Lobe-finned fish are bony fish that have fleshy lobes at the bases of their paired fins.

The limbs of a tetrapod (a four-limbed land vertebrate or one of its descendants) have the same structure as the fins of a lobe-finned fish.

Today, the only lobe-finned fish that are alive on Earth are lungfish and coelacanths.

Once, however, there were many different species of lobe-finned fish.

During the Devonian period (from about 416 million to about 360 million years ago) and the Carboniferous period (from about 360 million to about 300 million years ago), there were more species of lobe-finned fish than ray-finned fish on Earth.

The earliest lobe-finned fish that we know of lived toward the end of the Silurian period, about 416 million years ago.

Scientists used to think that coelacanths were extinct. However, in 1938, scientists identified a live coelacanth that was caught by fishermen in a boat off the eastern coast of South Africa. Since then, more live specimens have been identified.

It is thought that fishermen from Madagascar have been catching coelacanths for thousands of years.

Lobed Fins

Every bony fish has two sets of paired fins – pectoral fins and paired fins.

The pectoral fins lie on the sides of the fish’s body, while the pelvic fins lie below the pectoral fins, closer to the fish’s abdomen.

Each paired fin of a lobe-finned fish is connected to its body by a fleshy stalk, or lobe, that contains many bones and muscles.

The bones in each lobe have the same structure as the bones in a tetrapod’s limbs.

A lobe-finned fish can swivel each of its paired fins about in a shoulder socket or a hip socket, in the same way that a tetrapod can swivel its limbs.

In comparison, the paired fins of a ray-finned fish are connected to its body by several rod-like bones. A ray-finned fish’s fins are not as flexible as the fins of a lobe-finned fish.

Respiration

Coelacanths have very small gills. They move slowly to save conserve energy and stay in cold water, which has more dissolved oxygen than warm water.

There are six species of lungfish – the Australian lungfish, the South American lungfish, and four species of African lungfish.

South American lungfish and African lungfish have very small gills that do not function. They have two lungs. These lungfish must use their lungs to breathe air above the water’s surface or they will drown.

Fish that need to breathe air all the time or they will suffocate are known as an obligatory air breathers.

Australian lungfish have small gills and only one lung.

The Australian lungfish  uses both its gills and  its lungs for respiration. It relies on its lungs when there is too little oxygen in the water.

Fish that breathe air only when the oxygen level of the water becomes too low for them are known as facultative air breathers.

Lungfish breathe by coming to the surface of the water and sticking their nostrils out of the water.  A lungfish’s nostrils are connected to the mouth from inside.  This arrangement allows it to breathe air while keeping its mouth closed, so that it doesn’t swallow water.

In most bony fish, the nostrils are only used for smelling. They are not connected to the mouth.

Classification

Lobe-finned fish and the tetrapods all share a common ancestor.

Together, lobe-finned fish and tetrapods together make up the clade Sarcopterygii and may be called sarcopterygians.

A clade is a group of organisms with a common ancestor. No organism outside the clade is desecended from the common ancestor of the organisms within the clade.