoderm: the mesoderm is another embryonic layer that gives rise to other internal organs, such as the circulatory system.
When are frog tadpoles born? Very easy, when the entire embryonic period has concluded. Once the tadpole is perfectly formed and able to survive on its own in the wild, the eggs hatch and the frog tadpoles emerge into the water. It is no coincidence that frog eggs are submerged or very close to water: tadpoles need it to live, that is, at this stage they would not survive on dry land. The tadpole has the appropriate characteristics for this life: it has a caudal or tail fin, it lacks lateral appendages and its respiratory system is of the branchial type.
In ecology, it is estimated that the species that undergo metamorphosis, among others, do so to avoid competition from adults with their own offspring, occupying different ecological niches. In general, tadpoles live in water and are herbivores , while frogs live on the periphery of water and have a more extensive diet.
The characteristics that occur in the metamorphosis phase of the tadpole must disappear to give rise to the adult specimen. The metamorphosis of the frog begins , in which the tadpole will undergo at least the following changes:
- Appendage formation: frogs need two front and two rear legs (haunches) to live.
- Tail retraction: the tadpole’s tail disappears little by little.
- Development of the respiratory system: frogs have a mixed respiratory system in which the exchange of air is carried out by both the skin and the lungs. The gills of the tadpole disappear to give rise to lungs characterized by being light.
- Reattachment of the circulatory system: by changing the organs where oxygen is taken, the circulatory system adapts to be able to make a new distribution.
- Other changes: the eyes also change, the speech apparatuses develop, the skin becomes pigmented, etc.
Once all the necessary changes have been achieved, and remembering that many of the hatchlings will have died along the way, the adult individuals come to the surface . It is then when the ecological niche changes: they begin to live between the land and the water and hunt for food.
To complete the life cycle, we need fertile adults who can lay new consignments of eggs. The reproduction of frogs , in many cases, includes complex courtship rituals to obtain sexual partners.
Moreover, the copulation of frogs is often called “amplexus”, since the male covers the female and holds her under the forelimbs. It is a characteristic of many of the amphibians.
Once we have a fertilized female, she will deposit her eggs in a pool of water, thus closing the life cycle of a frog .