Lungfish fossils first appear at the beginning of
the Devonian period, about 400 million years
ago. Once distributed around the world, lungfish
now occur in only three areas (South America, Africa,
and Australia) that were once adjacent but
have since separated due to continental drift.
South American and African lungfish are more
closely related to one another. They have reduced
gills and two lungs and are true air breathers,
while Australian lungfish have one lung and rely
mostly upon their gills for respiration, gulping air
at the water surface to breathe only when dissolved
oxygen is in short supply.
African and South American Lungfish
There are two related families in this order. The African family, Protopteridae, has one genus (Protopterus) and four species (P. annectens, P. aethiopicus, P. dolloi, and P. amphibius). They live in the rivers, lakes, and swamps of East and Central Africa and grow to 2 meters (6.5 feet) in length. The South American family, Lepidosirenidae, has only one species, Lepidosiren paradoxa. It lives in swampy areas of the Amazon and ParanГЎ river basins and in the swamps of the Chaco region and grows to 1.2 meters (4 feet) in length. During spawning season, males dig holes for the eggs and guard both eggs and young after they hatch. The South American male develops branched, gill-like structures on his pelvic fins that may supply extra oxygen to himself as he incubates and to his offspring. Juveniles hatch with adhering organs to attach to plants. They have external gills like salamander young and undergo metamorphosis. Adults live on crustaceans, mollusks, and small fishes. They survive the dry season by resting quietly in moist burrows dug in the mud. If it becomes too arid, they seal their burrows with mud, secrete a protective covering of mucus, and estivate, sometimes for several months. During estivation, metabolism slows down to conserve energy and air. Once the rainy season begins, water enters the burrowand awakens the estivating fish.
The Australian lungfish was first discovered in 1870. This lungfish closely resembles fossil lungfishes except that its skull contains fewer, larger bones. It has remained unchanged for 100 million years or more, longer than any other vertebrate known. There is now only one living species of this order, Neoceratodus forsteri, and it occurs naturally in only two locations, the Burnett and Mary rivers in Queensland, Australia, although it is being introduced into other rivers in an effort to ensure its survival. During the summer, the rivers dry up, and the lungfish survive in small pools. They cannot estivate. Spawning takes place after the rainy season starts. The eggs are laid in masses of fifty to one hundred on aquatic plants. The young breathe through the gills and skin. They do not have adhering organs or external gills and do not undergo metamorphosis. They develop pectoral fins at about fourteen days and pelvic fins at about ten weeks, gradually assuming the adult shape. They feed on algae, but adults feed on small fishes, mollusks, crustaceans, insect larvae, and some plants and grow to a size of about 1.8 meters (6 feet) and forty-five kilograms (one hundred pounds). They have broad, flat heads with the mouth underneath, small eyes, and pointed tails. The body is olive green or brown above and pinkish below.
Order: Dipnomorpha or Dipnoi
Families: Lepidosirenidae (African and South American lungfish); Dipnorhynchidae, Dipteridae, Phaneropleuridae, Ctenodontidae, Conchopomidae, Sagenodontidae, Ceratodiae (Australian lungfish)
Geographical location: Africa, Australia, and South America
Habitat: Freshwater only (rivers, lakes, swamps, and marshes)
Gestational period: Australian lungfish, twenty-one to thirty days; African and South American lungfish, eleven to fifteen days
Life span: Unknown; larger fish may live to be many years old Special anatomy: Elongated bodies, continuous rear fins composed of connected dorsal, caudal, and anal fins; Australian lungfish-laterally compressed body, large scales, two pairs of flipperlike pectoral and pelvic fins, one lung; African and South American lungfish-eel-like body with small scales, two pairs of long, thin pectoral and pelvic fins, paired lungs
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