Fish
Lungfish
Dipnoi · also called salamanderfish
A lungfish is a fish that breathes. Not with a swim bladder pressed into service, but with real lungs, branched and blood-rich like a frog’s, and the African and South American species depend on them so completely that they will drown if held underwater. When their swamp dries, they wrap themselves in mud and mucus and wait — for months, or in extreme cases years.
Overview
Lungfish are the group Dipnoi, just six species — four in Africa, one in South America, one in Australia. They are lobe-finned fish, the same broad group as the coelacanth, and by most analyses the closest living fish relatives of land animals. They live in still fresh water, crush snails and clams with tooth plates, and all have at least one true lung.
Fins like limbs, and a walk
A lungfish has slender, muscular, limb-like fins on fleshy lobes, and the African species use them to walk and bound along the bottom in a salamander-like gait — a living picture of how fins became legs.
Waiting out the drought
When its swamp dries, an African lungfish burrows down, folds into a hook, and secretes a hardening mucus cocoon with a breathing tube to the surface, then burns its own muscle for fuel. It survives a dry season routinely, and there are records of revival after three to four years in dried mud.
Ancient, and huge in the genome
Lungfish go back 400 million years, the Queensland lungfish has barely changed in 100 million, and one captive individual lived from 1933 to 2017. Their genomes are among the largest of any animal — the marbled lungfish holds close to the record.
Status
Lungfish as a group are Not Evaluated. The African and South American species are reasonably secure across large ranges, but the Queensland lungfish is Endangered, native to just two river systems and threatened by dams that flood or dry its shallow spawning weed beds.
Lungfish: common questions
Do lungfish really have lungs?
Yes, genuine lungs, not just an air-holding swim bladder like some other fish. The lungfish lung develops from the gut wall, is divided internally into pockets that increase its surface area, and is served by its own blood supply that carries oxygen-poor blood to it and oxygen-rich blood away, the same arrangement as in a land animal. The Australian lungfish has one lung and uses it mainly as a backup when the water is warm and stagnant. The African and South American lungfish have paired lungs and depend on them, breathing air every few minutes to an hour, and their gills have shrunk so much that they cannot get enough oxygen from water alone and will drown if prevented from surfacing.
How does a lungfish survive a drought?
The African lungfish, and to a lesser extent the South American one, enters a dormant state called estivation. As its swamp dries, the fish digs a vertical burrow in the mud, folds itself into a U or hook with its snout pointing up, and its skin secretes copious mucus that dries into a tough, permeable cocoon around the body, with a small opening at the mouth for breathing. Inside, the fish slows its heartbeat and metabolism drastically and switches to burning its own muscle tissue for fuel, converting the toxic waste to a less harmful form. It can hold out for the length of a dry season routinely, and there are records of African lungfish reviving after being kept in dried mud for three to four years.
Are lungfish the ancestors of land animals?
Not the direct ancestors, but very close relatives of them. Land vertebrates, the tetrapods, evolved from lobe-finned fish in the Devonian period, and among living fish the lungfish are, by most genetic and anatomical analyses, the closest relatives of that tetrapod line, closer even than the coelacanth. The lungfish themselves are a separate branch that stayed in the water, and the actual fish-to-tetrapod transition ran through other, now extinct, lobe-fins such as Tiktaalik. But because lungfish have lungs, fleshy limb-like fins, and, in the African species, a bounding walk along the bottom, they give biologists a living model for the kind of animal from which legs and land life emerged.
Why is the Australian lungfish considered special and protected?
The Queensland, or Australian, lungfish is the most primitive of the six living species and has changed remarkably little in around 100 million years, so it is often called a living fossil and is regarded as a link to the deep history of vertebrates. Unlike its African and South American cousins it has a single lung, keeps functional gills, cannot estivate, and lays large eggs on aquatic plants rather than in a nest. Its natural range is tiny, just two rivers in southeast Queensland, and it is slow-growing and long-lived, so it recovers slowly from losses. It is listed as Endangered and is fully protected under Australian law, with dams on its rivers a particular concern because they alter the shallow, weedy shallows it needs to spawn.