Lungfish are freshwater vertebrates belonging to theclassDipnoi.[1] Lungfish are best known for retaining ancestral characteristics within theOsteichthyes, including the ability to breathe air, and ancestral structures withinSarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Lungfish represent the closest living relatives of thetetrapods (which includes living amphibians, reptiles, birds and mammals). The mouths of lungfish typically bear tooth plates, which are used tocrush hard shelled organisms.
Today there are only six known species of lungfish, living inAfrica,South America, andAustralia, though they were formerly globally distributed. The fossil record of the group extends into the EarlyDevonian, over 410 million years ago. The earliest known members of the group were marine, while almost all post-Carboniferous representatives inhabit freshwater environments.[2]
All lungfish demonstrate an uninterrupted cartilaginousnotochord and an extensively developed palatal dentition.Basal ("primitive") lungfish groups may retain marginal teeth and an ossified braincase, but derived lungfish groups, including all modern species, show a significant reduction in the marginal bones and a cartilaginous braincase. The bones of theskull roof in primitive lungfish are covered in amineralized tissue calledcosmine, but in post-Devonian lungfishes, the skull roof lies beneath the skin and the cosmine covering is lost. All modern lungfish show significant reductions and fusions of the bones of the skull roof, and the specific bones of the skull roof show nohomology to the skull roof bones ofray-finned fishes ortetrapods. During the breeding season, theSouth American lungfish develops a pair of feathery appendages that are actually highly modified pelvic fins. These fins are thought to improve gas exchange around the fish's eggs in its nest.[3]
Throughconvergent evolution, lungfishes have evolved internal nostrils similar to the tetrapods'choana,[4] and a brain with certain similarities toLissamphibian brain (except for the Queensland lungfish, which branched off in its own direction about 277 million years ago and has a brain resembling that of theLatimeria).[5]
The dentition of lungfish is different from that of any othervertebrate group. "Odontodes" on the palate and lower jaws develop in a series of rows to form a fan-shapedocclusion surface. These odontodes then wear to form a uniform crushing surface. In several groups, including the modernlepidosireniformes, these ridges have been modified to form occluding blades.
The modern lungfishes have a number of larval features, which suggestpaedomorphosis. They also demonstrate the largestgenome among the vertebrates.
Modern lungfish all have an elongate body with fleshy, pairedpectoral andpelvic fins and a single unpairedcaudal fin replacing thedorsal, caudal andanal fins of most fishes.
Lateral view of lungs of a dissectedspotted lungfish (Protopterus dolloi)
Lungfish have a highly specializedrespiratory system. They have a distinct feature in that their lungs are connected to the larynx and pharynx without a trachea. While other species of fish can breathe air using modified, vascularizedgas bladders,[6] these bladders are usually simple sacs, devoid of complex internal structure. In contrast, the lungs of lungfish are subdivided into numerous smaller air sacs, maximizing the surface area available forgas exchange.
Most extant lungfish species have two lungs, with the exception of the Australian lungfish, which has only one. The lungs of lungfish are homologous to the lungs of tetrapods. As in tetrapods andbichirs, the lungs extend from the ventral surface of theesophagus and gut.[7][8]
Of extant lungfish, only theAustralian lungfish can breathe through its gills without needing air from its lung. In other species, the gills are too atrophied to allow for adequate gas exchange. When a lungfish is obtainingoxygen from its gills, its circulatory system is configured similarly to the common fish. The spiral valve of theconus arteriosus is open, the bypass arterioles of the third and fourth gill arches (which do not actually have gills) are shut, the second, fifth and sixth gill arch arterioles are open, theductus arteriosus branching off the sixth arteriole is open, and the pulmonary arteries are closed. As the water passes through the gills, the lungfish uses a buccal pump. Flow through the mouth and gills is unidirectional. Blood flow through the secondary lamellae is countercurrent to the water, maintaining a more constant concentration gradient.
When breathing air, the spiral valve of the conus arteriosus closes (minimizing the mixing of oxygenated and deoxygenated blood), the third and fourth gill arches open, the second and fifth gill arches close (minimizing the possible loss of the oxygen obtained in the lungs through the gills), the sixth arteriole's ductus arteriosus is closed, and the pulmonary arteries open. Importantly, during air breathing, the sixth gill is still used in respiration; deoxygenated blood loses some of its carbon dioxide as it passes through the gill before reaching the lung. This is because carbon dioxide is more soluble in water. Air flow through the mouth is tidal, and through the lungs it is bidirectional and observes "uniform pool" diffusion of oxygen.
