The idea of fish with lungs might seem unusual, given their aquatic habitat. Most fish do not possess lungs like mammals or other land animals. However, some fish have evolved fascinating and diverse adaptations that allow them to breathe air, enabling them to survive in environments where typical gill respiration is insufficient.
Gills: The Primary Respiratory Organ
The vast majority of fish breathe using specialized organs called gills, located on either side of their head and typically covered by a bony plate known as the operculum. Water enters through the fish’s mouth and then flows over these gills. Gills are composed of delicate structures called gill filaments, which are further covered in tiny, plate-like tissues called lamellae, creating an extensive surface area for efficient gas exchange.
Within the gill filaments, a process known as countercurrent exchange takes place, which significantly enhances oxygen absorption. Blood flows through the capillaries in the lamellae in the opposite direction to the water passing over them. This opposing flow maintains a consistent concentration gradient along the entire respiratory surface, allowing oxygen to continuously diffuse from the water into the fish’s bloodstream. This highly efficient system allows fish to extract over 80% of the available oxygen from water, despite water containing far less dissolved oxygen than air.
Beyond Gills: Air-Breathing Adaptations
While gills are highly effective underwater, some fish have developed additional structures for breathing atmospheric air, distinct from true lungs. Certain species, such as gar and bowfin, possess a modified, vascularized swim bladder, allowing them to gulp air from the surface and absorb oxygen. Other fish, like electric eels and snakeheads, use specialized areas within their mouth or pharyngeal cavities for gas exchange, such as the heavily vascularized oral linings of electric eels.
Some fish can also absorb oxygen directly through their moist skin, a method used by mudskippers and the mangrove rivulus. Another adaptation is intestinal respiration, where fish like loaches and certain catfish swallow air, absorbing oxygen through a vascularized region of their gut before expulsion. These diverse modifications illustrate various evolutionary pathways to supplement or replace gill-based respiration.
The True Lungfish
Among the most unique fish are lungfish (Dipnoi), which possess true, homologous lungs. These lungs are structurally similar to those of terrestrial vertebrates and represent a significant evolutionary link. Six living species exist, found in freshwater environments across Africa, South America, and Australia.
The Australian lungfish typically has a single lung and primarily uses its gills, supplementing with air breathing only when water oxygen levels decrease. In contrast, African and South American lungfish possess paired lungs and are often obligate air-breathers, meaning they must surface for air to survive. These species can also aestivate, entering a dormant state within mud cocoons during dry periods, surviving for months or even years by breathing air.
Environmental Drivers for Air Breathing
The evolution of air-breathing in fish is closely connected to environmental pressures, particularly low-oxygen (hypoxic) aquatic environments. Stagnant waters, common in shallow lakes, swamps, and drying riverbeds, often have reduced dissolved oxygen due to decomposition and high temperatures. Seasonal droughts further intensify these conditions by concentrating fish and increasing competition for available oxygen.
In such challenging circumstances, breathing atmospheric oxygen provides a substantial survival advantage, allowing these fish to persist where gill-only breathers would perish. Air-breathing also enables some species to survive out of water for short periods, facilitating movement to new habitats when their current ones become inhospitable. These adaptations are crucial responses to water’s limitations as a respiratory medium, especially when conditions become unfavorable.