Air-Breathing Fish: Labyrinth Organs, Gut Breathing and Lungs

How some fish breathe atmospheric air: labyrinth organs, gut breathing in catfish and loaches, lungfish and lung-like swim bladders, and why such fish need surface access.

Why some fish breathe air

Water holds far less oxygen than air, and fish normally extract it with their gills. Vertebrates first breathed atmospheric air about 400 million years ago, and the ability evolved independently in several fish lineages. Today many species carry accessory respiratory organs (also called air-breathing organs, ABOs) that work alongside the gills and skin. Such organs are common in fish from poorly oxygenated waters, and all of them are thin, heavily vascularized surfaces where oxygen passes from swallowed air into the blood. Known types include labyrinth organs, modified parts of the digestive tract and gas bladders.

Facultative air breathers can live on gill breathing alone when oxygen is normal and turn to air only when the water becomes hypoxic.
Obligate air breathers must reach the surface even at normal oxygen levels, because their gills alone cannot take up enough oxygen.

Many fish without an air-breathing organ use aquatic surface respiration instead: in low-oxygen water they move up and ventilate their gills with the thin, better-oxygenated surface layer. In one study, all but four of 26 North American species without air-breathing organs did this under hypoxia.

The labyrinth organ

Labyrinth fish (suborder Anabantoidei, including the families Anabantidae, Helostomatidae and Osphronemidae) are named after this organ. It is a much-folded structure in a chamber above the gills, formed by an enlarged epibranchial bone of the first gill arch and covered with blood-rich respiratory tissue. The chamber is completely separate from the mouth cavity. The fish gulps air at the surface, and oxygen is absorbed from it through the folds. UF/IFAS extension material describes the organ as a modified gill used like a lung. Labyrinth fish include gouramis, paradisefish, the kissing gourami (Helostoma temminckii) and the Siamese fighting fish (Betta splendens).

Labyrinth fish are not born with a working labyrinth. Young fish rely on their gills, and the organ develops later, in some species only after sexual maturity. Its size depends on the habitat: species from oxygen-poor waters develop larger, more complex organs than those from fast, well-oxygenated streams. Snakeheads (Channidae), a related group, have a simpler suprabranchial organ that develops with age, and they can survive on land for several days if kept wet.

Air-breathing catfish and loaches

Catfish have evolved air breathing several times, using different organs. Airbreathing catfish (Clariidae, such as Clarias) have a tree-like dendritic organ in the suprabranchial chamber. This evolved in parallel to the labyrinth organ and is used during the seasonal drying of African and Asian swamps and rivers. Clariids are facultative air breathers.

Corydoras and other armoured catfish (Callichthyidae) breathe through the gut. According to UF/IFAS, these normally bottom-dwelling fish dash to the surface, gulp a bubble and take oxygen from it in the intestine. The weatherloach (Misgurnus anguillicaudatus) is a facultative air breather that also uses part of its digestive tract. It inhales through the mouth, passes the air to a thin-walled, highly vascularized posterior intestine where gas exchange takes place and releases the used gas through the anus. A 2024 study found that this gut breathing also helps move food through the intestine.

Lungfish and lung-like swim bladders

Living lungfish belong to three genera: Neoceratodus (Australia), Lepidosiren (South America) and Protopterus (Africa). Most have two lungs, while the Australian lungfish has one. The South American lungfish is an obligate air breather whose adult gills are greatly reduced and nearly non-functional. The Australian lungfish is the only lungfish that can rely on its gills without breathing air, and the African species fall in between. African and South American lungfish survive the dry season by burrowing into mud and aestivating. Their metabolism can fall to about one-sixtieth of normal, and they switch from excreting ammonia to excreting less toxic urea.

Some other fish use the swim bladder as a lung. The arapaima (Arapaima) has an enlarged swim bladder lined with lung-like tissue, depends on surface air and can live in oxbow lakes with dissolved oxygen as low as 0.5 ppm.

Aquarium implications

Surface access: air-breathing fish must be able to reach the surface freely. Obligate air breathers depend on it even in well-oxygenated water.
Humid air layer: for Betta splendens, Seriously Fish advises a well-fitting cover and a tank that is not filled to the brim. This leaves a layer of warm, humid air above the surface and keeps these good jumpers inside. When rearing fry, it recommends the tightest-fitting cover possible, because the fry need this layer to develop the labyrinth organ properly.
Gentle flow: the same source recommends avoiding strong water movement for this species, which lives in sluggish water.
Behaviour as a signal: regular trips to the surface are normal for labyrinth fish and Corydoras. In fish without air-breathing organs, gasping or hanging at the surface is aquatic surface respiration, a response to low oxygen.