African and South American lungfish are capable of surviving seasonal drying out of their habitats by burrowing into mud andestivating throughout the dry season. Changes inphysiology allow it to slow itsmetabolism to as little as one sixtieth of the normal metabolic rate, and protein waste is converted fromammonia to less-toxicurea (normally, lungfish excrete nitrogenous waste as ammonia directly into the water).
Burrowing is seen in at least one group of fossil lungfish, theGnathorhizidae.
Granddad, Shedd Aquarium, Chicago
Lungfish can be extremely long-lived. AQueensland lungfish called "Granddad"[10] at theShedd Aquarium inChicago was part of the permanent live collection from 1933 to 2017 after a previous residence at theSydney Aquarium;[11] at 109 years old,[12] it was euthanized following a decline in health consistent with old age.[11]
As of 2022, the oldest lungfish, and probably the oldest aquarium fish in the world is "Methuselah", an Australian lungfish 4 feet (1.2 m) long and weighing around 40 pounds (18 kg). Methuselah is believed to be female, unlikeits namesake, and is estimated to be over 90 years old.[10]
About 420 million years ago, during theDevonian, thelast common ancestor of lungfish andtetrapods split into two separate evolutionary lineages, with the ancestor of the extantcoelacanths diverging a little earlier from asarcopterygian progenitor.[13]Youngolepis andDiabolepis, dating to 419–417 million years ago, during Early Devonian (Lochkovian), are the currently oldest known lungfish, and show that the lungfishes had adapted to a diet including hard-shelled prey (durophagy) very early in their evolution.[14] The earliest lungfish were marine. Almost all post-Carboniferous lungfish inhabit or inhabited freshwater environments. There were likely at least two transitions amongst lungfish from marine to freshwater habitats. The last common ancestor of all living lungfish likely lived sometime between the LateCarboniferous[2] and theJurassic.[15] Lungfish remained present in the northernLaurasian landmasses into theCretaceous period.[16]
TheQueensland lungfish,Neoceratodus forsteri, isendemic to Australia.[17] Fossil records of this group date back 380 million years, around the time when thehigher vertebrate classes were beginning to evolve.[18] Fossils of lungfish belonging to the genusNeoceratodus have been uncovered in northernNew South Wales, indicating that the Queensland lungfish has existed in Australia for at least 100 million years, making it aliving fossil and one of the oldest living vertebrate genera on the planet.[18][19] It is the most primitive surviving member of the ancient air-breathing lungfish (Dipnoi) lineages.[18][20] The five other freshwater lungfish species,four in Africa andone in South America, are very different morphologically toN. forsteri.[18] The Queensland lungfish can live for several days out of the water if it is kept moist, but will not survive total water depletion, unlike its African counterparts.[17]
South American lungfish
TheSouth American lungfish,Lepidosiren paradoxa, is the single species of lungfish found inswamps and slow-moving waters of theAmazon,Paraguay, and lowerParaná Riverbasins in South America. Notable as anobligate air-breather, it is the sole member of its family native to the Americas. Relatively little is known about the South American lungfish,[21] also calledscaly salamander-fish.[22] When immature it is spotted with gold on a black background. In the adult this fades to a brown or gray color.[23] Its tooth-bearingpremaxillary andmaxillary bones are fused like other lungfish. South American lungfishes also share an autostylic jaw suspension (where thepalatoquadrate is fused to thecranium) and powerful adductor jaw muscles with the extant lungfish (Dipnoi). Like theAfrican lungfishes, this species has an elongate, almost eel-like body. It may reach a length of 125 centimetres (4 ft 1 in). Thepectoral fins are thin and threadlike, while the pelvic fins are somewhat larger, and set far back. The fins are connected to the shoulder by a single bone, which is a marked difference from most fish, whose fins usually have at least four bones at their base; and a marked similarity with nearly all land-dwelling vertebrates.[24] They have the lowest aquatic respiration of all extant lungfish species,[25] and their gills are greatly reduced and essentially non-functional in the adults.[26]
