Catfish belong to the diverse order Siluriformes, a group of fish that often lack the traditional scales seen on other aquatic species. Unlike most fish that rely solely on water passing over their gills, many species of catfish possess a unique biological adaptation for breathing air. The visible release of gas at the water surface, which confirms they make bubbles, is a byproduct of their highly effective method of extracting oxygen.
The Primary Source of Catfish Bubbles
The most common source of bubbles from catfish is facultative air breathing, often observed as a rapid dash to the water surface. Certain species, particularly those in the families Callichthyidae and Loricariidae, utilize a modified portion of their digestive tract for this purpose. When these fish gulp air, the atmospheric gas travels into the posterior intestine or stomach, which is lined with highly vascularized tissue.
This specialized lining functions similarly to a lung, allowing oxygen from the swallowed air to diffuse directly into the bloodstream. Once absorbed, the residual gas—composed mainly of nitrogen, carbon dioxide, and remaining oxygen—must be expelled. This expulsion is the bubble observers see, typically released through the gills or the anus, supplementing the oxygen absorbed through their gills.
Other air-breathing catfish, such as the Clariidae family, possess a different adaptation called a suprabranchial organ, or labyrinth organ. This complex structure is located above the gills and serves as a highly efficient air-breathing organ. These fish also gulp air, which is stored and used within this organ for gas exchange, resulting in a bubble release when the remaining gas is vented back into the water.
Why Catfish Inhale Atmospheric Air
Air gulping behavior is primarily driven by environmental conditions that cause low dissolved oxygen levels, making aquatic respiration insufficient. Catfish naturally inhabit environments prone to hypoxia, such as slow-moving rivers, stagnant pools, or warm, shallow water bodies. When dissolved oxygen drops below a certain point, the catfish must use their accessory breathing organs to survive.
High water temperatures further exacerbate this issue, as warmer water holds less dissolved oxygen, increasing the frequency of surface gulping. This behavior is considered facultative, meaning the fish can switch between aquatic and aerial breathing based on their physiological needs.
While hypoxia is the main trigger, air gulping can also occur in environments with normal oxygen saturation for other biological functions. For instance, some species use the swallowed air to regulate their buoyancy, adjusting the gas volume in their digestive tract to control their position in the water column. Increased physical activity or stress can also elevate metabolic demands, prompting a quick trip to the surface for a supplementary breath of air.
Gas Release from Digestive Processes
Although the most frequent bubbles result from aerial respiration, gas release is also connected to the digestive system. In species that absorb oxygen through the intestine, swallowing air assists with the movement of food through the gut. The air bubble helps clear the digestive tract, which must remain clear to maintain the efficiency of the gas exchange surface.
The residual gas expelled after oxygen absorption is primarily the remains of atmospheric air, though it is sometimes mistakenly identified as flatulence. True digestive gas, such as methane or hydrogen, is a minor source of external bubbles, sometimes released after a heavy meal.