While most fish are adapted to extract oxygen from water, some remarkable species have evolved the ability to breathe atmospheric air. This adaptation allows them to thrive in environments where dissolved oxygen levels can be extremely low or even absent for periods.
How Most Fish Breathe Underwater
Most fish breathe using gills, located behind the operculum, a protective flap, on either side of the fish’s head. Water enters through the fish’s mouth and flows over these gills, which are composed of delicate structures: gill arches, filaments, and tiny lamellae. These lamellae are rich in blood vessels, providing a large surface area for gas exchange.
Oxygen dissolved in the water diffuses across the lamellae into the bloodstream, while carbon dioxide from the blood diffuses into the water. This process is efficient due to countercurrent exchange, where blood flows through gill capillaries in the opposite direction to the water. This maintains a continuous concentration gradient, allowing the fish to absorb up to 90% of the oxygen from the water.
The Mechanics of Air Breathing in Fish
Fish capable of breathing air have evolved various organs distinct from their gills to extract oxygen directly from the atmosphere. These adaptations typically involve highly vascularized tissues, similar to lungs. One common adaptation is a modified swim bladder. In air-breathing fish like lungfish, this swim bladder has evolved into a lung-like structure with internal folds and extensive capillaries, allowing for direct gas exchange with air.
Other fish utilize different parts of their bodies for air breathing. Some species possess vascularized regions in their gut, such as the stomach or intestine, to absorb oxygen from swallowed air. These are often called “gut air breathers.” Another adaptation is the labyrinth organ, found in fish like gouramis and bettas. This structure is located above the gills and is lined with a vascularized membrane, enabling these fish to take gulps of air from the surface. Some fish can also perform cutaneous respiration, absorbing oxygen directly through their skin.
Diverse Examples of Air-Breathing Fish
Air-breathing capabilities are found across diverse fish species, each adapted to specific environmental challenges. Lungfish, found in Africa, South America, and Australia, possess modified swim bladders that function as lungs. African and South American lungfish are obligate air-breathers, meaning they must regularly surface for air and can drown if denied access.
Gouramis and betta fish utilize their labyrinth organs to breathe atmospheric air. This adaptation is useful in their natural habitats, which often include stagnant, oxygen-poor waters. Snakehead fish also possess a suprabranchial organ, allowing them to survive in low-oxygen conditions and move over land. Walking catfish, common in Southeast Asia, have specialized arborescent organs derived from their gill arches, enabling them to breathe air and travel short distances out of water.
Beyond Breathing: Surviving Out of Water
While air-breathing fish extract oxygen from the atmosphere, some fish can survive out of water for short periods without being primary air-breathers. The duration a fish can survive outside water depends on factors including species, moisture retention, and tolerance for low oxygen. Most aquatic fish, like trout or bass, quickly suffocate out of water because their gills collapse, reducing the surface area for gas exchange.
However, some non-air-breathing species can endure brief terrestrial exposure. They might do this by retaining water in their gill chambers, having a protective slime layer to prevent desiccation, or tolerating oxygen deprivation. Certain eels, for instance, can absorb oxygen through their skin (cutaneous respiration). This short-term survival, often aided by moisture retention and reduced metabolic rates, differs from the sustained atmospheric respiration performed by specialized air-breathing fish.