Catfish do not possess true lungs like mammals, but many species have evolved a highly adaptive respiratory system. This unique flexibility allows them to survive in environments where other fish cannot. Their ability to take in oxygen from two sources—water and air—is a remarkable adaptation that distinguishes them within the aquatic world.
How Catfish Utilize Gills
Catfish, like nearly all fish, primarily rely on gills for aquatic respiration, which is the process of extracting dissolved oxygen from the water. The gills are composed of delicate filaments and lamellae, which are richly supplied with blood vessels. Water flows over these gill structures, and oxygen moves from the water into the bloodstream through a process called diffusion.
The efficiency of water breathing is maximized by countercurrent exchange. This system ensures that blood flows in the opposite direction to the water passing over the gills. Because of this opposing flow, the blood constantly encounters water with a slightly higher oxygen concentration, maintaining a diffusion gradient. This arrangement allows the gills to extract up to 90% of the available oxygen from the water.
Specialized Accessory Breathing Organs
To supplement their gills, many catfish species have developed specialized accessory breathing organs (ABOs) that function as a form of “lung.” These organs are not homologous to land vertebrate lungs but allow for the direct utilization of atmospheric oxygen. In species like the walking catfish (Clarias batrachus), these structures are highly vascularized extensions of the gill arches called arborescent or dendritic organs.
These tree-like organs are housed within a suprabranchial chamber located above the gills. When a catfish gulps air, oxygen is forced into this chamber and diffuses across the dense network of blood vessels. The structure is modified specifically for aerial gas exchange. Other species use different methods, such as a modified respiratory gas bladder, a vascularized lining of the mouth, or their intestines for air breathing. The skin, when kept moist, also contributes to gas exchange, known as cutaneous respiration.
Environmental Drivers for Air Breathing
The evolution of these accessory breathing organs is directly linked to the challenging environments where many catfish species thrive. Air-breathing is an adaptation to aquatic hypoxia, a condition where the dissolved oxygen levels in the water are severely low. Catfish frequently inhabit stagnant ponds, muddy rivers, and shallow swamps, where high water temperatures and decomposition consume large amounts of oxygen.
When oxygen concentrations drop, catfish increase their air-breathing frequency. For example, the African catfish (Clarias gariepinus) may more than double its air-breathing rate when oxygen levels drop significantly. This bimodal respiration enables certain species, such as the walking catfish, to survive short terrestrial excursions and migrate between drying water bodies. The ability to breathe air allows them to escape inhospitable aquatic conditions and find new, more oxygenated habitats.