Amphibians are a class of vertebrate animals that represent an evolutionary bridge between fully aquatic and terrestrial life. The name Amphibia is derived from Greek words meaning “double life,” reflecting the characteristic two-part existence of most species. These organisms are four-limbed vertebrates, or tetrapods, that share physical and reproductive traits setting them apart from reptiles, birds, and mammals. Modern amphibians belong to the subclass Lissamphibia, encompassing nearly 8,000 species that inhabit a wide variety of freshwater and moist terrestrial environments globally. They lack specialized egg structures, such as a shell, which prevents them from being fully independent of water for reproduction.
Life Cycle and Water Dependence
The life history of most amphibians is linked to water, particularly for the early developmental stages. Reproduction generally involves external fertilization, where the female lays unshelled eggs in the water or a moist location, and the male releases sperm over them. This strategy ensures the non-amniotic eggs remain hydrated and provides a necessary medium for fertilization.
The fertilized eggs hatch into an aquatic larval form, such as the familiar tadpole, adapted for life underwater. Larvae possess gills for breathing, a tail for propulsion, and a lateral line system, resembling their fish ancestors. This larval stage undergoes metamorphosis, which prepares the organism for a terrestrial existence.
Metamorphosis involves significant changes, including the resorption of the gills and tail, the development of limbs, and the maturation of lungs for breathing air. While the adult form is capable of moving onto land, its reliance on a moist environment persists. The requirement for water means that adults must return to aquatic or extremely damp habitats to breed.
A few species have evolved adaptations that bypass the free-swimming larval stage, a process called direct development. In these cases, the young hatch from the egg as miniature versions of the adult, eliminating the need for standing water. The majority of amphibians remain dependent on environmental moisture for the survival of their early life stages.
Permeable Skin and Specialized Respiration
A defining feature of amphibians is their thin, scaleless skin, which is highly permeable to both water and gases. This integument facilitates cutaneous respiration, allowing oxygen to be absorbed directly into the bloodstream through a dense network of capillaries beneath the surface. Some species, such such as lungless salamanders, rely entirely on their skin and the lining of their mouth for gas exchange.
The skin must remain moist for gas diffusion to occur effectively, which is why amphibians possess numerous mucous glands that secrete a protective, hydrating film. This moisture also allows amphibians to absorb water directly through their skin, a process sometimes aided by specialized “drinking patches.” The trade-off for this permeable skin is a high vulnerability to desiccation and the easy absorption of environmental toxins and pollutants.
Beyond mucous glands, the skin often contains granular glands that produce defensive or toxic secretions. These glands are a defense mechanism against predators, with the potency of the toxins varying widely across species. The reliance on the external environment for both respiration and hydration means that amphibians are highly sensitive to changes in habitat quality, serving as indicators of ecosystem health.
Internal Systems and Ectothermy
Amphibians are ectothermic organisms, meaning they do not generate sufficient internal heat to maintain a constant body temperature. This dependence on external conditions requires them to regulate their temperature behaviorally, often by basking in the sun to warm up or retreating to water or shade to cool down.
When temperatures drop significantly, their metabolic rate slows dramatically, which can lead to periods of dormancy like hibernation during winter. Their circulatory system reflects a transitional stage between fish and fully terrestrial vertebrates. The adult amphibian heart has three chambers: two atria and a single ventricle.
The single ventricle receives both oxygenated blood from the lungs and deoxygenated blood from the body, resulting in a partial mixing of the two. While this mixing reduces efficiency compared to the four-chambered heart of mammals, it is adequate for the amphibian’s lower metabolic demands. The supplementary oxygen gained through their skin helps compensate for the less efficient internal circulation.
The Three Major Living Groups
The Class Amphibia is comprised of three distinct orders, illustrating a wide range of body forms and ecological strategies.
Anura (Frogs and Toads)
This is the largest group, characterized by the lack of a tail in the adult stage. Anuran bodies are squat, with long, powerful hind limbs adapted for leaping and swimming.
Caudata (Salamanders and Newts)
These species retain a tail throughout their adult lives. They generally have an elongated body shape and four limbs of roughly equal size, moving with a side-to-side undulation. While some are fully aquatic, others are terrestrial, often residing in damp forest environments.
Gymnophiona (Caecilians)
This is the most specialized group. Caecilians are limbless, elongated, and typically burrowing amphibians that superficially resemble large earthworms or snakes. They are found primarily in tropical regions and possess specialized skulls for pushing through soil.