The transition from an aquatic environment to air presents a profound physiological challenge for most fish. Their respiratory system is finely tuned to extract dissolved oxygen from water, a medium that holds approximately 33 times less oxygen than air. This specialization requires a highly efficient gas exchange surface, which becomes a fatal liability when the fish is removed from the water. The shift from liquid to gas fundamentally alters the mechanics of their breathing apparatus, leading to suffocation despite abundant atmospheric oxygen.
How Fish Breathe Underwater
Aquatic respiration involves specialized organs called gills, located on either side of the fish’s pharynx and often protected by a bony flap called the operculum. Gills are composed of comblike filaments, which are further lined with tiny, plate-like folds known as lamellae. This complex, layered structure creates an immense surface area necessary for efficient gas exchange in water, where oxygen is scarce.
Fish continually pull oxygenated water over these structures by pumping the water over the gills. The blood flowing through the capillaries of the lamellae moves in the opposite direction to the water flow, a process called countercurrent exchange. This opposing flow ensures the blood constantly encounters water with a higher oxygen concentration along the respiratory surface. This design maintains a concentration gradient, allowing the fish to extract a substantial amount of available oxygen, often achieving an efficiency of 80% or more.
Why Gills Cannot Function in Air
When a fish is taken out of the water, the delicate architecture of the gills immediately begins to fail. The fine structures of the gill lamellae are designed to be separated and supported by the buoyancy and density of water. Without this support, the fragile, paper-thin lamellae immediately collapse and stick together. This physical collapse drastically reduces the total functional surface area, preventing sufficient oxygen from diffusing into the bloodstream.
Concurrently, the exposed gill tissues quickly begin to dry out, a process known as desiccation. Respiratory surfaces must remain moist for oxygen to dissolve and diffuse across the membranes. Drying out further hinders the already compromised gas exchange. The fish effectively suffocates, not from a lack of environmental oxygen, but from the physical failure of its specialized respiratory organ.
Fish That Can Survive Out of Water
Not all fish are condemned to this fate, as some species have evolved specialized accessory respiratory organs to utilize atmospheric oxygen. These adaptations allow them to survive in environments where water oxygen levels are low, such as stagnant ponds, or to move across land for short distances.
The African lungfish, for example, possesses true lungs—modified swim bladders—that allow it to breathe air and survive prolonged drought by burrowing into the mud. Other species have developed highly vascularized chambers near the gills to take in gulps of air. Labyrinth fish, including the betta and gourami, use a complex, folded organ called the labyrinth organ to absorb oxygen directly from the air. Mudskippers utilize a combination of cutaneous respiration through their moist skin and modified gill chambers that hold water while they are active on land.