The term amphibian, derived from Greek, literally means “living a double life.” This reflects the unique biological strategy of this class of vertebrates, which bridges the gap between aquatic and terrestrial life forms. Amphibians were among the earliest vertebrates to evolve four limbs, or tetrapods, representing an evolutionary step toward life on land approximately 340 million years ago. Their life cycle typically includes an aquatic, gill-breathing larval stage and a terrestrial, air-breathing adult stage, tying their reproduction directly to wet environments. This reliance on water distinguishes them from other land-dwelling vertebrates.
Frogs and Toads
Frogs and toads belong to the order Anura, meaning “without tail,” a defining characteristic of the adult form. This group is the most diverse of all amphibians, accounting for nearly 90% of all known species. Their bodies are specialized for jumping (saltatorial locomotion), made possible by a short, stout body and disproportionately long, powerful hind limbs. Fertilization is typically external, occurring when the female releases eggs into the water as the male simultaneously releases sperm during a mating embrace called amplexus.
Prominent examples demonstrate the diversity within this order. The Poison Dart Frog, found in Central and South American rainforests, uses vivid aposematic coloration to warn predators of the potent toxins secreted through its skin. The American Bullfrog, one of the largest species in North America, is an aggressive predator that uses its sticky tongue to capture prey. The Cane Toad, native to South and Central America, is known for its large size and toxic parotoid glands that secrete a milky venom.
Salamanders and Newts
Salamanders and newts constitute the order Caudata, distinguished by their elongated bodies and the presence of a tail throughout their lives. Unlike Anura species, Caudata typically possess four short, equally sized limbs, giving them a lizard-like appearance as they walk or crawl. The largest amphibian, the Chinese Giant Salamander, belongs to this group, reaching lengths of up to 1.8 meters.
A remarkable trait of this order is their extraordinary ability for tissue regeneration, unmatched among other adult tetrapods. Salamanders can repeatedly regenerate complex structures like limbs, tails, and even portions of their spinal cord and heart. This capacity is studied extensively, focusing on the formation of a mass of undifferentiated cells, called a blastema, at the site of injury. The Axolotl, a Mexican salamander, is famous for neoteny, retaining its aquatic, gilled larval form into adulthood and bypassing metamorphosis.
Caecilians
The least familiar group of amphibians is the Caecilians, belonging to the order Gymnophiona, which means “naked snake.” These animals are entirely limbless, possessing elongated, worm-like or snake-like bodies, with lengths up to 1.5 meters in species like Caecilia thompsoni. Their primary habitat is fossorial—living burrowed beneath the soil or in streambeds—which is why they are rarely encountered.
Caecilians are mostly found in the tropics of South and Central America, Africa, and Southern Asia. Their subterranean lifestyle has led to several specialized physical adaptations. They have a heavily ossified, bullet-shaped skull that assists in burrowing through dense substrate. Most species also have reduced or absent eyes, often covered by skin or bone, limiting their vision to light and dark perception. A unique feature is a pair of chemosensory tentacles located on the head, which they extend to sense their surroundings in the dark earth.
Defining Biological Traits of Amphibians
The class Amphibia is unified by shared biological traits linked to their transitional lifestyle. A fundamental characteristic is metamorphosis, the dramatic transformation from an aquatic larva (like a tadpole with gills) to a terrestrial adult with lungs. This change is regulated by hormonal signals, allowing the animal to shift ecological niches and reducing competition between life stages.
Their highly permeable, glandular skin is generally smooth and moist due to mucus secretion. This skin plays a dual role, functioning as a primary or secondary surface for gas exchange in a process known as cutaneous respiration. However, this high permeability means they readily absorb and lose water rapidly, making them susceptible to dehydration and environmental contaminants. All amphibians are ectothermic, meaning they are cold-blooded and rely on external sources, such as sunlight or warm substrates, to regulate their internal body temperature.