Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the

Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...

, including the ability to breathe air, and ancestral structures within

Sarcopterygii Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...

, including the presence of lobed fins with a well-developed internal skeleton. Lungfish represent the closest living relatives of the

tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct therapsid ...

s. Today there are only six known species of lungfish, living in

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, South America, and Australia. The fossil record shows that lungfish were abundant since the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

. While

vicariance Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

would suggest this represents an ancient distribution limited to the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leav ...

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

, the

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

record suggests advanced lungfish had a widespread freshwater distribution and the current distribution of modern lungfish species reflects

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed an ...

of many lineages subsequent to the breakup of

Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

and

Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...

. Lungfish have historically been referred to as salamanderfish, but this term more often refers to ''

Lepidogalaxias salamandroides ''Lepidogalaxias salamandroides'' is a species of small fish of Western Australia. It is the only member of the family Lepidogalaxiidae and genus ''Lepidogalaxias''. Common names for this fish include salamanderfish and Shannon mudminnow. Althou ...

''.

Anatomy and morphology

All lungfish demonstrate an uninterrupted cartilaginous

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...

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 the

skull roof The skull roof, or the roofing bones of the skull, are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In comparat ...

in primitive lungfish are covered in a

mineralized tissue Mineralized tissues are biological tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusc shells, deep sea sponge ''Euplectella'' species, radiolarians, diatom ...

called

cosmine Cosmine is a spongy, bony material that makes up the dentine-like layers in the scales of the lobe-finned fishes of the class Sarcopterygii. Fish scales that include layers of cosmine are known as cosmoid scales. Description As traditionally ...

, but in post-

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

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 no homology to the skull roof bones of

ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...

es or

tetrapods Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsid ...

. During the breeding season, the South 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. Through convergent evolution, lungfishes have evolved internal nostrils similar to the tetrapods'

choana The choanae (singular choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the throat in tetrapods, including humans and other mammals (as well as crocodili ...

, and a brain with certain similarities to the

Lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...

n 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 the ''

latimeria ''Latimeria'' is a rare genus of fish which contains the only living species of coelacanth. It includes two extant species: the West Indian Ocean coelacanth (''Latimeria chalumnae'') and the Indonesian coelacanth (''Latimeria menadoensis''). Th ...

''). The dentition of lungfish is different from that of any other

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

group. "

Odontode Odontodes, or dermal teeth, are hard structures found on the external surfaces of animals or near internal openings. They consist of a soft pulp surrounded by dentine and covered by a mineralized substance such as enamel, a structure similar to th ...

s" on the palate and lower jaws develop in a series of rows to form a fan-shaped

occlusion Occlusion may refer to: Health and fitness * Occlusion (dentistry), the manner in which the upper and lower teeth come together when the mouth is closed * Occlusion miliaria, a skin condition * Occlusive dressing, an air- and water-tight traum ...

surface. These odontodes then wear to form a uniform crushing surface. In several groups, including the modern lepidosireniformes, these ridges have been modified to form occluding blades. The modern lungfishes have a number of larval features, which suggest

paedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...

. They also demonstrate the largest

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gen ...

among the vertebrates. Modern lungfish all have an elongate body with fleshy, paired

pectoral Pectoral may refer to: * The chest region and anything relating to it. * Pectoral cross, a cross worn on the chest * a decorative, usually jeweled version of a gorget * Pectoral (Ancient Egypt), a type of jewelry worn in ancient Egypt * Pectorali ...

and

pelvic The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

fins and a single unpaired

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as s ...

replacing the dorsal, caudal and

anal Anal may refer to: Related to the anus *Related to the anus of animals: ** Anal fin, in fish anatomy ** Anal vein, in insect anatomy ** Anal scale, in reptile anatomy *Related to the human anus: ** Anal sex, a type of sexual activity involving ...

fins of most fishes.

Lungs

Lungfish have a highly specialized

respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gr ...

. 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, vascularized

gas bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wi ...

s, 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 for gas 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

s. As in tetrapods and

bichir Bichirs and the reedfish comprise Polypteridae , a family of archaic ray-finned fishes and the only family in the order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The Diversity of Fishes. West Sussex, UK: Blackwell P ...

s, the lungs extend from the ventral surface of the

esophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to th ...

and gut.

