A swim bladder is usually a large air sac with a simple structure.

The lungs of a lungfish are divided up into  many small air sacs, so that the surface area available for gas exchange is increased.

Lungfish and tetrapods inherited their lungs from a common ancestor.

Tetrapods consist of four-limbed land vertebrates and animals that descended from four-limbed land vertebrates.

Human beings are tetrapods.

The earliest lungfish that we know of lived during the Devonian period, which occurred from around 416 million to around 360 million years ago.

There were once many species of lungfish, which lived all over the world.

Today, there are only six living species of lungfish: the South American lungfish (Lepidosiren paradoxa), which is also known as the Amazonian lungfish, the Australian lungfish (Neoceratodus forsteri), which is also known as the Queensland Lungfish, and four species of African lungfish.

The four living African lungfish species are the East African lungfish (Protopterus amphibius), also known as the gilled African lungfish, the West African lungfish(Protopterus annectens), also known as the Tana lungfish, the marbled lungfish (Protopterus aethiopicus), also known as the leopard lungfish, and the spotted lungfish(Protopterus dolloi), also known as the slender lungfish.

The South American lungfish and the four species of African lungfish are more closely related to each other than they are to the Australian lungfish.

African and South American lungfish belong to the order Lepidosireniformes.

The Australian lungfish belongs to the order Ceratodontiformes. It is the only living member of the Ceratodontiformes. There were once many other species of Ceratodontiformes on Earth, but they are now extinct.

The marbled lungfish has the largest genome of any animal, as far as we know today. A genome is the entire set of hereditary information that is encoded in an organism’s DNA (or RNA, in some types of viruses).

Physical Characteristics

Lungfish have long, eel-like bodies.

They can be up to 6 feet long and weigh up to 25 pounds.

Like most bony fish, lungfish have paired pectoral and pelvic fins.

There is one pectoral fin on each side of a bony fish’s body.

The pelvic fins lie below the pectoral fins, nearer to the fish’s abdomen.

In addition to two sets of paired fins, most bony fish have a caudal fin (tail fin), one or more dorsal fins (back fins) and an anal fin, which lies on the fish’s underside, behind the anus and in front of the tail.

The earliest lungfish had a caudal fin, two dorsal fins and an anal fin.

However, in the lungfish that are alive today, the caudal fin, dorsal fins and anal fin are fused together.

A lungfish retains its notochord throughout its life.

A notochord is a flexible rod that runs along the inside of a chordate’s body and provides it with support.

In most vertebrates, the notochord is replaced by a spinal column when the vertebrate is still an embryo.

Respiration

Lungfish usually live in shallow lakes, streams, ponds or swamps, where they can easily obtain oxygen from the air above the surface.

A lungfish’s nostrils are connected to its mouth from the inside. It can breathe air through its nostrils with its mouth closed. This way, it can come to surface of the water and breathe air without swallowing water.

The nostrils of most bony fish are not connected to their mouths. Bony fish usually use their nostrils only for smelling. They use only their gills for breathing.

African lungfish and South American lungfish have two lungs. They also have gills, but they are very small and cannot be used for breathing.

These lungfish will suffocate if they cannot breathe air.

Fish that must breathe air all the time, or they will drown, are known as obligatory air breathers.

An Australian lungfish has gills and one lung.

It can breathe water through its gills and air through its lungs.

Australian lungfish breathe through their lungs when the oxygen level in the water they are in becomes too low to support their metabolic needs. They tend to use their lungs more when they are more active.

Australian lungfish do not need to use their lungs if the oxygen level in the water is high enough.

Fish that breathe air only when there is not enough oxygen in the water to meet their needs are known as facultative air breathers.

Behavior

Lungfish are primarily carnivorous.

They eat small invertebrates, such as worms, insects, mollusks and crustaceans, as well as fish and amphibians.

Some people say that they have seen lungfish eating plants.

African lungfish and South American lungfish estivate during the dry season, when the shallow bodies of water in which they live dry up. During estivation, an animal’s metabolism slows down dramatically.

During the dry season, a South American lungfish will dig a hole in the mud at the bottom of a dry waterbed and remain covered in mud until the water level rises again.

African lungfish also estivate in mud at the bottom of dried up bodies of water.  They cover their bodies with mucus that dries out as the water level goes down. When an African lungfish is estivating, this dried out mucus forms a protective cocoon around its body.

Australian lungfish do not estivate. As the oxygen level in the water decreases, an Australian lungfish will spend more and more time breathing air until, eventually, it is breathing only air through its lungs and not using its gills at all.

Life Cycle

Lungfish practice external fertilization. The females lay eggs (are oviparous.)

South American and African lungfish build nests, which consists of depressions in mud at the bottom of shallow bodies of water. They line their nests with plant matter. The males protect the eggs in the nest.

During breeding season, male South American lungfish develop feathery structures in their pelvic fins that filter gases in the air. These structures take in carbon dioxide and expel oxygen. The males use them to increase oxygen levels in their nests.

Australian lungfish do not build nests. The female lays her eggs on plants growing at the bottom of the water. The parents do not guard the eggs.

Lungfish undergo metamorphosis.

The larvae breathe through gills.

During metamorphosis, the gills of African and South American lungfish atrophy. The lungfish stop breathing through gills and begin breathing through lungs.

After metamorphosis, Australian lungfish continue to breathe through gills, as long as there is enough oxygen in the water, but they can also breathe through lungs.