Lungfish are an ancient and unique group of freshwater fish whose lineage dates back over 400 million years, making them one of the oldest living vertebrates on Earth. They possess highly developed, functional lungs, which is why they are considered the closest living relatives to tetrapods—the four-limbed vertebrates that include amphibians, reptiles, birds, and mammals. This respiratory adaptation provides a biological snapshot of the evolutionary phase where aquatic life began to acquire structures for breathing air.
The Dual Breathing System
The lung is a complex organ structurally comparable to the lungs of terrestrial vertebrates. It is considered homologous to the swim bladder found in most bony fish, having evolved to primarily function in gas exchange rather than buoyancy control. The lung connects to the pharynx, or throat, via a glottis-like opening, allowing the fish to surface and gulp atmospheric air.
Internally, the lung is partitioned into numerous small, vascularized air sacs, which significantly increase the surface area available for oxygen absorption into the bloodstream. This aerial respiration is coupled with a modified circulatory system that routes deoxygenated blood to the lung before sending it to the rest of the body, mirroring the pulmonary circuit of land animals.
The reliance on aerial respiration varies significantly among species. African and South American lungfish are obligate air-breathers, meaning their gills are often too reduced to meet their metabolic oxygen demand, even in well-oxygenated water. They must regularly surface to breathe, or they will drown. In contrast, the Australian lungfish is a facultative air-breather, relying mostly on its gills and only using its single lung to supplement oxygen uptake when the water becomes warm or stagnant.
Global Species of Lungfish
The six living species of lungfish are found on three continents, reflecting a distribution pattern that originated before continental drift. They are categorized into three main geographical groups. The Australian lungfish (Neoceratodus forsteri) is the most primitive extant species and possesses a single, unpaired lung. This fish is endemic to the Burnett and Mary River systems in Queensland.
The African lungfish (Protopterus), which includes four species, and the South American lungfish (Lepidosiren paradoxa), a single species, belong to a more derived group. Both possess paired lungs, which are two separate sacs running parallel to each other. The African species are distributed throughout the continent’s tropical wetlands, while the South American species is found in the Amazon and Paraná basins.
Surviving Extreme Environments
The necessity of the lung system is most evident in estivation, a state of metabolic dormancy triggered by environmental stress. When seasonal floodplains and swamps in Africa and South America dry up, the water becomes stagnant and oxygen-depleted (anoxia).
The African lungfish initiates estivation by burrowing tail-first into the drying mud, creating a subterranean chamber. As the mud hardens, the fish secretes mucus from its skin, which dries to form a parchment-like cocoon. This protective shell encases the body, with a small tube leading from the mouth to the surface, allowing the fish to continue breathing air.
In this dormant state, the fish drastically lowers its metabolic rate, sometimes by 60% or more, allowing it to survive without food or water for extended periods. African lungfish have been known to remain in this state for up to four years until the rains return and the water levels rise again. The South American species also burrows and estivates, though it generally does not form the full, hard mucus cocoon seen in its African counterparts.