Perfusion of water

Of extant lungfish, only the

Australian lungfish The Australian lungfish (''Neoceratodus forsteri''), also known as the Queensland lungfish, Burnett salmon and barramunda, is the only surviving member of the family Neoceratodontidae. It is one of only six extant lungfish species in the world. ...

can breathe through its gills without needing air from its lungs. In other species, the gills are too atrophied to allow for adequate

gas exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...

. When a lungfish is obtaining

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well ...

from its gills, its circulatory system is configured similarly to the common fish. The spiral valve of the

conus arteriosus The infundibulum (also known as ''conus arteriosus'') is a conical pouch formed from the upper and left angle of the right ventricle in the chordate heart, from which the pulmonary trunk arises. It develops from the bulbus cordis. Typically, the i ...

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, the

ductus arteriosus The ''ductus arteriosus'', also called the ''ductus Botalli'', named after the Italian physiologist Leonardo Botallo, is a blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta. It all ...

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.

Perfusion of air

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.

Ecology and life history

Lungfish are

omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...

, feeding on fish,

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arborea ...

s and plant matter. They have an intestinal spiral valve rather than a true

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

. African and South American lungfish are capable of surviving seasonal drying out of their habitats by burrowing into mud and estivating throughout the dry season. Changes in

physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...

allow it to slow its

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...

to as little as one sixtieth of the normal metabolic rate, and protein waste is converted from

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

to less-toxic

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...

(normally, lungfish excrete nitrogenous waste as ammonia directly into the water). Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. Lungfish can be extremely long-lived. A

Queensland lungfish The Australian lungfish (''Neoceratodus forsteri''), also known as the Queensland lungfish, Burnett salmon and barramunda, is the only surviving member of the family Neoceratodontidae. It is one of only six extant lungfish species in the world. ...

at the

Shedd Aquarium Shedd Aquarium (formally the John G. Shedd Aquarium) is an indoor public aquarium in Chicago, Illinois, in the United States. Opened on May 30, 1930, the aquarium was for some time the largest indoor facility in the world. Today it holds about ...

in Chicago was part of the permanent live collection from 1933 to 2017 after a previous residence at the

Sydney Aquarium SEA LIFE Sydney Aquarium (formerly Sydney Aquarium) is a public aquarium that features a large variety of Australian aquatic life, displaying more than 700 species comprising more than 13,000 individual fish and other sea and water creatures fr ...

; at perhaps 90 years old, it was euthanized following a decline in health consistent with old age. A primitive Australian fish living in a San Francisco museum is believed to be the oldest living aquarium fish in the world. Methuselah is a four-foot-long (1.2-meter) Australian lungfish, weighing around 40 lb (18.1 kg).

Evolution

About 420 million years ago, during the

, the

last common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of both lungfish and the

s split into two separate evolutionary linages, with the ancestor of the extant

coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...

s diverging a little earlier from a

sarcopterygian Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...

progenitor. The fossil of Youngolepis from about 415 million years ago already show significant modification of their jaws and jaw muscles towards the specialized lungfish feeding mechanism. ''

Diabolepis ''Diabolepis'' (or ''Diabolichthys'') is an extinct genus of very primitive lungfish which lived about 400 million years ago, in the Early Devonian period of South China. ''Diabolepis'' is the most basal known dipnoan. A rather small fish, the ...

'', a 400 million year old lungfish, is the oldest known unmistakingly durophagous vertebrate, and shows that the lungfishes had adapted to a diet including hard-shelled prey very early in their evolution. The transition could have taken as little as 7 million years. It is worth noting that, despite the name "lungfish", fish evolved lungs before lungfish, or even

lobe-finned fish Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...

. The common ancestor of lobe-finned and

had lungs, but in most surviving branches of ray-finned fish these evolved into

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wi ...

s used for flotation, instead of breathing. Some, like the

s, do retain their lungs, and several other traits that appear to have been common to lobe-finned and ray-finned fish. While the coelacanth shares some traits with the tetrapods (reptiles, amphibians, birds and mammals), the lungfish have other tetrapod traits which are absent in the coelacanth. Evidence suggests that the tetrapods are related more closely to lungfish than to coelacanths.

