Amphibians comprise the most primitive group of vertebrates Their name means “double life” (amphi=both and bios=life) and They are ectothermic animals, that is, they depend on external heat sources to control their internal balance. In addition, they are anamniotes, like fish; this means that their embryos lack a membrane that surrounds them: the amnion.
On the other hand, the evolution of amphibians and their passage from water to land occurred over millions of years. Their ancestors lived about 350 million years ago, in the late Devonian, and their bodies were stocky and their legs were broad and flattened with many toes. These were Acanthostega and Icthyostega, which were the predecessors of all the tetrapods we know today. They have a worldwide distribution, although they are not present in desert regions, in polar and Antarctic areas and in some oceanic islands. Continue reading this article on our site and you will learn about all the characteristics of amphibians, their peculiarities and lifestyles.
What are amphibians?
Amphibians are tetrapod vertebrate animals, that is, they have bones and four limbs. This is a most peculiar group of animals, since they undergo a metamorphosis that allows them to go from the larval phase to the adult phase, which also means that throughout their lives they have various mechanisms for breathing.
Types of amphibians
There are three types of amphibians, which are classified as follows:
- Amphibians of the order Gymnophiona: in this group there are only caecilians, whose body resembles a worm, but with four very short extremities.
- Amphibians of the order Caudata: are all those amphibians that have a tail, such as salamanders and newts.
- Amphibians of the order Anura: they do not have a tail and are the best known. Some examples are frogs and toads.
Main characteristics of amphibians
Among the characteristics of amphibians, the following stand out:
The metamorphosis of amphibians
Amphibians have certain peculiarities in their way of life. Unlike the rest of the tetrapods, they undergo a process called metamorphosis in which the larva, that is, the tadpole, becomes an adult and passes from gill breathing to pulmonary. During this process, all kinds of structural and physiological changes occur, in which the organism prepares to pass from aquatic to terrestrial life.
The amphibian egg is placed in the water, so when the larva hatches it has gills to breathe, a tail and a circular mouth for feeding. After some time spent in the water, it will be ready for metamorphosis, where it undergoes dramatic changes ranging from disappearance of tail and gills, as in some salamanders (Urodelos), to profound changes in organ systems, as in frogs (Anurans). Also the following happens:
- Development of forelimbs and hind limbs.
- Development of a bony skeleton.
- Lung growth.
- Differentiation of ears and eyes.
- Skin changes.
- Development of other organs and senses.
- Neural development.
However, some salamander species can do without metamorphosis and reach the adult stage with larval characteristics, such as the presence of gills, so it will look like a small adult. This process is called neoteny.
The skin of amphibians
All modern amphibians, that is, Urodelos or Caudata (salamanders), Anuros (frogs) and Gimnofiona (caecilians), are collectively called Lissanphibia, and this name derives from the fact that these animalshave no scales on their skin , so they are “naked”. They do not have another skin covering like the rest of the vertebrates, be it hair, feathers or scales, with the exception of caecilians, whose skin is covered by a type of "dermal scale".
On the other hand, its skin is very thin, which facilitates skin respiration, it is permeable and is provided with a rich vascularization, pigments and glands (in some cases toxic) that allow them to protect themselves against abrasion from the environment and against other individuals, by acting as their first line of defense.
Many species, such as dendrobatids (poison arrow frogs), have very bright colors that allow them to give a “warning” to their predators, since they are very striking, but this coloration is almost always associated with poisonous glands. This, in nature, is called animal aposematism, which is basically warning coloration.
Amphibian skeleton and limbs
This animal group has a great variation in terms of its skeleton with respect to other vertebrates. During their evolution they have lost and modified many bones of the forelimbs, but in the case of the waist, it is much more developed.
The forelegs have four toes and the hind legs five, and are elongated for the function of jumping or swimming, except in caecilians, which have lost their hind limbs due to their fossorial lifestyle. On the other hand, depending on the species, the hind legs can be adapted both for jumping and swimming, but also for walking.
Mouth of amphibians
The mouth of amphibians is characterized by having the following:
- Weak teeth.
- Large and wide mouth.
- Muscularized and fleshy tongue.
The tongue of amphibians makes it easier for them to feed, and in some species they can project them outwards to capture their prey.
Feeding of amphibians
Answering the question of what amphibians eat is a bit ambiguous, since amphibian feeding varies according to age, being able to feed on aquatic vegetation in the larval stage, and on small invertebrates in the adult stage, such as:
- Worms.
- Insects.
- Spiders.
There are also predatory species that can feed on small vertebrates, such as fish and mammals, for example, escuerzos (they are found within the group of anurans) that are stalking hunters and can often suffocate trying to swallow prey that is too large.
Amphibian Respiration
Amphibians have gill respiration (in their larval stage) and cutaneousthanks to their thin and permeable skin, which allows them to transfer gases. However, adults also have pulmonary respiration, and in most species they combine both forms of respiration throughout their lives.
On the other hand, some species of salamanders completely lack pulmonary respiration, so they use only gas exchange through the skin, and this is often folded, so that the surface exchange rate increases.
For more information, you can consult this other article on our site about Where and how do amphibians breathe?
Reproduction of amphibians
Amphibians have the separate sexes, that is, they are dioecious, and in some cases there is sexual dimorphism, which means that the male and female are distinguishable. Mainly, fertilization is external in anurans and internal in urodeles and gymnophyans. They are oviparous and the eggs are deposited in water or on moist soil to avoid desiccation, but in the case of salamanders, the male leaves a package of sperm on the substrate, called a spermatophore, to be later collected by the female.
Amphibian eggs are laid inside foamy masses produced by the parent, and may in turn be protected by agelatinous membrane which also protects them against pathogens and predators. Many species have parental care, although they are scarce, and it is limited to transporting the eggs inside their mouths or the tadpoles on their backs and moving them if there is a predator nearby.
In addition, they have a cloaca, just like reptiles and birds, and it is through this single conduit that reproduction and excretion occur.
Other characteristics of amphibians
In addition to the characteristics mentioned above, amphibians are also distinguished by the following:
- Three-chambered heart: they have a three-chambered heart, with two atria and one ventricle and a double circulation through the heart. Your skin is highly vascularized.
- They fulfill ecosystem services: since many species feed on insects that can be pests for some plants or vectors of diseases such as mosquitoes.
- They are good bioindicators: some species can provide information about the environment in which they live, since they accumulate toxic substances or pathogens in their skins. This has caused their populations to decline in many regions of the planet.
- Great diversity of species: there are more than 8,000 species of amphibians in the world, of which more than 7,000 correspond to anurans, some 700 species of urodeles and more than 200 correspond to gymnophynas.
- Endangered: A significant number of species are vulnerable or in danger of extinction due to the destruction of their habitat and a disease called chytridiomycosis, caused by a pathogenic chytrid fungus, Batrachochytrium dendrobatidis, which is drastically destroying its populations.
