While most fish are obligate water breathers, the biological reality of fish respiration outside of water is nuanced. Evolution has equipped certain species with adaptations allowing them to survive in terrestrial environments for periods of time. Understanding this requires first examining the standard mechanism by which fish extract oxygen from water. This article explains the fundamental biology of aquatic respiration and details the specialized organs that enable some fish to defy aquatic limitations.
The Mechanism of Aquatic Respiration
Aquatic respiration relies on the structure of the gills, which are fine, feathery organs located behind the fish’s head. Water flows into the mouth and is actively pumped over these respiratory surfaces before exiting through the operculum, or gill cover. This constant, directed flow, achieved through the buccal pump, ensures a steady supply of dissolved oxygen.
The process is challenging because water holds approximately 30 times less dissolved oxygen than air. To compensate, each gill arch supports numerous thin filaments covered in hundreds of tiny, sheet-like folds called lamellae. This arrangement drastically increases the respiratory surface area, making oxygen uptake efficient despite the low concentration.
The lamellae contain a dense network of capillaries where gas exchange occurs. Their efficacy stems from the countercurrent exchange system, where blood flows through the lamellae opposite to the water passing over them. This mechanism maintains a concentration gradient across the respiratory surface, ensuring oxygen continually diffuses from the water into the blood.
Why Gills Fail When Exposed to Air
This specialized gill structure is precisely why most fish cannot breathe effectively when removed from water. When water is absent, the fine filaments and lamellae collapse inward, sticking together due to the physical force of surface tension. This clumping instantly and severely reduces the total surface area available for gas exchange.
The collapsed lamellae effectively block the pathway for oxygen diffusion into the capillaries, causing immediate respiratory distress. Although the atmosphere contains a greater concentration of oxygen than water, the fish cannot access it because the respiratory surfaces are not functional. The physical adhesion means the fish suffocates from the inability to present the tissue to the gas, not from a lack of oxygen.
Furthermore, the thin tissues of the lamellae are prone to rapid desiccation, or drying out. Gas exchange requires the respiratory surface to be constantly bathed in moisture, as oxygen must first dissolve in liquid before passing across the membrane. Once the permeable gill tissues dry, the mechanism ceases to function, leading quickly to respiratory failure.
Specialized Respiratory Adaptations for Air
Evolutionary pressures, often stemming from environments with low dissolved oxygen or seasonal drying, have driven some fish lineages to develop specialized structures that bypass the limitations of standard gills. These accessory breathing organs allow certain species to extract oxygen directly from the atmosphere, often supplementing their regular aquatic respiration, enabling survival in harsh conditions.
Modified Swim Bladder
One adaptation involves the modification of the swim bladder, typically used for buoyancy control. In ancient fish like gars and the South American arapaima, the swim bladder has evolved into a highly vascularized, lung-like structure. The fish surfaces to gulp air, and oxygen is absorbed into the bloodstream through the walls of this modified organ, which is histologically similar to the lungs of terrestrial vertebrates.
Branchial Organs
A distinct solution is the development of branchial organs, which are highly vascularized structures located in a suprabranchial chamber above the standard gills. The arborescent labyrinth organ found in species such as climbing perch and gouramis is a complex fold of bony plates. This organ offers a large, moist surface area resistant to collapse when exposed to air, allowing the fish to take a bubble of air from the surface for steady oxygen extraction.
Cutaneous Respiration
A third strategy is cutaneous respiration, involving the direct absorption of oxygen through the skin. While this method is supplementary for most fish, species like the swamp eel rely heavily on this process when water is severely oxygen-poor. This requires the fish to remain damp or in muddy conditions, as the skin must be perpetually moist for oxygen to diffuse across the permeable epidermal layer.
These air-breathing organs are optimized for oxygen uptake but are less efficient at releasing carbon dioxide, which is highly soluble and easily expelled in water. Consequently, air-breathing fish often still rely on their standard gills or skin for the bulk of carbon dioxide elimination. This dual requirement means they must maintain access to water, even if only to expel metabolic waste gases.
Notable Examples of Air-Breathing Fish
The African lungfish (Dipnoi lineage) utilizes sophisticated atmospheric breathing to survive extreme drought conditions. This species uses a pair of true, lung-like organs derived from the swim bladder. When its river or pond habitat dries completely, the lungfish burrows deep into the mud and secretes a mucus cocoon, entering a state of suspended animation called aestivation. It relies almost entirely on air-breathing during this dormancy, maintaining a metabolic rate low enough to survive for periods that can last up to four years until the water returns.
In contrast, the mudskipper is truly amphibious, spending significant time out of water feeding and interacting on tropical mudflats. These unique fish possess dual respiratory capabilities, using their gills and moist skin to extract oxygen from both air and water. The mudskipper uses its muscular pectoral fins to “walk” across land, periodically gulping air and maintaining a reservoir of water in its enlarged gill chambers. This water keeps the branchial tissues damp, allowing for aquatic respiration on land alongside cutaneous and buccal respiration.
The walking catfish, found in warm, oxygen-depleted waters of Southeast Asia, employs a complex, arborescent branchial organ similar to the labyrinth organ. Their effective use of atmospheric oxygen allows them to actively travel short distances overland, particularly after heavy rains, to find new or better-oxygenated bodies of water, demonstrating terrestrial mobility.