Lungfish
Lungfish are unique freshwater fish belonging to the class Dipnoi, found primarily in Africa, Australia, and South America. These ancient creatures, with fossils dating back to the Devonian period approximately 400 million years ago, are known for their ability to breathe air, a trait that allows them to survive in oxygen-poor waters. There are three families of lungfish: Lepidosirenidae, which includes one South American species; Protopteridae, which comprises four African species; and Neoceratodontidae, represented by the Australian lungfish.
Lungfish have distinctive elongated bodies and specialized fins, with some species featuring both lungs and gills. The South American and African lungfish are true air breathers, while the Australian lungfish primarily relies on its gills but can gulp air when necessary. These fish exhibit fascinating behaviors, such as building nests during spawning and estivating during dry conditions, where they enter a dormant state to conserve energy. The Australian lungfish is particularly notable for being the last of its lineage and can live for over a century, showcasing remarkable longevity. Overall, lungfish are a vital part of the aquatic ecosystem, reflecting a significant evolutionary link between fish and terrestrial vertebrates.
Lungfish
Lungfish Facts
Classification:
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Dipnoi
Order: Ceratodontiformes
Families: Lepidosirenidae (one South American lungfish species); Protopteridae (four African lungfish species); Neoceratodontidae (one Australian lungfish species)
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: twenty to twenty-five years
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
Lungfish fossils first appeared at the beginning of the Devonian period, about 400 million years ago. Once distributed worldwide, lungfish are found in only three areas in the twenty-first century (South America, Africa, and Australia). These areas 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
The Protopteridae family consists of four lungfish species in the genre Protopterus, including 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 Lepidosirenidae family contains the single South American lungfish (Lepidosiren paradoxa), sometimes called the American mud-fish or scaly salamanderfish. It lives in swampy areas of the Amazon and Paraná 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 mucus covering, and estivate, sometimes for several months. During estivation, metabolism slows down to conserve energy and air. Once the rainy season begins, water enters the burrow and awakens the estivating fish.
Australian Lungfish
The endangered Australian lungfish (Neoceratodus forsteri) is the only living member of its family Neoceratodontidae. First discovered in 1870, this lungfish closely resembles fossil lungfish, except its skull contains fewer, larger bones. It has remained unchanged for 100 million years or more, longer than any other known vertebrate. It occurs naturally only in the Burnett and Mary Rivers in Queensland, Australia. However, 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 a 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 about 1.8 meters (6 feet) and 45 kilograms (100 pounds). They have broad, flat heads with mouths underneath, small eyes, and pointed tails. The body is olive-green or brown above and pinkish below.
Australian lungfish live for about twenty-five years on average. However, one lungfish housed at the San Francisco museum nicknamed Methuselah lived for more than one hundred years.
Principal Terms
Continental Drift: theory that the continents have moved slowly apart from an early landmass, explaining why many species appear to be closely related while separated by wide expanses of ocean
Estivation: similar to hibernation; period of reduced activity or dormancy triggered by dry and/or hot environmental conditions
Metamorphosis: pronounced developmental change in form or structure of an animal after birth or hatching, as in the transformation of tadpoles into frogs
Swim Bladder (air bladder): an internal organ evolved from the gut that allows a fish to regulate its vertical position in the water column (maintain its balance)
Tetrapods: four-legged vertebrates (amphibians, reptiles, birds, and mammals)
Bibliography
Brownstein, Chase Doran, et al. “The Biogeography of Extant Lungfishes Traces the Breakup of Gondwana.” Journal of Biogeography, vol. 50, no. 7, 2023, pp. 1191–98, doi.org/10.1111/jbi.14609.
Carroll, Robert L. Vertebrate Paleontology and Evolution. W. H. Freeman, 1988.
Long, John A. The Rise of Fishes: Five Hundred Million Years of Evolution. The Johns Hopkins UP, 1995.
"Methuselah: Oldest Aquarium Fish Lives in San Francisco and Likes Belly Rubs." Guardian, 26 Jan. 2022, www.theguardian.com/us-news/2022/jan/26/methuselah-oldest-aquarium-fish-lives-san-francisco. Accessed 1 Oct. 2024.
Moyle, Peter B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology. 4th ed., Prentice Hall, 2000.
Roush, Wade. “‘Living Fossil’ Fish Is Dethroned.” Science, vol. 277, no. 5331, Sept. 1997, p. 1436.
Weinberg, Samantha. A Fish Caught in Time: The Search for the Coelacanth. HarperCollins, 2000.
“West African Lungfish.” National Geographic Society, 19 Oct. 2023, education.nationalgeographic.org/resource/west-african-lungfish. Accessed 1 Oct. 2024.