Marbled lungfish
Themarbled lungfish,Protopterus aethiopicus, is found in Africa. The marbled lungfish is smooth and elongated with deeply embeddedscales, and (starting from the head end) is cylindrical for much of its length. The tail is very long and tapers at the end. They are the largest of the African lungfish species as they can reach a length of up to 200 cm.[27] The pectoral and pelvic fins are also very long and thin, almost spaghetti-like. The newly hatched young have branched external gills much like those of newts. After 2 to 3 months the young transform (calledmetamorphosis) into the adult form, losing theexternal gills for gill openings. These fish have a yellowish gray or pinkish toned ground color with dark slate-gray splotches, creating a marbling or leopard effect over the body and fins. The color pattern is darker along the top and lighter below.[28] The marbled lungfish'sgenome contains 133 billionbase pairs, making it the largest known genome of anyvertebrate. The onlyorganisms known to have more base pairs are theprotistPolychaos dubium and the flowering plantParis japonica at 670 billion and 150 billion, respectively.[29]
Gilled lungfish
Thegilled lungfish,Protopterus amphibius is a species of lungfish found inEast Africa.[30][31] It generally reaches only 44 centimetres (17 inches) long, making it the smallestextant lungfish in the world.[32] This lungfish is uniform blue, or slate grey in colour. It has small or inconspicuous black spots, and a pale grey belly.[33]
West African lungfish
Thewest African lungfish,Protopterus annectens, is a species of lungfish found in West Africa.[34][35][36] It has a prominentsnout and smalleyes. Its body is long andeel-like, some 9–15 times the length of the head. It has two pairs of long, filamentousfins. Thepectoral fins have a basal fringe and are about three times the head length, while itspelvic fins are about twice the head length. In general, three externalgills are insertedposterior to thegill slits and above the pectoral fins. It hascycloid scales embedded in the skin. There are 40–50 scales between theoperculum and theanus and 36–40 around the body before the origin of thedorsal fin. It has 34–37 pairs ofribs. Thedorsal side is olive or brown in color and theventral side is lighter, with great blackish or brownish spots on the body and fins except on its belly.[37] They reach a length of about 100 cm in the wild.[38]
Recent molecular genetic analyses strongly support a sister relationship of lungfishes and tetrapods (Rhipidistia), with coelacanths branching slightly earlier.[42][43]
The relationships among lungfishes are significantly more difficult to resolve. While Devonian lungfish had enough bone in the skull to determine relationships, post-Devonian lungfish are represented entirely by skull roofs and teeth, as the rest of the skull iscartilaginous. Additionally, many of the taxa already identified may not bemonophyletic.
^Purkerson, M.L. (1975). "Electron microscopy of the intestine of the African lungfish,Protopterus aethiopicus".The Anatomical Record.182 (1):71–89.doi:10.1002/ar.1091820109.PMID1155792.S2CID44787314.
^abLake, John S. (1978).Australian Freshwater Fishes. Nelson Field Guides. Melbourne: Thomas Nelson Australia Pty. Ltd. p. 12.
^abcdAllen, G.R.; Midgley, S.H.; Allen, M. (2002). Knight, Jan; Bulgin, Wendy (eds.).Field Guide to the Freshwater Fishes of Australia. Perth, W.A.: Western Australia Museum. pp. 54–55.
^Frentiu, F.D.; Ovenden, J.R.; Street, R. (2001). "Australian lungfish (Neoceratodus forsteri: Dipnoi) have low genetic variation at allozyme and mitochondrial DNA loci: A conservation alert?".Conservation Genetics. 2.2:63–67.doi:10.1023/A:1011576116472.S2CID22778872.
^Guenther, Konrad (1931).A Naturalist in Brazil. Translated byMiall, Bernard. Houghton Mifflin Company. pp. 275, 399.The record of a year's observation of her flora, her fauna, and her people.
^Brien, P. (1959). Ethologie du Protopterus dolloi(Boulenger) et de ses larves. Signification des sacs pulmonaires des Dipneustes. Ann. Soc. R. Zool. Belg. 89, 9–48.
^Poll, M. (1961). Révision systématique et raciation géographique des Protopteridae de l’Afrique centrale. Ann. Mus. R. Afr. Centr. Sér. 8. Sci. Zool. 103, 3–50.
Palmer, Douglas, ed. (1999).The Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures, A visual who's who of prehistoric life. Great Britain: Marshall Editions Developments Limited. p. 45.
