Amphibians are vertebrates adapted to a “dual life,” typically inhabiting both aquatic and terrestrial environments. The term “amphibian” originates from Greek, meaning “both lives,” reflecting this characteristic. They bridge the evolutionary gap between water-dwelling fish and land-dwelling reptiles. Amphibians are ectothermic, meaning their body temperature is regulated by their external surroundings.
Defining Characteristics of Amphibians
Amphibians possess several defining biological features. A primary characteristic is their complex life cycle, which often involves metamorphosis. Most amphibians begin as aquatic larvae, such as tadpoles, breathing through gills and adapted to living in water. As they mature, these larvae undergo significant transformations, developing legs and lungs, which enable them to transition to a more terrestrial adult form. Some species can also breathe through their skin and the lining of their mouth.
Their skin is moist, permeable, and typically lacking scales. This permeable skin allows for the exchange of oxygen and carbon dioxide directly with the environment. Many amphibians also have specialized glands in their skin that secrete mucus to keep it moist or produce toxins for defense against predators. As ectotherms, their body temperature fluctuates with their surroundings, requiring them to seek out suitable environmental conditions to regulate their internal temperature. Reproduction for most amphibians involves laying gelatinous eggs, often in water or damp environments, which lack the protective shells found in reptiles.
Major Groups of Amphibians and Examples
The class Amphibia includes over 7,500 species across three main orders.
Anura (Frogs and Toads)
This order includes frogs and toads, characterized by their tailless adult stage and adaptations for jumping. Frogs typically have smooth, moist skin, long hind legs for leaping, and often bulging eyes. They commonly lay their eggs in clusters. Examples include the bullfrog and various tree frogs. Toads, while also anurans, generally have drier, warty skin, shorter legs more suited for walking or short hops, and often lay their eggs in long chains. The cane toad is a well-known example.
Caudata (Salamanders and Newts)
This order comprises salamanders and newts, distinguished by their elongated bodies and the retention of a tail throughout their adult lives. These amphibians typically move by walking and possess moist skin. Newts are a subgroup within salamanders, often more aquatic, with some species developing webbed feet for swimming. Notable examples include the spotted salamander, the axolotl, and various newt species like the Eastern newt.
Gymnophiona (Caecilians)
This order includes caecilians. These amphibians are limbless and have a worm-like or snake-like appearance, often with segmented rings around their bodies. Caecilians are predominantly burrowing animals, living hidden in soil or streambeds. Many species have small or absent eyes. Their unique adaptations include a bullet-shaped skull for digging and sensory tentacles near their nostrils to navigate their subterranean environments.
Distinguishing Amphibians from Other Animals
Amphibians are sometimes confused with other animal groups, particularly reptiles and fish, yet key differences set them apart.
A primary distinction is their skin: amphibians possess moist, permeable skin lacking scales, which is crucial for respiration. In contrast, reptiles have dry, scaly skin that provides protection against water loss, enabling them to inhabit arid environments. This difference in skin also affects their habitat preferences, with most amphibians needing to stay near water.
Their reproductive strategies also differ. Amphibians lay soft, jelly-like eggs requiring water or damp conditions. Reptiles, however, lay eggs with leathery or hard shells adapted to survive on land. Amphibians undergo metamorphosis, transforming from an aquatic larval stage with gills to an adult form that typically breathes air with lungs or through their skin. Fish breathe exclusively through gills throughout their lives, while reptiles hatch as miniature versions of the adults, without a larval stage.