Extant lungfish

The

, ''Neoceratodus forsteri'', is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...

to Australia. Fossil records of this group date back 380 million years, around the time when the

higher vertebrate Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disting ...

classes were beginning to evolve. Fossils of lungfish belonging to the genus ''Neoceratodus'' have been uncovered in northern

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , e ...

, indicating that the Queensland lungfish has existed in Australia for at least 100 million years, making it a

living fossil A living fossil is an extant taxon that cosmetically resembles related species known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living fossi ...

and one of the oldest living vertebrate genera on the planet. It is the most primitive surviving member of the ancient air-breathing lungfish (Dipnoi) lineages. The five other freshwater lungfish species, four in Africa and one in South America, are very different morphologically to ''N. forsteri''. 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. F de Castelnau-poissonsPl50

The South American lungfish, ''Lepidosiren paradoxa'', is the single species of lungfish found in

swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...

s and slow-moving waters of the

Amazon Amazon most often refers to: * Amazons, a tribe of female warriors in Greek mythology * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon River, in South America * Amazon (company), an American multinational technology co ...

Paraguay Paraguay (; ), officially the Republic of Paraguay ( es, República del Paraguay, links=no; gn, Tavakuairetã Paraguái, links=si), is a landlocked country in South America. It is bordered by Argentina to the south and southwest, Brazil to ...

, and lower

Paraná River The Paraná River ( es, Río Paraná, links=no , pt, Rio Paraná, gn, Ysyry Parana) is a river in south-central South America, running through Brazil, Paraguay, and Argentina for some ."Parana River". Encyclopædia Britannica. Encyclopædia Br ...

basins in South America. Notable as an

obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism tha ...

air-breather, it is the sole member of its family native to the Americas. Relatively little is known about the South American lungfish, or ''scaly salamander-fish''. When immature it is spotted with gold on a black background. In the adult this fades to a brown or gray color. Its tooth-bearing premaxillary and

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

ry bones are fused like other lungfish. South American lungfishes also share an autostylic jaw suspension (where the

palatoquadrate In some fishes, the palatoquadrate is the dorsal component of the mandibular arch, the ventral one being Meckel's cartilage. The palatoquadrate forms from splanchnocranium in various chordates including placoderms and acanthodians. See also * H ...

is fused to the

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

) and powerful adductor jaw muscles with the extant lungfish (Dipnoi). Like the

African lungfish ''Protopterus'' is the genus of four species of lungfish found in Africa. ''Protopterus'' was formerly thought to be the sole genus in the family Protopteridae, but more recent studies have classified it with '' Lepidosiren'' in the family Lep ...

es, this species has an elongate, almost eel-like body. It may reach a length of . The

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

s 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. The gills are greatly reduced and essentially non-functional in the adults. Marbled lungfish 1

The

marbled lungfish The marbled lungfish (''Protopterus aethiopicus'') is a lungfish of the family Protopteridae. Also known as the leopard lungfish, it is found in Eastern and Central Africa, as well as the Nile region. At 133 billion base pairs, it has the larg ...

, ''Protopterus aethiopicus'', is found in Africa. The marbled lungfish is smooth, elongated, and cylindrical with deeply embedded

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...

. 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.Fishbase.org
/ref> 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 (called

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insec ...

) into the adult form, losing the

external gills External gills are the gills of an animal, most typically an amphibian, that are exposed to the environment, rather than set inside the pharynx and covered by gill slits, as they are in most fishes. Instead, the respiratory organs are set on a frill ...

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. The marbled lungfish's

contains 133 billion

base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...

, making it the largest known genome of any

. The only

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fung ...

s known to have more base pairs are the

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclu ...

Polychaos dubium ''Polychaos dubium'' is a freshwater amoeboid and one of the larger species of single-celled eukaryote. Like other amoebozoans, ''P. dubium'' moves by means of temporary projections called pseudopods. ''P. dubium'' reportedly has one of the l ...

'' and the flowering plant ''

Paris japonica is a Japanese species of plants in the genus ''Paris'' in the family Melanthiaceae.Ohwi, J. (1984). Flora of Japan (in English): 1-1067. Smithsonian Institution, Washington, D.C.. It is native to sub-alpine regions of Japan. A slow growing pe ...

'' at 670 billion and 150 billion, respectively.

