Six species of lungfish are the best-known fish with true lungs, but they aren’t the only ones. Bichirs, a family of freshwater fish from Africa, also have paired lungs that develop from the same embryonic tissue as human lungs. Beyond these groups, several other fish breathe air using modified swim bladders, though those organs aren’t technically lungs. The distinction matters because lungs and swim bladders grow from different sides of the embryonic gut, lungs from the ventral (bottom) side and swim bladders from the dorsal (top) side.
The Six Living Lungfish Species
Lungfish belong to an ancient group called Dipnoi, and only six species survive today, split across three continents. Four species live in Africa (genus Protopterus), one in South America (Lepidosiren paradoxa), and one in Australia (Neoceratodus forsteri). All six trace their geographic distribution to the breakup of Gondwana, the supercontinent that split into the southern landmasses roughly 180 million years ago. Their current homes reflect where the pieces ended up.
These six species vary widely in how much they depend on their lungs. The African and South American lungfish have paired lungs and are obligate air breathers, meaning they will drown if denied access to the surface, even in well-oxygenated water. The Australian lungfish is a different story: it has only a single lung and breathes primarily through its gills, surfacing to gulp air only during heavy exertion or when water quality drops.
African Lungfish and Estivation
The four African lungfish species are famous for surviving droughts that would kill virtually any other fish. When their swamps and rivers dry up, they burrow into the mud and secrete a cocoon of mucus around their bodies. Inside this cocoon, they enter a state called estivation, a deep torpor where all movement and feeding stops. Their lungs become their sole source of oxygen, pulling air through a small breathing hole in the hardened mud.
This isn’t a brief nap. African lungfish can remain in estivation for as long as four years. During that time, they face a serious biochemical problem: without water flowing over their gills, they can’t flush out ammonia, a toxic waste product of metabolism. To survive, they switch their waste processing to produce urea instead, which is far less toxic and can be stored safely in their tissues. They also slow their metabolism dramatically to avoid poisoning themselves. The entire process is a remarkable example of a fish living more like a hibernating land animal than anything aquatic.
South American Lungfish
The South American lungfish, Lepidosiren paradoxa, is the sole species on its continent. It lives in swampy, low-oxygen waters across the Amazon and Paraná river basins and can grow up to 1.5 meters long. Its body is elongated and dark, built for slow movement through muddy environments rather than open-water swimming. Like its African relatives, it is an obligate air breather with paired lungs.
Its fins are unusual. Rather than the broad, paddle-like fins of the Australian lungfish, the South American species has thin, whisker-like pectoral and pelvic fins. These are nearly useless for propulsion but function as sensory organs, detecting touch and chemical signals in murky water where vision is limited. Males develop another surprising adaptation during breeding season: their pelvic fins sprout temporary respiratory filaments, essentially gill-like structures that release oxygen into the water around their eggs.
The Australian Lungfish Stands Apart
Neoceratodus forsteri is often called a “living fossil,” and its anatomy explains why. It has a single lung rather than the paired lungs found in African and South American species, and it relies on its gills for most of its oxygen needs. Air breathing kicks in only as a backup, during periods of vigorous activity or when dissolved oxygen in the water drops. This makes it a facultative air breather: it can breathe air but doesn’t need to in order to survive under normal conditions.
It also looks markedly different from its relatives. The Australian lungfish has large, fleshy, paddle-shaped fins and heavy scales, giving it a more robust, prehistoric appearance. It’s found only in a handful of river systems in Queensland, Australia, and is protected by law. Unlike the African species, it cannot estivate and will die if its habitat dries out completely.
Bichirs: The Other Fish With True Lungs
Bichirs (family Polypteridae) are a group of freshwater fish native to African rivers and lakes, and they genuinely have lungs, not modified swim bladders. Histological studies of the Senegal bichir (Polypterus senegalus) show that its lungs develop through a mechanism nearly identical to that of land-dwelling vertebrates. The lungs grow from the ventral side of the foregut, use the same genetic toolkit (including a key regulatory element called Tbx4 C-LME), and share structural similarities with tetrapod lungs at the tissue level.
This finding has reshaped how scientists think about lung evolution. Because bichirs are ray-finned fish (the group that includes most fish alive today) while lungfish are lobe-finned fish (the group that includes the ancestors of all land vertebrates), the fact that both lineages have true lungs suggests their common ancestor already had them. That pushes the origin of lungs back to at least 400 million years ago, before the two major branches of bony fish diverged. In other words, lungs likely came before swim bladders, not the other way around. Swim bladders appear to be a later modification where the breathing organ migrated to the top of the gut and was repurposed for buoyancy control.
Fish That Breathe Air Without True Lungs
Several other fish species breathe air but use a modified swim bladder or other specialized structure rather than a true lung. These are worth knowing about because they’re sometimes confused with lung-bearing fish.
- Arapaima (Arapaima gigas): One of the largest freshwater fish in the world, found in the Amazon basin. It is an obligate air breather that must surface every 10 to 20 minutes. Its swim bladder is heavily vascularized and functions like a lung, but it develops from the dorsal side of the gut.
- Bowfin (Amia calva): A North American freshwater fish that uses its “respiratory swim bladder” to supplement gill breathing. It’s a facultative air breather, turning to its swim bladder mainly in warm, low-oxygen water. Most of its carbon dioxide is still expelled through the gills into the water.
- Electric eel (Electrophorus electricus): An obligate air breather that takes in oxygen through its highly vascularized mouth cavity. It will drown without surface access.
- Snakehead (Channa argus) and swamp eel (Monopterus albus): Both are obligate air breathers found in Asia. Snakeheads use a structure called a suprabranchial organ above their gills, while swamp eels absorb oxygen through specialized tissue in their throat.
The key distinction is anatomical origin. True lungs bud from the underside of the embryonic throat. Swim bladders, even highly efficient respiratory ones, bud from the top. Both can exchange gases effectively, but they represent different evolutionary paths from a shared starting point.
Why Lungs Evolved in Fish First
It’s a common misconception that lungs evolved when fish first crawled onto land. The fossil and genetic evidence tells a different story. Lungs appeared in aquatic fish during the Devonian period, roughly 400 million years ago, long before any vertebrate walked on land. The early bony fish that developed lungs lived in shallow, warm waters where dissolved oxygen was often scarce. Being able to gulp air at the surface was a survival advantage in those environments, not a stepping stone to terrestrial life.
Fossil lungfish from the Devonian show curved rib structures consistent with enlarged lung cavities, and even some marine species from that period show anatomical hints of air-breathing capacity. Over time, most ray-finned fish lineages converted their ventral lung into a dorsal swim bladder, optimized for buoyancy rather than breathing. But in lungfish and bichirs, the original lung persisted, giving us a window into what the respiratory system of our own distant ancestors looked like before they ever left the water.