Amphibians are unique vertebrates defined by their “double life,” a name derived from the Greek word amphibios. This refers to the transition most species make from an aquatic existence to a terrestrial or semi-terrestrial adult form. Their respiratory strategy is highly flexible, depending on their life stage, the surrounding environment, and the temperature of the water. They have evolved specialized respiratory surfaces, allowing them to absorb oxygen from both air and water.
Larval Gills for Water Breathing
The journey for most amphibians begins entirely underwater, where their survival depends on extracting dissolved oxygen using gills. The gills function as the primary means of gas exchange during this developmental period.
Larval salamanders often possess feathery, external gills that project directly into the water, maximizing the surface area for oxygen uptake. In contrast, most frog tadpoles develop internal gills, hidden beneath a protective flap of skin called the operculum. Water is drawn into the mouth, passed over the dense capillary beds within the internal gills, and then expelled through a small opening. This process efficiently transfers oxygen into the bloodstream, sustaining the growing larva until metamorphosis begins.
Cutaneous Respiration: Breathing Through the Skin
For adult amphibians, the ability to breathe underwater shifts to cutaneous respiration, or skin breathing. This method is vital for species that spend significant time submerged or hibernate. It involves the direct exchange of gases—oxygen uptake and carbon dioxide release—across the entire surface of the skin.
The skin is uniquely structured to facilitate this exchange, being thin, non-keratinized, and highly permeable. Immediately beneath the outer layer lies an extensive, dense network of capillaries known as the subepidermal capillary plexus. This vascular arrangement brings blood extremely close to the surface, creating a minimal diffusion distance.
Oxygen from the water dissolves into the moist skin surface and diffuses directly into the bloodstream. Simultaneously, carbon dioxide diffuses out of the blood and into the water. This continuous process allows submerged amphibians to meet a substantial portion of their oxygen needs without surfacing, especially when resting.
The effectiveness of this mechanism relies on the skin remaining constantly moist, which is maintained by mucus secreted from specialized glands. The amount of oxygen supplied by cutaneous respiration can fluctuate widely, contributing anywhere from zero to one hundred percent of the total oxygen required.
Why Lungs Are Ineffective Underwater
Although adult amphibians develop lungs during metamorphosis, these organs are designed specifically for extracting oxygen from the air, not water. The internal structure of amphibian lungs is relatively simple, resembling sac-like structures with limited internal partitioning. This simple design provides a much smaller surface area for gas exchange.
The primary limitation of the lungs underwater is that water holds significantly less dissolved oxygen than air. Unlike the specialized gills of fish, amphibian lungs are not structured to process the large volumes of water necessary to capture sufficient oxygen. Drawing water into the lungs would be ineffective and block the air-breathing mechanism.
Adult amphibians use their lungs primarily when on land, utilizing a positive-pressure process called buccal pumping. This involves lowering the floor of the mouth to draw in air through the nostrils, then closing the nostrils and contracting the throat muscles to force the air into the lungs. The lining of the mouth, or buccal cavity, can also facilitate some minor gas exchange, supplementing the skin when the animal periodically gulps air near the surface.
Diversity in Amphibian Breathing Strategies
The reliance on these different respiratory surfaces varies dramatically across the three main orders of amphibians: Anura (frogs and toads), Caudata (salamanders and newts), and Gymnophiona (caecilians).
A striking example of specialization is found in the Plethodontidae, the family of lungless salamanders. These species have completely lost their lungs and rely entirely on cutaneous and buccopharyngeal respiration for all gas exchange, whether on land or in water. Their moist skin and the vascularized lining of their mouth cavity perform all the work typically done by both lungs and skin.
Conversely, some highly aquatic species have evolved exaggerated skin features to maximize aquatic oxygen absorption. The Titicaca water frog, for example, possesses extensive folds of loose, wrinkled skin that greatly increase its surface area. This adaptation allows the frog to extract enough dissolved oxygen from the cold, well-oxygenated lake water to remain submerged indefinitely, rarely needing to surface.