The

gilled lungfish The gilled lungfish (''Protopterus amphibius''), also known as the East African lungfish, is a species of African lungfish. It is found in the swamps and flood plains of East Africa, and has been positively identified in Kenya, Somalia and Mozamb ...

, ''Protopterus amphibius'' is a species of lungfish found in

East Africa East Africa, Eastern Africa, or East of Africa, is the eastern subregion of the African continent. In the United Nations Statistics Division scheme of geographic regions, 10-11-(16*) territories make up Eastern Africa: Due to the historical ...

. It generally reaches only long, making it the smallest

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extin ...

lungfish in the world. This lungfish is uniform blue, or slate grey in colour. It has small or inconspicuous black spots, and a pale grey belly. LepidosirenFord

The

west African lungfish The West African lungfish (''Protopterus annectens''), also known as the Tana lungfish or simply African lungfish, is a species of African lungfish. It is found in a wide range of freshwater habitats in West and Middle Africa, as well as the nor ...

''Protopterus annectens'' is a species of lungfish found in West Africa. It has a prominent

snout A snout is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw. In many animals, the structure is called a muzzle, rostrum, or proboscis. The wet furless surface around the nostrils of the nose of many mammals is c ...

and small

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...

s. Its body is long and eel-like, some 9-15 times the length of the head. It has two pairs of long, filamentous

fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...

. The

s have a basal fringe and are about three times the head length, while its

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two e ...

s are about twice the head length. In general, three external

gills A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

are inserted posterior to the

gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In co ...

s and above the pectoral fins. It has

cycloid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...

s embedded in the skin. There are 40-50 scales between the operculum and the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, de ...

and 36-40 around the body before the origin of the

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through co ...

. It has 34-37 pairs of

rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ch ...

s. The dorsal side is olive or brown in color and the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

side is lighter, with great blackish or brownish spots on the body and fins except on its belly. They reach a length of about 100 cm in the wild . Protopterus dolloi Boulenger2

The

spotted lungfish The spotted lungfish or slender lungfish (''Protopterus dolloi'') is a species of lungfish from Middle Africa, where found in the Congo, Kouilou-Niari and Ogowe river basins. It is one of four extant species in the genus Protopterus. Habitat ...

, ''Protopterus dolloi'', is a species of lungfish found in Africa. Specifically, it is found in the Kouilou-Niari Basin of the Republic of the Congo and Ogowe River basin in

Gabon Gabon (; ; snq, Ngabu), officially the Gabonese Republic (french: République gabonaise), is a country on the west coast of Central Africa. Located on the equator, it is bordered by Equatorial Guinea to the northwest, Cameroon to the north ...

. It is also found in the lower and Middle

Congo River Basin The Congo River ( kg, Nzâdi Kôngo, french: Fleuve Congo, pt, Rio Congo), formerly also known as the Zaire River, is the second longest river in Africa, shorter only than the Nile, as well as the second largest river in the world by discharge ...

s.Fishbase.org
/ref> ''Protopterus dolloi'' can aestivate on land by surrounding itself in a layer of dried

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

. It can reach a length of up to 130 cm.

Taxonomy

The relationship of lungfishes to the rest of the

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

is well understood: * Lungfishes are most closely related to '' Powichthys'', and then to the Porolepiformes. * Together, these taxa form the

Dipnomorpha Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, in ...

, the sister group to the

Tetrapodomorpha The Tetrapodomorpha (also known as Choanata) are a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advance ...

. * Together, these form the

Rhipidistia Rhipidistia, also known as Dipnotetrapodomorpha, is a clade of lobe-finned fishes which includes the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, ...

, the sister group to the

s. Recent molecular genetic analyses strongly support a sister relationship of lungfishes and tetrapods (

), with coelacanths branching slightly earlier. 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 is

cartilaginous Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck and ...

. Additionally, many of the taxa already identified may not be

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

. Current phylogenetic studies support the following relationships of major lungfish taxa: Class Osteichthyes, subclass Sarcopterygii, order Dipnoi. In 2017, Anne Kemp, Lionel Calvin, and Guillaume Guinot found a different classification for more derived lungfish. In this new analysis, all extant lungfish belong in the suborder Ceratodontoidei, and despite their similar appearance there is no distinct clade containing only '' Ceratodus'', ''

Neoceratodus ''Neoceratodus'' is a genus of lungfish in the family Neoceratodontidae. The extant Australian lungfish (''Neoceratodus forsteri'') is the only surviving member of this genus, but it was formerly much more widespread, being distributed throughou ...

