Amphibians are vertebrates defined by a unique life cycle that often bridges aquatic and terrestrial environments. Their respiratory organs change dramatically depending on the stage of life and the specific species. As the first vertebrates to move onto land, their methods of gas exchange are diverse, shifting between water-based and air-based respiration throughout their development. This characteristic transition from a fully aquatic juvenile form to a semi-aquatic or terrestrial adult form necessitates varied breathing mechanisms.
Gills in the Larval Stage
The vast majority of amphibians begin life as aquatic larvae (e.g., frog tadpoles or salamander newts), where gills are the primary respiratory organ. These gills are necessary for extracting dissolved oxygen from the surrounding water, functioning similarly to the gills of fish.
Types of Larval Gills
Larval amphibians typically possess one of two types of gills: external or internal. External gills are visible as feathery tufts protruding from the sides of the head, commonly seen in salamander larvae. Internal gills, typical of most frog tadpoles, are hidden within a protective flap of skin called the operculum, with water passing over them and exiting through a spiracle opening.
The Respiratory Transition During Metamorphosis
The transition from a water-dwelling larva to a land-capable adult is marked by metamorphosis, a profound reorganization of the amphibian’s body. During this period, the aquatic respiratory structures are systematically broken down and replaced by air-breathing organs. The gills undergo degeneration and are resorbed as the animal shifts toward a more terrestrial existence.
Concurrently, paired lungs develop and mature, becoming the dominant organ for oxygen intake from the air. This shift is not instantaneous; many larvae begin taking occasional air breaths using their developing lungs before the gills are fully lost. This temporary overlap, known as bimodal breathing, allows the animal to use both aquatic and aerial respiration.
Adult Respiration: Lungs and Cutaneous Breathing
Once metamorphosis is complete, adult amphibians rely on a combination of lungs and skin for gas exchange, leaving the gills behind. The lungs are relatively simple, sac-like structures, lacking the complex internal subdivision found in mammals or birds. Due to this simpler structure and the absence of a diaphragm, adult amphibians breathe air using a method known as buccal pumping.
Buccal pumping involves muscular contractions of the mouth and throat floor to force air into the lungs via positive pressure. The skin, a secondary respiratory surface, often plays an equally significant role through a process called cutaneous respiration. To facilitate this, amphibian skin is thin, highly permeable, and rich in underlying capillaries. The skin must remain moist for oxygen to diffuse across the surface, and it can contribute substantially to overall oxygen uptake, especially when the animal is submerged.
Specialized Amphibians and Permanent Gills
While the loss of gills is the typical pattern, two significant groups of amphibians defy this general rule. The first exception involves species that retain their larval features, including gills, into adulthood, a phenomenon known as neoteny or paedomorphosis. The Mexican Axolotl and the Mudpuppy (Necturus) are prime examples, living their entire lives in water with prominent external gills, even after reaching sexual maturity.
The second major deviation is the family Plethodontidae, commonly known as the lungless salamanders. This large family undergoes metamorphosis and loses its larval gills but never develops lungs. Instead, these adults rely almost entirely on cutaneous respiration, exchanging oxygen and carbon dioxide across the skin and the moist lining of the mouth. The existence of these lungless species, which thrive without both gills and lungs, proves that gills are not a universal feature of all adult amphibians.