The class Amphibia, which literally translates to “double life,” is a group of vertebrates that includes frogs, toads, salamanders, newts, and caecilians. These animals represent a significant evolutionary transition, being the first tetrapods to move from strictly aquatic environments to land. While often mistaken for reptiles, amphibians possess several unique biological features that universally define their entire class. All 8,000-plus known species share a combination of characteristics that tie them to moist habitats, making them a distinctive group among backboned animals.
Permeable and Glandular Skin
All amphibians possess a unique integumentary system characterized by thin, smooth skin that lacks scales and is highly permeable to both water and gases. This skin serves a dual function, acting as a protective barrier and as a primary respiratory surface, a process known as cutaneous respiration. This method allows for gas exchange, supplementing the action of their lungs, and is so effective that some lungless salamanders rely on it entirely for oxygen intake.
The skin must remain continuously moist for oxygen to diffuse across the surface and into the bloodstream. This dependency on moisture is maintained by numerous mucous glands that constantly secrete a watery film across the skin. While this permeability allows them to absorb water rather than drinking, it also makes them highly vulnerable to dehydration in dry environments and susceptible to absorbing environmental toxins and pollutants.
Amphibians also possess granular glands embedded in their skin, which are primarily defensive in function. These glands secrete a variety of compounds, ranging from distasteful substances to potent neurotoxins, serving to deter predators. The combination of a thin, respiration-focused epidermis and these specialized glands is a defining trait.
The Obligate Metamorphic Life Cycle
A universal characteristic of amphibians is their complex life cycle, which involves a developmental transition known as metamorphosis. This biological process typically begins with the laying of eggs that lack a hard shell and the protective membranes of an amniotic egg. This absence of a self-contained, waterproof egg means amphibian eggs must be laid directly in water or extremely moist environments to prevent desiccation.
The egg hatches into an aquatic larval form adapted for life in water, typically possessing gills, a tail, and lacking limbs. Metamorphosis is the transformation that converts this aquatic larva into a terrestrial or semi-terrestrial adult, involving the reabsorption of gills and tail, the development of lungs for air-breathing, and the growth of limbs. The process is regulated by hormones, primarily thyroxine.
While some species, known as direct developers, have evolved to skip the free-swimming larval stage and hatch as miniature versions of the adult, they still retain the fundamental dependency on a highly humid environment for their embryonic development. This requirement for moisture during reproduction, stemming from the lack of an amniotic egg, fundamentally separates amphibians from all other tetrapods.
Ectothermy and Temperature Regulation
All amphibians are ectotherms, meaning their internal body temperature is regulated primarily by external sources rather than by internal metabolic heat generation. Their body temperature fluctuates with the surrounding environment. Amphibians rely on behavioral thermoregulation to maintain a temperature range that allows for optimal biological function.
To warm up, an amphibian may bask in sunlight or rest on a warm surface, absorbing heat from the environment. If temperatures become too high, they seek out shade, burrow into the cooler ground, or retreat into water to prevent overheating. This reliance on external heat sources limits their distribution, preventing them from thriving in areas with consistently high or low temperatures. Their permeable skin, while aiding in respiration, also contributes to evaporative cooling but risks dehydration.