'', and their closest allies. New analyses of the relationships of the recently discovered ''Persephonichthys chthonica'' and modern lungfishes robustly places both taxa within dipterid-grade dipnoans rather than within a clade containing Late Devonian ‘phaneropleurids’ and common Late Paleozoic lungfishes such as '' Sagenodus''. Monophyly of post-Devonian lungfishes is not supported and the Carboniferous-Permian taxon ''Sagenodus'' is found to be incidental to the origins of modern lungfishes, suggesting widespread convergence in Late Paleozoic lungfishes.Pardo, Jason D et al. “An exceptionally preserved transitional lungfish from the lower permian of Nebraska, USA, and the origin of modern lungfishes.” PLOS ONE vol. 9,9 e108542. 29 Sep. 2014, doi:10.1371/journal.pone.0108542

Timeline of lungfish genera of the Devonian

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Lochkovian The Lochkovian is one of three faunal stages in the Early Devonian Epoch. It lasted from 419.2 ± 3.2 million years ago to 410.8 ± 2.8 million years ago. It marked the beginning of the Devonian Period, and was followed by the Pragian Stage. It is ...

from: -411.2 till: -407 color:earlydevonian text:

Pragian The Pragian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.8 ± 2.8 million years ago to 407.6 ± 2.8 million years ago. It was preceded by the Lochkovian Stage and followed by the Emsian Stage. The most important La ...

from: -407 till: -397.5 color:earlydevonian text:

Emsian The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after the ...

from: -397.5 till: -391.8 color:middledevonian text:

Eifelian The Eifelian is the first of two faunal stages in the Middle Devonian Epoch. It lasted from 393.3 ± 1.2 million years ago to 387.7 ± 0.8 million years ago. It was preceded by the Emsian Stage and followed by the Givetian Stage. North American ...

from: -391.8 till: -385.3 color:middledevonian text:

Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in Fra ...

from: -385.3 till: -374.5 color:latedevonian text:

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during t ...

from: -374.5 till: -359.2 color:latedevonian text:

Famennian The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used b ...

bar:eratop from: -416 till: -359.2 color:devonian text:

PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left color:earlydevonian bar:NAM1 from:-416 till:-411.2 text:

color:earlydevonian bar:NAM2 from:-416 till:-359.2 text: Devonesteus color:earlydevonian bar:NAM3 from:-416 till:-359.2 text: Grossipterus color:earlydevonian bar:NAM4 from:-416 till:-359.2 text: Palaedaphus color:earlydevonian bar:NAM5 from:-411.2 till:-407 text: Uranolophus color:earlydevonian bar:NAM6 from:-411.2 till:-359.2 text:

Dipterus ''Dipterus'' (from el, δίς , 'two' and el, πτερόν 'wing') is an extinct genus of lungfish from the middle Devonian period of Europe and North America. In most respects, ''Dipterus'', which was about long, closely resembled modern l ...

color:earlydevonian bar:NAM7 from:-407 till:-403.83 text: Sponysedrion color:earlydevonian bar:NAM8 from:-407 till:-391.8 text: Dipnorhynchus color:middledevonian bar:NAM9 from:-397.5 till:-385.3 text: Melanognathus color:middledevonian bar:NAM10 from:-397.5 till:-385.3 text: Stomiahykus color:middledevonian bar:NAM11 from:-397.5 till:-359.2 text: Rhinodipterus color:middledevonian bar:NAM12 from:-391.8 till:-359.2 text: Ganorhynchus color:middledevonian bar:NAM13 from:-387.5 till:-359.2 text: Conchodus color:latedevonian bar:NAM14 from:-385.3 till:-381.7 text: Chirodipterus color:latedevonian bar:NAM15 from:-385.3 till:-381.7 text: Pillararhynchus color:latedevonian bar:NAM16 from:-385.3 till:-374.5 text: Griphognathus color:latedevonian bar:NAM17 from:-385.3 till:-374.5 text: Holodipterus PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:period from: -416 till: -411.2 color:earlydevonian text: