Most fish extract dissolved oxygen from water using their gills, but many species must sometimes, or always, seek out atmospheric air to survive. This behavior of coming to the surface to gulp air is an evolutionary adaptation, allowing fish to thrive in environments that would suffocate most aquatic relatives. The need to breathe air separates fish into groups: those forced to the surface by poor water quality and those whose biology depends on the air above the water. These “air-breathing fish” have developed specialized organs to absorb oxygen directly from the atmosphere, bypassing their gills when necessary.
Environmental Factors That Force Surface Breathing
The most common reason a fish that typically uses gills rises to the surface is aquatic hypoxia, meaning the water contains a low level of dissolved oxygen (DO). This surface behavior, sometimes called “piping” or “aquatic surface respiration,” is a temporary measure adopted by fish otherwise reliant on water-breathing, making them facultative air breathers. They are only forced to the surface by external stressors, not by an inherent biological requirement.
Low DO is often caused by high water temperatures and biological activity. Warm water holds less oxygen than cold water, and this reduced solubility quickly creates oxygen-poor conditions, especially during summer months. High temperatures also increase the metabolic rate of the fish, causing them to need more oxygen just as the supply dwindles.
Decomposition of organic material, such as pollution or algae blooms, consumes oxygen, particularly at night when aquatic plants are not producing oxygen through photosynthesis. When DO levels fall below a species-specific threshold, fish move to the water’s surface, where a thin layer of water is constantly re-oxygenated by the air. This action is an attempt to use the oxygen-rich surface film, or to gulp atmospheric air, to stave off suffocation.
Fish That Must Breathe Atmospheric Gas
A different group of species are obligate air breathers, meaning they have an inherent biological requirement to access atmospheric gas regardless of water quality. For these fish, air breathing is a normal, continuous part of their respiration, not a response to environmental stress. If they are denied access to the surface, they will drown, even in water fully saturated with oxygen.
This need is an evolutionary adaptation to life in challenging habitats, such as stagnant, muddy, or seasonally drying tropical waters. The gills of these obligate air breathers are often reduced or insufficient to meet their full oxygen demand, forcing reliance on specialized air-breathing organs.
Examples of obligate breathers include the African Lungfish and the Arapaima of the Amazon, which synchronizes its surface gulps. Aquarium favorites, like the Betta fish, are also obligate air breathers and must regularly rise for a breath. For these fish, the atmosphere is their primary source of oxygen, with the water serving a secondary role for waste excretion, such as carbon dioxide.
Specialized Organs for Air Absorption
The ability of these fish to breathe air stems from specialized anatomical structures that have evolved independently in numerous fish families. These organs are highly vascularized (rich in blood vessels), allowing for the efficient transfer of oxygen from the gulped air directly into the bloodstream. The most well-known example is the labyrinth organ found in species like Gourami and Betta.
The labyrinth organ is a complex, folded structure of bony plates and mucous membrane located above the gills, designed for atmospheric gas exchange. When the fish gulps air, oxygen is trapped within these chambers and absorbed across the vascularized tissue before the spent air is expelled. Another common adaptation involves the swim bladder, which functions as a primitive lung in air-breathing species, though it is typically used for buoyancy control in most fish.
In fish such as the Arapaima and the Bowfin, the swim bladder is highly vascularized and connected to the esophagus, allowing the fish to gulp air and fill the organ for direct gas exchange. This modified organ is so large in the Arapaima that it envelops the animal’s kidneys. Other species, such as certain catfish, have evolved highly vascularized skin or specialized linings in the mouth cavity, called the buccal cavity, to serve as respiratory surfaces, absorbing oxygen directly across the moist membrane.