Lungfish: Anatomy, Habitat, and Unique Biology

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-19

A lungfish is a lobe-finned freshwater fish that breathes air with a true lung, an organ that develops as a ventral outgrowth of the foregut. Six living species survive today in three families on three southern continents, and their genomes and anatomy make them the closest living relatives of tetrapods, the four-limbed vertebrates that include amphibians, reptiles, birds, and mammals.

That relationship is the reason lungfish matter far beyond their small species count. They sit on the sarcopterygian branch of the vertebrate tree, alongside coelacanths and tetrapods, and they retain features that help biologists reconstruct how fins became limbs and how water breathing gave way to air breathing. They are not ancestral tetrapods. They are the surviving sister lineage, a separate branch that kept many ancient traits while tetrapods went their own way [1][2].

What Is a Lungfish?

Lungfish belong to the class Sarcopterygii, the lobe-finned fishes, and within that group to the order Dipnoi. The name Dipnoi means "two breaths," a reference to the dual respiratory setup that defines the group: gills for water and lungs for air. The living species are all freshwater animals, and all of them can or must surface to gulp air.

The fossil record of dipnoans stretches back roughly 400 million years to the Devonian period. Early lungfish were more diverse than the modern survivors, and some lived in marine habitats. A Devonian genus called Rhinodipterus, recovered from marine limestones in Australia, shows skeletal features tied to air breathing, which places the origin of aerial respiration in lungfish at around 375 million years ago, near the start of the Late Devonian [3]. That finding matters because it undercuts the old idea that air breathing evolved only after lungfish moved into freshwater. Air breathing appears to have arisen in both marine and freshwater settings, and a global decline in oxygen levels during the Middle Devonian, combined with the metabolic cost of living in low-oxygen water, is a more likely driver [3].

Air breathing itself has evolved independently many times across fishes. At least 49 known families of fish breathe air in some form, using a wide variety of organs [4]. Lungfish are unusual not because they breathe air but because their air-breathing organ is a genuine lung, homologous with the lungs of tetrapods and with the lungs of ray-finned fishes such as bichirs [5].

The Three Living Families

Three genera survive, one per continent, and each has a distinct respiratory strategy. Getting these three right is the fastest way to understand lungfish diversity.

Neoceratodus: The Australian Lungfish

Neoceratodus forsteri lives in slow-flowing rivers and pools of southeastern Queensland, Australia. It has a single lung and well-developed gills, and it is the least dependent on air of the three genera. Under normal conditions it can meet its oxygen needs through the gills alone, and it surfaces for air mainly when water oxygen is low, when it is active, or when water temperature is high. Its fins are limb-like and paddle-shaped, and the genome study of 2024 noted that Neoceratodus fins still resemble those of extinct relatives and have stayed phenotypically static for about 100 million years [1].

Lepidosiren: The South American Lungfish

Lepidosiren paradoxa inhabits the Amazon and Paraná river basins and associated wetlands. It has paired lungs and is an obligate air breather, meaning it must surface to breathe air to survive. Its gills are reduced and cannot support it alone. Its paired fins are thin and filamentous, a derived condition rather than a primitive one. The 2024 genome paper described a secondary loss of limb-like appendages on the Lepidosiren and Protopterus lineage, which is a useful reminder that these fishes are not frozen ancestors but animals with their own evolutionary history [1].

Lepidosiren has the largest animal genome sequenced to date, about 91 gigabases, roughly 30 times the size of the human genome and more than twice the size of the Australian and African lungfish genomes. The expansion comes from enlarged intergenic regions and introns packed with repetitive content, about 90 percent, and the genome grew extremely fast over the past 100 million years, adding the equivalent of one human genome every 10 million years [1].

Protopterus: The African Lungfish

Protopterus is the most species-rich genus, with four recognized species including P. annectens, P. aethiopicus, P. dolloi, and P. amphibius. All have paired lungs and reduced gills, and all are obligate air breathers as adults. They live in rivers, swamps, and floodplain pools across sub-Saharan Africa, habitats that routinely dry out. This genus is famous for estivation, a dormancy state that carries it through the dry season buried in dried mud.

The respiratory split between gills and lungs changes with body size in Protopterus. In P. amphibius, small specimens of 3 to 7 grams got about 70 percent of their oxygen from water, while grown specimens of around 255 grams got only 10 to 15 percent from water. Combined oxygen uptake fell from about 60 milliliters of oxygen per kilogram per hour in the smallest fish to about 30 in the largest, while aquatic uptake alone changed eightfold across the same range [6]. Young African lungfish are therefore much more dependent on gills than adults, and the shift toward air breathing is a normal part of growth.

