Amphibians are vertebrates known for inhabiting both aquatic and terrestrial environments during different life stages. This dual existence has led to unique respiratory adaptations. Unlike many animals that rely on a single primary method for gas exchange, amphibians employ multiple strategies to acquire oxygen from diverse habitats.
Skin Breathing
Amphibians use their skin for gas exchange, a process known as cutaneous respiration. Their skin is thin and permeable, with a rich network of blood vessels directly beneath its surface. This design facilitates the direct absorption of oxygen from the environment into the bloodstream and the release of carbon dioxide. Cutaneous respiration is an important method for both aquatic and terrestrial amphibians, often supplementing or even replacing lung or gill breathing.
For this method to function effectively, the amphibian’s skin must remain moist. Amphibians secrete mucus to maintain skin hydration, which is important for efficient gas diffusion. If the skin dries out, gas exchange becomes impaired, potentially leading to suffocation for species heavily reliant on this method. This dependency on moist environments means amphibians are often found in aquatic or semi-aquatic habitats. Some species, like the hellbender salamander, rely on cutaneous respiration for over 90% of their oxygen uptake and carbon dioxide excretion, with specialized skin folds increasing surface area.
Lung Breathing
Adult amphibians use simple, sac-like lungs for pulmonary respiration. These lungs differ from those of mammals and birds, lacking complex internal structures and extensive surface area. Amphibians lack a diaphragm, so they use buccal pumping to force air into their lungs. This involves lowering the floor of the mouth to draw air in, then closing the nostrils and raising the mouth floor to push the air into the lungs.
This method of lung inflation is less efficient than the aspiration system seen in mammals, where a diaphragm creates negative pressure to draw air in. Lung breathing is used by terrestrial adult amphibians, but it is often supplemented by skin breathing due to the lungs’ simpler structure. The contribution of lungs to overall gas exchange can vary among species and may change seasonally.
Gill Breathing
Branchial respiration, or gill breathing, is observed in the larval stages of most amphibians, such as tadpoles. These aquatic larvae possess gills, which can be external and feathery or internal and covered by an operculum. Gills are highly vascularized structures that efficiently extract dissolved oxygen from water and release carbon dioxide.
As amphibians undergo metamorphosis, most species resorb their gills as they develop lungs and transition to a terrestrial lifestyle. Some amphibians, such as axolotl, exhibit neoteny, retaining their external gills throughout their adult lives. Even in larval stages, while gills are present, the skin often accounts for a greater percentage of total oxygen and carbon dioxide exchange.
Mouth and Throat Breathing
Amphibians use buccopharyngeal respiration, absorbing oxygen through the moist lining of their mouth and throat cavity. This method involves a rich network of blood vessels within the oral and pharyngeal membranes, facilitating gas exchange. Buccopharyngeal respiration often works in conjunction with lung breathing as part of the buccal pumping mechanism, helping to move air.
This form of breathing can also act as a supplementary respiratory surface, especially when the amphibian is submerged or less active. It allows for some gas exchange even when the mouth is closed, with air entering through the nostrils. Lungless salamanders, for instance, rely entirely on their skin and the membranes of their mouth and throat for respiration.