Lungfish are freshwater lobe-finned fish classified under the class Dipnoi and are the closest living relatives of the tetrapods. They have elongated, eel-like bodies with paired pectoral and pelvic fins, along with a caudal fin. Unlike most fish, lungfish can breathe air, thanks to their sac-like lungs in addition to gills. The name Dipnoi, meaning ‘two breathing structures,’ highlights their unique ability to respire through both lungs and gills.
Another unique trait of these fish is their mouth, which bears dermal tooth plates called odontodes on the palate and lower jaws. This type of dentition helps them crush hard-shelled animals, like crustaceans and mollusks, which form a major part of their diet.
Currently, there are 6 extant species of lungfish spread across Africa, South America, and Australia. The Queensland lungfish is found in Australia, whereas the South American lungfish is endemic to South America. The remaining four species, the West African, spotted, gilled, and marbled lungfish, are found in Africa.
The length of lungfish varies across the different species. Here is a list of the 6 living species, along with their body length.
| Species | Body Length |
|---|---|
| Queensland Lungfish | Around 3.3 ft (100 cm) |
| South American Lungfish | Up to 4 ft 1 in (125 cm) |
| Marbled Lungfish (Largest) | Up to 6.5 ft (200 cm) |
| Gilled Lungfish (Smallest) | Around 17 in (44 cm) |
| West African Lungfish | About 3.2 ft (100 cm) |
| Spotted Lungfish | Around 2.7 ft (85 cm) |
They have an elongated body with paired pectoral and pelvic fins, along with a single unpaired caudal fin. Their notochord is continuous and made of cartilage, and they have well-developed palatal teeth. As members of Sarcopterygii, the fins of lungfish are lobed, and they also have a well-developed, bony endoskeleton.
This lungfish has a stout body with a flattened head and small eyes. Both their pelvic and pectoral fins are fleshy and paddle-like.
The body color ranges from olive-green to dull brown on the back, while the sides, tail, and fins vary from yellow to orange. During the breeding season, males develop bright red sides, which are likely to attract females.
This species has an eel-like body with thin, threadlike pectoral fins and relatively larger pelvic fins. Unlike most fish, which have at least four bones at the base of each fin, all of its fins are connected to the shoulder by a single bone.
When immature, this lungfish is black in color with spots of gold on the body. However, as it grows, it fades into a gray or brown shade.
They have smooth, cylindrical bodies with deeply embedded scales. Their tail is considerably long, tapering at the end.
The base body color is yellow, pink, or gray, speckled with dark slate-gray splotches, giving this species a marbled (hence the name) or leopard-like look.
This species is typically blue or slate grey; however, it is flecked with tiny, almost inconspicuous black spots. Its belly is lighter gray in color.
Their eel-like bodies are approximately 5 to 9 times the length of their heads. The fins are long and filamentous, with the pectoral fins featuring a distinct basal fringe. The pelvic fins are about twice the length of the head, while the pectoral fins are three times as long.
They have cycloid scales on their bodies. There are 40 to 50 scales between the operculum and anus and 36 to 40 scales just before the dorsal fin.
While their dorsal surface is olive or brown in color, the ventral part is lighter and spotted except on the belly.
Like the West African lungfish, this species, too, has an eel-like body. It is generally brown, and despite being called ‘spotted,’ black spots are found only in their juveniles.
In addition to gills, lungfish possess primitive lungs, like those of amphibians. These lungs are modified swim bladders and are directly connected to the alimentary canal (larynx and pharynx) without an intermediate trachea. All extant lungfish have two lungs except for the Queensland lungfish, which has one. Internally, these lungs are partitioned into smaller air sacs, which help increase the surface area for gaseous exchange.
Of all extant species, only the Queensland lungfish breathes exclusively through the gills and not the lungs. In other species, the gills are so atrophied that they require supplementary pulmonary circulation for adequate gaseous exchange.