Lungfish Anatomy

Lungfish anatomy is a mix of primitive sarcopterygian features and specializations unique to the group. The skull, the fins, the gills, and the lung all carry information about the fish-tetrapod transition.

The Lung

The lung of a lungfish is a ventral outgrowth of the foregut, the embryonic tube that also gives rise to the esophagus and stomach. This developmental origin is the key anatomical fact. A lung buds off the floor of the gut, while a swim bladder in most ray-finned fishes buds off the roof or the dorsal side. That difference in position and blood supply is why lungs and swim bladders are not simply the same organ under two names.

The vascular plumbing that serves the lung is elaborate. In Protopterus and Lepidosiren, a short muscular vessel called the ductus connects the systemic and pulmonary circulations, and its structure closely resembles the ductus arteriosus of mammals. It is richly innervated, with myelinated and unmyelinated fibers arising at least partly from the vagus nerve, and neurons sit in its outer wall. Perfusing the ductus with hypoxic saline makes it dilate, while alpha-agonists constrict it. Dopamine and prostaglandin E2 are potent dilators [7]. A separate vasomotor segment sits on the pulmonary artery near its junction with the ductus, positioned much like the equivalent segment in amphibians and reptiles, and it constricts in response to acetylcholine [7]. This is a control system built for switching blood flow between water-breathing and air-breathing circuits.

Gills and Skin

Lungfish are not lung-only animals. The Australian lungfish relies heavily on gills. The African and South American species have reduced gills and use them less, but gills still contribute, especially in young fish [6]. Skin also participates in gas exchange in many air-breathing fishes, and many amphibious vertebrates are trimodal breathers at some stage, using skin, gills, and lung in various combinations [4].

During estivation, the gills of Protopterus annectens undergo dramatic remodeling. The secondary lamellae collapse, and the animal shifts to exclusive aerial ventilation as the lungs become vascularized and expanded [8]. At the molecular level, aestivating gills and lungs show highly similar transcriptome profiles, and the lungs maintain significantly higher bioactivity during dormancy, which suggests the main respiratory organ can effectively transform during estivation [9].

Fins and Limbs

The paired fins of Neoceratodus are fleshy, lobed, and supported by a skeleton that resembles the limb bones of early tetrapods. This is the anatomy that makes lungfish useful for studying the fin-to-limb transition. The genome work of 2024 confirmed that Neoceratodus fins have remained phenotypically static for about 100 million years, while the Lepidosiren and Protopterus lineage lost limb-like appendages secondarily [1]. Lungfish chromosomes also conservatively reflect the ancestral tetrapod karyotype despite massive genome expansion, and transposable elements remain active in all lungfish genomes today [1].

Respiration: How Lungfish Breathe

Breathing air and breathing water are mechanically different problems. Air is about 1,000 times less dense and 50 times less viscous than water, and it holds hundreds of times more oxygen. Air breathing does not require the body support that walking on land demands, so the evolutionary changes needed for air breathing may have been smaller than those needed for terrestrial locomotion [10].

Even so, the mechanics differ. Researchers used X-ray reconstruction of moving morphology to compare suction feeding, an aquatic buccal pumping behavior, with the aerial buccal pumping used for lung ventilation in Protopterus annectens. Both behaviors show an anterior-to-posterior wave of expansion followed by an anterior-to-posterior wave of compression. The pectoral girdle and cranial rib rotate consistently during both, and the muscle between them shortens during buccal expansion. The major cranial bones keep the same basic functions in both modes [10]. In plain terms, the same head machinery that pulls water through the mouth can be repurposed to push air into the lungs.

Ventilation itself is tightly controlled. In Lepidosiren paradoxa, the normal breathing pattern consists of isolated ventilatory cycles, each made of an expiration followed by about 2.4 buccal inspirations. Expiratory and inspiratory tidal volumes reach a maximum of roughly 35 milliliters per kilogram, which means the fish can exchange nearly all of its lung air in a single cycle. Hypoxia increases pulmonary ventilation mainly by raising breathing frequency rather than by changing tidal volume. Expiratory time shortens at 30 degrees Celsius, and carbon dioxide release and air convection requirement both respond significantly to severe hypoxia at 7 percent oxygen [11].

Control of breath timing also resembles that of mammals. In Protopterus annectens, inflating the lungs at the start of a breath shortens the breath, and the relationship between intrapulmonary pressure and breath duration is curvilinear, similar to the relationship between tidal volume and inspiratory duration in mammals. Cutting the vagus nerve largely abolishes this effect [12]. The neural circuits for respiratory timing appear to have been in place long before mammals existed.

