Amphibians are the oldest terrestrial vertebrates out there. But unlike most of the animals that make up this classification, amphibians go through a metamorphosis during their development, that is, a series of deep morphological changes that distinguish the specific stages of their life cycle.
Thus, the first part of the life cycle of the amphibians takes place in the water, and the second part on the earth. This dual condition is reflected in the etymology of its name: from the Greek amphi (both) and bio (life), that is, it has two different lives.
Amphibians have four mobile limbs, are ectothermic (regulate their temperature from the environment), or cold-blooded reptiles and fish.
1. Zoological classification
Animals of the amphibian class are recognized as being tetrapods (four legs), anamniotas (their eggs do not have a protective membrane) and going through a metamorphosis in their reproductive cycle. That includes toads, frogs, salamanders and caecilians.
The group is divided, in turn, into three sub categories or clades: Gymnophiona (vermiform excavators, without limbs, such as caecilians), Caudata (they have tail and four legs, like salamanders and newts) and Anura (without tail, as toads and frogs)
The origin of amphibians is uncertain in phylogenetic terms:
Some suppose that they derive from the group of the temnospondils, primitive tetrapods of the Carboniferous period, precursors also of many dinosaurs.
Another assumption would make them descendants of the aquatic tetrápodos lepospóndilos also of the Carboniferous period, abundant in Europe and North America.
A third possibility is that both inheritances were true and it was a group with individuals of multiple origin.
The debate would determine if the category should include all the anamniota tetrapods, or only the modern amphibians, known as lissanfibios.
3. Reproductive cycle
The amphibians are oviparous, that is, they reproduce through the laying of fertilized eggs during the copulation between male and female.
In general, the laying of eggs occurs in aquatic environments and in large quantities, as the tadpole hatches from them, a larva that grows over time and mutates its physical properties to resemble the adult. Then he leaves the water and becomes a terrestrial adult capable of continuing the cycle.
The embryos formed in the amphibian eggs soon give way to the larval forms called tadpole, whose physical characteristics are adapted to the aquatic environment: long tails, absence of legs, round mouth and gills.
Throughout its life, the tadpole feeds more or less omnivorously until its process of change begins and its legs form as it loses its tail. In the later stages, not only the body resembles that of the adult formed, but the gills are lost and pulmonary respiration is activated, a stage prior to the abandonment of aquatic life.
Then, once constituted as a young adult, the amphibian leaves the water and lives the rest of its existence as a terrestrial animal, with legs and lungs, but usually requiring a very high humidity margin to survive.
In the case of caudate amphibians or gymnophions, the metamorphosis is much less radical than in the anurans, since the tadpole and the adult are much more similar.
During its aquatic stage, tadpoles can be herbivores or basically feed on everything, depending on the species. Some may even tend to cannibalism if the conditions are too adverse.
In their adult stages, amphibians are mostly carnivores, becoming important predators for insects, arthropods, worms and smaller species of amphibians.
The skin of amphibians is quite particular. It does not have any type of scales, hair or protective shell, is permeable to water, has a very high vascular content and is constituted mostly by glands. Thanks to it, amphibians can stay humid in a terrestrial context, at the same time as breathing (cutaneous breathing) and controlling their body temperature.
In many, in addition, the skin of the amphibians secretes toxic substances that serve as defense apparatus, and contain colorful pigments to alert of their poison, or cryptic to camouflage it with the environment.
The skin of amphibians is usually renewed through a molting process, during which the animal ingests the outer layers that are about to be discarded.
7. Privileged habitat
For the most part, amphibians prefer humid and warm terrestrial environments, such as humid tropical or subtropical zones, as long as the aquifer presence is sweet and not salty. The salt concentration in the oceans is too high for them.
The anurans are those with the greatest planetary spread, although their greatest diversity is concentrated in the tropics, while the salamanders prefer the temperate zones of the north. The cecílidos, in turn, prefer the tropical heat, although they are really scarce in number and of rare appearance. The only regions devoid of amphibian life are the driest deserts, the oceanic islands and the Arctic and Antarctic poles.
There are about 7000 proven species of amphibians, which places them among the most numerous animals in the world. Of that number, more than 6,000 are species of toads and frogs, about 650 of salamanders and axolotls, and only about 200 of caecilians.
9. Mythical animals
Amphibians have been intriguing animals for human culture since time immemorial, in which they were linked with abundance and vital proliferation, at the same time as with alchemical nigredo and decomposing elements.
During the Middle Ages, they were considered to be regular ingredients of spells and witchcraft, or they were associated with the rains of animals, in which providence punished the behavior of some human group by making them rain frogs from the sky.
In contemporary children’s stories, the toad occupied a central place as the creature in which witches used to convert princes; spell that only a princess kiss could tear off. Another common place attributed to the transmission of warts is a frog.
The salamanders were also considered mythical animals, supposedly born of fire or embers, or even immune to it. It was said that touching a salamander infected fever. As will be seen, contact with amphibians has always been a disturbing fact.
10. Ecological threats
Since 1911 the main ecologists have warned of a radical decline in the number of amphibians that populate the world, which could be one of the greatest threats to the planet’s biodiversity.
Climate change, the destruction of habitat and the introduction of species that violate the trophic chain are the main factors for the extinction of amphibians by human hands.
It is estimated that 85% of threatened mammal species (around 100) today receive little or no ecological protection, despite the fact that 33 amphibians are found in the list of the 100 most threatened species worldwide, and 3 of them in the list of the ten species at present risk of extinction.