Respiration
Lungfish have a dual breathing system, allowing them to respire through both gills (in water) and lungs (in air). When using their gills, the spiral valve in the conus arteriosus (a muscular chamber at the base of the heart) remains open. During this process, the third and fourth gill arches close while the second, fifth, and sixth remain open. Since the lungs are inactive, the pulmonary arteries stay closed, and water flows unidirectionally through the mouth and gills.
In contrast, when lungfish breathe through their lungs, the third and fourth gill arches open, along with the pulmonary arteries. The sixth gill arch also remains open, allowing air to move in and out of the mouth in a tidal pattern.
In primitive lungfish, the bones of the skull roof were covered in a mineralized tissue called cosmine. However, in present-day species, the roof lies beneath the skin, and the cosmine layer is absent. In these species, most bones of the skull are significantly reduced and fused.
These fish have a dentition unique among all vertebrates. They possess dermal tooth plates or odontodes on the palate and lower jaws. These teeth form a fan-shaped occlusion surface that eventually wears out and smoothens to form a uniform crushing surface.
The name of the class, Dipnoi, derives from the Greek word dipnoos, which translates to ‘two breathing structures.’
The 6 living species of lungfish are grouped under 3 families, each having a single genus.
The closest bony fish relatives of lungfish are the prehistoric groups Powichthys and Porolepiformes.
| Period | Event |
|---|---|
| Devonian Period (~420 million years ago) | The last common ancestor (LCA) of lungfish and tetrapods diverged into two separate lineages. |
| Early Devonian Period (419 to 417 million years ago) | The earliest lungfish, such as Youngolepis and Diabolepis, appeared.These species were marine and had adapted to a durophagous diet, feeding on hard-shelled animals. |
| Middle to Late Devonian (382.7 to 358.9 million years ago) | Fossils of Scaumenacia and Phaneropleuron date back to this period.These species had a much-reduced first dorsal fin, followed by a large second dorsal fin. |
| Carboniferous to Early Permian (323.2 to 251.9 million years ago) | Dipterus existed during this time, with leaf-like pectoral and pelvic fins resembling the modern Queensland lungfish. |
| Late Carboniferous to Jurassic Periods(250 to 143 million years ago) | Lungfish had transitioned to freshwater habitats.The last common ancestor of all extant lungfish appeared during this time. |
Their numbers kept decreasing, thereby leading to a gradual loss of diversity.
The six extant species of lungfish are distributed in Africa, South America, and Australia.
These fish are omnivorous and generally feed on insects, worms, crustaceans, mollusks, amphibians, and other fish. They also consume aquatic plants, their roots, fruits, seeds, and flowers.
These fish are generally long-lived. As of 2022, a Queensland lungfish named Methuselah has been unofficially designated as the oldest living species. It is estimated to be over 90 years old. However, another individual, the Granddad (also a Queensland lungfish), was 109 years old when it was euthanized in 2017.
The South American lungfish have an average lifespan of about 8 years in the wild, though they have been found to survive up to 15 years in aquariums. In contrast, the average lifespan of the West African lungfish is about 20 years in the wild.
African lungfish and the Queensland lungfish breed between the onset of monsoon and the last half of winter. The former dig pit-like nests at the bottom of the water bodies and lay multiple eggs (about 0.14 in). In the South American lungfish, notably, the males remain in the nest and constantly guard the eggs before they hatch into larvae.
The eggs hatch into larvae almost a week after they are laid. The larvae possess bright red, tuft-like external gills, which help in breathing till their lungs develop.
However, the Queensland lungfish lays gelatinous eggs that hatch into larvae lacking external gills. Instead, these larvae respire through internal gills within their bodies.
The eggs and juveniles of these fish are highly vulnerable to predation. They are commonly preyed upon by larger fish, such as jewfish and tilapia, as well as crustaceans and birds like wood ducks. In contrast, adult lungfish face no significant natural predators.
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