Chemoreception supports all of this. Respiratory control depends on feedback from peripheral and central receptors that sense oxygen partial pressure, carbon dioxide partial pressure, and pH. The specific chemoreceptors have not been conclusively identified, but in water-breathing teleosts, neuroepithelial cells in gill tissue are implicated as oxygen-sensitive receptors that trigger cardiorespiratory reflexes. Protopterus, along with Polypterus and Amia, has these cells in gills or lungs [4].

Habitat and Ecology

All living lungfish are freshwater animals, but the habitats differ sharply by continent. Neoceratodus lives in permanent rivers and pools where water usually holds enough oxygen for gill breathing. Lepidosiren lives in Amazon and Paraná basin wetlands that flood and dry on a seasonal cycle. Protopterus lives in African rivers, swamps, and floodplain pools that may dry completely for months.

These habitats create different respiratory pressures. In oxygen-poor, warm, stagnant water, gills alone cannot meet demand, and air breathing becomes essential. This is likely why obligate air breathing evolved in the South American and African lineages while the Australian species retained facultative air breathing.

Water chemistry adds another challenge. Tropical air-breathing fishes can face high environmental ammonia when stranded in shrinking puddles during the dry season, when confined in a burrow, or after agricultural fertilization. At low environmental ammonia, ammonia excretion is impeded and endogenous ammonia accumulates. At high environmental ammonia, the partial pressure gradient reverses and the fish both retains its own ammonia and takes up ammonia from the water [13]. Air-breathing fishes have modified gill morphology and accessory breathing organs, which tends to reduce branchial ammonia excretion, so they need other strategies to handle the toxic load [14].

Estivation in African Lungfish

Estivation is a dormancy state that lets some animals survive hot, dry conditions. In African lungfish, it is the most dramatic survival strategy in the group. When a pool dries, Protopterus burrows into the mud, curls its body, and secretes mucus that hardens into a cocoon around it. The fish remains inside this cocoon in dried mud for months, without food or water.

The metabolic changes are substantial. Aestivating African lungfish detoxify ammonia to urea and accumulate it, then excrete the stored urea after returning to water [14]. In Protopterus dolloi, fish kept in water excrete mostly ammonia, but after 30 days of estivation in air they release massive amounts of urea on return to water. Urea excretion follows a biphasic pattern, with an immediate rise, a fall, and a second larger peak at about 12 hours that can last several days. Peak rates reach 2,000 to 6,000 micromoles of nitrogen per kilogram per hour, two to three orders of magnitude above rates in most fish and comparable only to species that use facilitated diffusion urea transporters [15].

Muscle does not waste away during this long dormancy. Myostatin, a factor linked to disuse muscle atrophy in mammals, shows a rise in transcript and protein during the induction and early maintenance phases of estivation in P. annectens, then returns to control levels after 6 months, which may prevent runaway muscle breakdown [16]. Brain cells manage ion balance through sodium-potassium ATPase isoforms. Protein abundance of one isoform rises after 12 days of estivation, likely to maintain cellular sodium and potassium concentrations and regulate cell volume, while messenger RNA for another isoform falls after 6 months, consistent with suppressed transcription to save energy [17]. Broader transcriptome work confirms that estivation triggers widespread, tightly controlled changes in gene expression [9].

Signal molecules coordinate the organ remodeling. After 6 months of estivation, nitric oxide synthase, Akt, and Hsp-90 expression fall in the gills while nitric oxide synthase and Hsp-90 rise in the lungs. After 6 days of arousal, gill values return to freshwater levels [8]. The animal is not simply shutting down. It is actively rebuilding its respiratory system for a different world.

Why Lungfish Matter for Understanding Tetrapods

Lungfish and coelacanths are the only living sarcopterygian fishes, and the phylogenetic position of lungfish relative to the last common ancestor of tetrapods makes them uniquely informative [2]. Transcriptome evidence indicates that lungfish, not coelacanths, are the closest relatives of land-adapted vertebrates [2]. Protein-coding genes in lungfish evolve very slowly, which fits their reputation as living fossils, while transposable elements remain active and diverse, which may help explain the enormous genome expansion [2].

The 2024 genome study tied these molecular patterns directly to the Devonian transition. Lungfish genomes inform the molecular and developmental basis of the fish-tetrapod transition, and the conserved karyotype plus the static Neoceratodus fins show that some ancient features persisted for a very long time [1].

