The ability of certain fish to survive outside of water represents one of the most remarkable instances of evolutionary adaptation within vertebrates. These species have developed the capacity to use atmospheric oxygen, a function typically reserved for terrestrial animals, distinguishing them from the vast majority of strictly aquatic fish that rely on dissolved oxygen. This evolutionary transition was primarily driven by environmental necessity, particularly the challenge of surviving in low-oxygen water bodies like stagnant swamps, drying riverbeds, or intertidal zones. The development of specialized structures allows these fish to exploit ecological niches that are inaccessible to their water-dependent relatives.
Physiological Adaptations for Air Breathing
The survival of these amphibious fish hinges on internal biological structures that facilitate the direct extraction of oxygen from the air. Unlike gills, which are highly efficient at gas exchange in water but collapse or dry out in air, these organs are designed to remain moist and highly vascularized for aerial respiration. This adaptation often involves significant modifications to the fish’s anatomy, creating a supplementary respiratory system.
One common mechanism involves the development of accessory respiratory organs (AROs), which are often intricate, maze-like structures located in the head or gill region. For instance, anabantoids, such as gouramis, possess a labyrinth organ, a complex folding of tissue that is richly supplied with blood vessels and sits within a cavity above the gills. Similarly, some catfish have suprabranchial chambers or dendritic (tree-like) organs derived from the gill arches that act as primitive lungs for gulping air. These AROs function by holding a bubble of air against a large, thin, blood-rich surface, allowing for efficient oxygen diffusion.
Another specialized method utilizes the swim bladder, an organ primarily for buoyancy control in most fish, which is modified into a lung-like structure. In lungfish, this organ is highly sacculated with internal folds, increasing the surface area for gas exchange, and is connected to the pharynx. The African and South American lungfish are so dependent on this modified organ that they are considered obligate air-breathers and will drown if prevented from accessing the surface to gulp air. This adaptation is particularly significant as the lungfish’s lung-like organ is considered homologous to the lungs of all terrestrial vertebrates.
A third strategy, known as cutaneous respiration, involves absorbing oxygen directly through the skin, the lining of the mouth, or the throat. This method is only effective when the fish remains moist, which is why species relying heavily on skin breathing are often found in highly humid environments like mudflats. The skin in these areas has a high density of capillaries near the surface to maximize the uptake of atmospheric oxygen.
Movement and Drought Survival Strategies
Survival on land requires not only the ability to breathe air but also the means to navigate terrestrial environments and withstand desiccation. The movement capabilities of these fish are a radical departure from the typical aquatic locomotion seen in finned species.
Terrestrial mobility is achieved through the modification of fins and unique body movements. Mudskippers, for example, have robust, muscular pectoral fins that function like a pair of forelegs, allowing them to “crutch” or walk across mudflats. Other fish, such as the walking catfish, employ a snake-like wriggling motion, using their stiffened pectoral fins and body musculature to pivot and propel themselves across wet ground. These specialized forms of locomotion enable them to move between drying pools of water or traverse land to find new habitats.
To survive prolonged dry periods, some species have evolved the strategy of aestivation, a state of dormancy analogous to hibernation. The African lungfish exhibits this behavior by burrowing deep into the mud as water recedes. Once buried, the fish secretes a layer of mucus from its skin, which hardens into a protective, papery cocoon, leaving only a small opening to the surface for breathing. Inside this cocoon, the fish significantly slows its metabolism, allowing it to survive without food or water for months, or even up to four years, until the rains return.
Key Species That Live Out of Water
The mudskipper, a small amphibious fish found in the intertidal zones of Africa and Indo-Pacific regions, spends most of its life on land. This fish uses its crutch-like pectoral fins for terrestrial movement and relies on a combination of cutaneous respiration and holding water in its enlarged gill chambers to breathe air. It also constructs J-shaped burrows where it can retreat during high tide and low humidity, maintaining a pocket of air within the chamber.
The walking catfish (Clarias species) utilizes accessory respiratory organs in the form of suprabranchial organs, which are tree-like structures situated above the gills. This organ allows the catfish to gulp atmospheric air and survive in stagnant, oxygen-poor water. True to their name, these fish use their strong bodies and fins to wriggle across short distances on land, typically moving between bodies of water during the dry season.