Amphibian Definition

Amphibians are animals that live both in water and on land. Their name refers to “double life”, from the fact that these animals reproduce and spend a period of their existence in the water: they have a unique life cycle among vertebrates. The adults, which can be aquatic or terrestrial, mate and lay their eggs in the water. After some time, the eggs hatch into larvae, known as tadpoles. The best-known members of amphibians are frogs and toads, and animals that are traded as pets, such as axolotls and newts.

Based on the traditional classification of the animal kingdom, amphibians belong to the group of vertebrates, aquatic or terrestrial, which are characterized by having an internal skeleton of bones, cartilage, or a combination of both types of tissues. The classic groups of vertebrates are fish, amphibians, reptiles, birds, and mammals.

The double life of amphibians

Tadpoles live in water, breathe through gills like fish and are unable to breathe air, since they do not have lungs: tadpoles die if they are not in water or in a very humid environment.

Over time, the larvae grow into adults, lose their gills and develop lungs to breathe air, many also develop legs to walk on land. Internally, the tadpole’s body undergoes important changes for its “new” life as an adult on earth.

Tadpoles are herbivorous and feed on algae that they scrape from the stones at the bottom of their pond. Many of the adult amphibians are insectivorous or carnivorous. During metamorphosis the intestine undergoes changes to adapt to the change in diet. In frogs and toads, a sticky, retractile tongue develops, adapted for hunting insects.

This process of profound body reorganization is called metamorphosis and it is unique in vertebrates, only amphibians have a metamorphosis in their life cycle.

Characteristics of adult amphibians

The adult is an animal with lungs, which needs to breathe air. Many amphibians are excellent swimmers and spend a lot of time in the water, but they need to surface to breathe air, since they no longer have gills that allow them to breathe underwater.

Adult amphibians are known for their smooth, wet and slippery skin and for inhabiting wet places or near bodies of water. The skin allows them to absorb oxygen from the air or water (as long as it is moist), and works as a supplement to lung respiration. The preference for places near water is understandable because of the amphibians’ dependence on water for reproduction.

amphibian evolution

Amphibians were the first group of terrestrial vertebrates with 4 legs (tetrapods) instead of fins. These early tetrapods evolved from a group of fleshy-finned fish and began walking the land in the Devonian period, about 400 million years ago.

From then on, the diversity of these first tetrapods began to increase, giving rise to two groups: one of them gave rise to modern amphibians and the other to reptiles.

Current diversity of amphibians: urodeles/anurans, caudates, and gymnophions

In the Earth’s geological past, the diversity of amphibians was greater than today, many of these species have already become extinct, and are recognized three groups of modern amphibians: the urodeles/anurans (frogs and toads), the caudates (salamanders, newts and axolotls) and the gymnastics (animals in the form of snakes that live mostly in tropical jungles, commonly known as “caecilians”).

The anurans (frogs and toads) They are characterized because during metamorphosis, the tadpoles lose their tails. They are the only group of amphibians whose adults do not have a tail. Their bodies are short and broad, sometimes wider than they are long, giving them the appearance of stocky animals. This last characteristic is more marked in toads than in frogs, which are usually longer than they are wide, and for this reason they have a more slender appearance than toads.

The skin is smooth and moist to the touch, but in the case of toads it often has warts or folds. The skin may have poison glands, which are activated if handled and can cause serious poisoning in animals and people. They have large, bulging eyes on the top of their heads. The mouth is wide and its retractable tongue. During the adult stage they are insectivorous. Many anurans use vocalizations to communicate, especially during the breeding season, when males call females with a characteristic croak.

Anurans, like the rest of the amphibians, have four legs, but their hind legs are long and muscular and are adapted to their mode of locomotion on land: jumping. Between their fingers they have skin membranes that allow them to swim when in water.

Anurans are small animals. The largest known frog can weigh 2-3 kg and measure 30 cm as an adult, but most frogs and toads range between 10 and 15 cm, and some are even smaller than that.

The caudate amphibians They are a group of animals that are characterized by having a tail for life. They are shaped like lizards and include salamanders, newts, and axolotls. They are distributed almost exclusively in the Northern Hemisphere and are common in North America, Europe, and Asia. Some caudates are traded worldwide as pets, such as the Mexican axolotl (Ambystoma mexicanun).

Unlike anura, they have four short and generally equal legs: caudates walk slowly on land. There are completely aquatic species, and in these cases the hind legs are less developed or almost non-existent.

They have a broad head (like anurans) but the body is elongated. Adult caudates are carnivorous and their diet is based on insects, worms, crustaceans, molluscs and other small animals. Another difference with the anurans is that they do not have well-developed ears, and although they are able to hear, do not use sounds to communicateexcept for a few species.

Most salamanders are small, less than half a meter, but the giant salamanders of China and Japan can measure more than 1.5 meters and are the largest living amphibians.

The gymnophion amphibians They are a group of animals that have an elongated and cylindrical body, without limbs or tail. Their appearance is reminiscent of worms or snakes, but they are not related to them. Gymnophions live underground, in humid tropical regions, where they feed on small invertebrates.

Their eyes are very small and are covered by a skin membrane so they are almost blind.

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