The critical caveat is that lungfish are not ancestral tetrapods. They are the closest living relatives, which is a different claim. The last common ancestor of lungfish and tetrapods lived in the Devonian and is extinct. Living lungfish have had roughly 400 million years of their own evolution since that split, and they carry their own derived traits, including reduced gills, filamentous fins in two genera, and gigantic genomes. They are a window into the transition, not a snapshot of the ancestor.

Air breathing itself should not be read as a tetrapod invention shared with lungfish. Air breathing in lungfishes is not considered a shared specialization with tetrapods. It evolved independently [3]. The same is true of other air-breathing structures across fishes, which have arisen many times in different lineages [4]. Bichirs, for example, are ray-finned fishes that breathe air, share lung structure similarities with sarcopterygians, and inhale through large paired spiracles on top of the head. Spiracle-mediated aspiration accounts for up to 93 percent of all air breaths in four Polypterus species, and the size and position of those spiracles resemble those of some stem tetrapods, which suggests spiracular air breathing may have been important during the water-to-land transition [18].

Common Mistakes and Limitations

The most common mistake is calling lungfish ancestral tetrapods. They are the closest living relatives of tetrapods, not the ancestors. The distinction matters because it changes how you read every comparison between a lungfish fin and a tetrapod limb.

A second mistake is treating the lung as a modified swim bladder. The lung is a ventral outgrowth of the foregut. The swim bladder of most ray-finned fishes has a different developmental origin and a different blood supply. The two organs are not interchangeable, even though the homology question across vertebrates is still actively discussed [5].

A third mistake is assuming all lungfish estivate. Only Protopterus is known for the mucus cocoon and months of dormancy in dried mud. Neoceratodus does not estivate. Lepidosiren shows seasonal dormancy in some habitats, but the classic cocoon story belongs to the African genus.

A fourth mistake is assuming all lungfish are obligate air breathers. Neoceratodus can meet its oxygen needs through gills under normal conditions. Lepidosiren and Protopterus are obligate air breathers as adults, but young Protopterus get most of their oxygen from water [6].

A fifth mistake is reading genome size as a measure of complexity. Lepidosiren has the largest animal genome sequenced, about 91 gigabases, but the expansion comes largely from repetitive content in intergenic regions and introns, not from a proportional increase in protein-coding genes [1].

Several questions remain open. The specific chemoreceptors that initiate respiratory reflexes have not been conclusively identified [4]. The mechanisms driving respiratory organ remodeling during estivation are not fully known [8]. The homology relationships between sarcopterygian lungs, actinopterygian lungs, and actinopterygian gas bladders are still under discussion [5]. Individual animals vary, and any clinical or husbandry decision about a specific fish needs a qualified veterinarian or aquatic animal specialist.

Frequently Asked Questions

How many species of lungfish are alive today?

Six species survive, divided among three genera: Neoceratodus in Australia, Lepidosiren in South America, and Protopterus in Africa.

Do all lungfish have two lungs?

No. Neoceratodus has a single lung. Lepidosiren and Protopterus have paired lungs.

Are lungfish the ancestors of land animals?

No. They are the closest living relatives of tetrapods, not their ancestors. The last common ancestor of lungfish and tetrapods is extinct.

Can lungfish breathe through their skin?

Skin contributes to gas exchange in many air-breathing fishes, and many amphibious vertebrates use skin, gills, and lungs in combination at some life stage [4].

How long can an African lungfish estivate?

African lungfish can remain torpid without food and water for extended periods, and studies have examined animals after 6 months of estivation [16][17].

What is the mucus cocoon?

It is a hardened mucus envelope that Protopterus secretes around its body when it burrows into drying mud for estivation.

Why do lungfish have such large genomes?

Genome expansion in lungfish comes mainly from repetitive content in intergenic regions and introns, and transposable elements remain active in all lungfish genomes today [1].

Do lungfish use their gills at all?

Yes. Neoceratodus relies heavily on gills. Lepidosiren and Protopterus have reduced gills, but gills still contribute, especially in young African lungfish [6].

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14. Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes.
15. Greatly elevated urea excretion after air exposure appears to be carrier mediated in the slender lungfish (Protopterus dolloi).
16. Molecular characterization of myostatin from the skeletal muscle of the African lungfish, Protopterus annectens, and changes in its mRNA and protein expression levels during three phases of aestivation.
17. Brain Na+/K+-ATPase α-subunit isoforms and aestivation in the African lungfish, Protopterus annectens.
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