Lungfish

Lungfish, common name for primitive, airbreathing fishes that live in freshwater in some tropical areas of Australia, Africa, and South America. Only six species of lungfish survive today. Fossils show that lungfish were much more widespread and diverse in the distant past. Scientists think lungfish are closely related to the ancestors of the earliest vertebrates (animals with backbones) that adapted to live on land.

The name lungfish refers to the specialized “lung” that serves as the creature’s main organ for breathing. The lung allows the fish to gulp air as an adaptation to low-oxygen water environments such as swamps or bodies of water that dry up. Although lungfish also have gills, these organs are relatively small compared to those of other fish. The single lung is actually a modified swim bladder, the air-filled organ that most fish use to help them float at a particular depth to save energy when swimming. In lungfish the modified swim bladder can also absorb oxygen—other fish get oxygen by passing water through their gills. In some lungfish, the lung also removes carbon dioxide waste when the lungfish is very active. When resting, lungfish usually excrete carbon dioxide through gills or skin, similar to other fish.

The African and South American lungfishes must rise to the surface to breathe and can drown without air. The Australian lungfish can get the oxygen it needs from water through its gills unless conditions become stagnant; it then uses its lung to breathe air. The young of African and South American lungfishes have true external gills, but these degenerate with age.

Lungfish have elongated bodies with two pairs of fleshy, limblike fins in front and in back. The tail has a tapering fin along the top and bottom. They are generally brownish and mottled in color. Adults range from 61 cm to 1 m (24 in to 4 ft) in length, although some species are up to 2 m (6 ft) long and may weigh 45 kg (100 lbs). Their fan-shaped teeth form crushing plates, adapted for a diet of fish, insects, mollusks, worms, crustaceans, and plant material. Some species can be territorial and aggressive. African and South American species build nests where the male protects the eggs until they hatch. The Australian lungfish does not build a nest or protect its eggs, however.

The African and South American lungfish can estivate (become dormant) during dry periods or droughts, usually for only a few months but possibly for more than four years. The fish burrow into the mud and secrete a covering of mucus around themselves. This “cocoon” hardens, leaving a small closable breathing vent. The fish reduce their metabolism and become inactive. Their protective cocoon softens when wet, as at the end of the dry season, and the fish can then reemerge and live in the water again.

The lungfish and its cousin the coelacanth are often referred to as “living fossils”—animals that have changed little for hundreds of millions of years. Both are the last survivors of an ancient group called lobe-finned fishes, named for the scale-covered lobe at the base of their fins. The lobe-finned fishes were the dominant group of fishes during much of the Paleozoic period, and the earliest fossils belong to the Devonian Period. One branch of lobe-finned fishes gave rise to land vertebrates during the late Devonian Period, evolving into amphibians. Neither lungfish nor coelacanths are considered direct ancestors of land vertebrates, but can provide important clues about how early land vertebrates lived. The similarities between lungfish found in Africa and in South America provide evidence that the two continents were once connected. (See Plate Tectonics.) The Australian lungfish is considered more primitive than the African and South Americans forms.

People catch lungfish as food in parts of Africa and South America. Lungfish are also sometimes kept as aquarium pets, although they require large tanks. The Australian lungfish has protected status in Australia and internationally under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). The Australian lungfish must be released if caught in the wild and wild specimens cannot be exported, but it may be bred in captivity for the pet trade.

Scientific classification: Lungfishes make up the subclass Dipnoi of the class Osteichthyes. The Australian lungfish is classified as Neoceratodus forsteri, and the South American lungfish as Lepidosiren paradoxa. African lungfishes are classified in the genus Protopterus.