The ability for an animal to actively breathe in two distinct environments—water and air—is known as bimodal respiration. This complex physiological strategy requires specialized organs that efficiently extract oxygen from media with vastly different chemical properties. Bimodal respiration involves the continuous or intermittent exchange of gases with both the surrounding water and the atmosphere. This dual capacity allows organisms to survive in environments where oxygen availability fluctuates, such as stagnant ponds or shorelines.
Amphibians: Respiration Through Transformation
Amphibians offer the most recognized example of bimodal respiration, demonstrating a complete transformation in their breathing apparatus. The aquatic larval stage, such as a tadpole, relies on gills to extract dissolved oxygen from the water. During metamorphosis, the gills degenerate, and simple, sac-like lungs develop, preparing the organism for a terrestrial existence. The adult amphibian primarily uses these lungs to breathe air.
Even after developing lungs, the skin remains a highly active respiratory surface throughout the amphibian’s life, a process called cutaneous respiration. The thin, permeable skin is densely supplied with capillaries, allowing for the diffusion of oxygen and the excretion of carbon dioxide directly across the surface. This skin breathing is active both when the animal is submerged and when it is on land.
For gas exchange to occur on land, the skin must remain consistently moist to keep respiratory gases dissolved, enabling them to cross the cell layers. Mucous glands constantly secrete moisture to prevent the surface from drying out. Some salamanders, like the Plethodontidae family, rely solely on their skin and the lining of their mouth for gas exchange, having entirely lost their lungs.
Fish with Dual Capabilities
Certain fish have developed accessory breathing organs to supplement standard gill respiration with atmospheric oxygen, necessary for surviving hypoxic (low-oxygen) aquatic conditions. Lungfish are prominent examples, possessing a modified swim bladder that functions as a true lung, homologous to those of terrestrial vertebrates. African and South American lungfish rely so completely on air breathing that they will drown if prevented from reaching the surface.
Other species, such as gourami and Siamese fighting fish, have evolved the labyrinth organ, a specialized, highly vascularized structure located in a chamber above the gills. This organ allows the fish to gulp air at the surface and absorb oxygen directly into the bloodstream. This adaptation provides an advantage when water becomes warm or stagnant, causing dissolved oxygen concentration to plummet.
In catfish, the swim bladder is also modified to function as an air-breathing organ, complementing gill function. These accessory organs are typically only used when the fish’s oxygen demands exceed what the gills can extract from the water. This ability to switch between water and air breathing allows these fish to inhabit environments inaccessible to obligate water-breathers.
Specialized Adaptations in Reptiles and Invertebrates
Bimodal respiration manifests in unique ways among reptiles and invertebrates, often involving highly localized or temporary respiratory surfaces. Certain freshwater turtles employ cloacal respiration, especially when hibernating or remaining submerged for extended periods. They actively pump water into two specialized sacs, called bursae, located within their cloaca. These bursae are lined with blood vessel-rich papillae that function like aquatic gills, absorbing dissolved oxygen.
While this method is inefficient for high metabolic activity, it sustains the turtle’s greatly reduced metabolic rate in cold water. This adaptation allows them to remain submerged beneath ice or in oxygen-poor mud for months without surfacing.
Marine snakes, which are air-breathing reptiles, utilize cutaneous gas exchange to supplement their primary lung function during long dives. Their highly permeable and vascularized skin enables them to absorb approximately 20 to 30 percent of their total oxygen requirement directly from the surrounding seawater. This skin breathing also serves as a major route for excreting carbon dioxide while submerged.
Semi-aquatic crabs maintain bimodal respiration by modifying their branchial chambers for aerial gas exchange. They use the chamber lining, known as the branchiostegite, as a respiratory surface. Crabs retain a small amount of water in the chamber to keep their true gills moist, allowing them to extract oxygen from the air while simultaneously using the gills to excrete carbon dioxide into the residual water.
The Physiological Challenge of Bimodal Respiration
The fundamental challenge of bimodal respiration lies in the vast difference in the physicochemical properties of air and water. Air contains about 30 times more oxygen than the same volume of water. However, water is a far denser medium, requiring significantly more energy to move over a respiratory surface. Furthermore, the specialized, delicate structure of gills collapses and desiccates rapidly in air, making them useless for aerial breathing.
Conversely, a terrestrial lung is inefficient underwater because the water pressure is too great to allow for the necessary changes in volume to move air. The lung is also not designed to absorb dissolved oxygen. These differences force bimodal animals to develop two distinct organ systems or a single system capable of functioning optimally in both media. The partitioning of gas exchange is complex because carbon dioxide is highly soluble in water. Therefore, aquatic respiration is often the preferred route for CO2 elimination, even when air breathing supplies most of the oxygen.
For vertebrates, the circulatory system must adapt to the respiratory mode, particularly in species with an incompletely divided heart, such as reptiles and lungfish. These animals utilize physiological mechanisms known as cardiovascular shunts. Shunts actively redirect blood flow, bypassing the pulmonary circuit when the animal relies on aquatic respiration. This shunting ensures that blood is not needlessly pumped through a non-functional lung while submerged. It also ensures that oxygenated blood from the air-breathing organ is distributed efficiently to the body.