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Lungfish

 

Lungfish

Lungfishes
Temporal range: Early Devonian–Recent
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Queensland lungfish
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sarcopterygii
Clade: Dipnomorpha
Subclass: Dipnoi
J. P. Müller, 1844
Orders

Lungfish (also known as salamanderfish[1]) are freshwater fish belonging to the subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton.

Today, lungfish live only in Africa, South America and Australia. While vicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinent Gondwana, the fossil record suggests advanced lungfish had a widespread freshwater distribution and the current distribution of modern lungfish species reflects extinction of many lineages subsequent to the breakup of Pangaea, Gondwana and Laurasia.

Contents

  • Anatomy and morphology 1
  • Lungs 2
    • Perfusion of water 2.1
    • Perfusion of air 2.2
  • Ecology and life history 3
  • Extant lungfish 4
  • Taxonomy 5
  • Timeline of genera 6
  • See also 7
  • References 8
  • Further reading 9
  • External links 10

Anatomy and morphology

All lungfish demonstrate an uninterrupted cartilaginous notochord 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 in primitive lungfish are covered in a mineralized tissue called cosmine, 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 no homology to the skull roof bones of ray-finned fishes or tetrapods. 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.[2]

Through convergent evolution, lungfishes have evolved internal nostrils similar to the tetrapods' choana.[3]

The dentition of lungfish is different from that of any other vertebrate group. "Odontodes" on the palate and lower jaws develop in a series of rows to form a fan-shaped occlusion 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. They also demonstrate the largest genome among the vertebrates.

Modern lungfish all have an elongate body with fleshy, paired pectoral and pelvic fins and a single unpaired caudal fin replacing the dorsal, caudal and anal fins of most fishes.

Lungs

Lateral view of lungs of a dissected Protopterus dolloi

Most extant lungfish species have two lungs, with the exception of the Australian lungfish, which only has one. The lungs connect to the pharynx. The lungs of lungfish are homologous to the lungs of tetrapods. As in tetrapods and bichirs, the lungs extend from the ventral surface of the esophagus and gut.[4][5]

While other species of fish can breathe air via modified, vascularized gas 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 for gas exchange.

Perfusion of water

Of extant lungfish, only the Australian lungfish can respire through its gills. In other species, the gills are too atrophied to allow for adequate gas exchange. When a lungfish is obtaining oxygen from its gills, its circulatory system is configured similarly to the common fish. The spiral valve of the conus 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, the ductus 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.

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 though 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, feeding on fish, insects, crustaceans, worms, molluscs, amphibians and plant matter. They have an intestinal spiral valve rather than a true stomach.[7]

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 allow it to slow its metabolism to as little as 1/60th of the normal metabolic rate, and protein waste is converted from ammonia to less-toxic urea (normally, lungfish excrete nitrogenous waste as ammonia directly into the water).

Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. It has been proposed both that burrowing is plesiomorphic for lungfish, and that gnathorhizids are directly ancestral to modern Lepidosireniformes, but the similarity possibly is simply due to convergent or parallel evolution.

Lungfish can be extremely long-lived. A Queensland lungfish at the Shedd Aquarium in Chicago has been part of the permanent live collection since 1933.[8]

Extant lungfish

Extant lungfishes
Order Family Species Image Comments
Ceratodonti-
formes
Cerato-
dontidae
Queensland lungfish The Queensland lungfish, Neoceratodus forsteri, is endemic to Australia.[9] Fossil records of this group date back 380 million years, around the time when the higher vertebrate classes were beginning to evolve.[10] Fossils of lungfish almost identical to this species have been uncovered in northern New South Wales, indicating that the Queensland lungfish has remained virtually unchanged for well over 100 million years, making it a living fossil and one of the oldest living vertebrate genera on the planet.[10] It is the most primitive surviving member of the ancient air-breathing lungfish (Dipnoi) lineages.[10][11] The five other freshwater lungfish species, four in Africa and one in South America, are very different morphologically to N. forsteri.[10] 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.[9]
Lepidosireni-
formes
Lepido-
sirenidae
South American lungfish The South American lungfish, Lepidosiren paradoxa, is the single species of lungfish found in swamps and slow-moving waters of the Amazon, Paraguay, and lower Paraná River basins in South America. Notable as an obligate air-breather, it is the sole member of its family Lepidosirenidae. Relatively little is known about the South American lungfish.[1] and scaly salamander-fish.[12] When immature it is spotted with gold on a black background. In the adult this fades to a brown or gray color.[13] Its tooth-bearing premaxillary and maxillary bones are fused like other lungfish. South American lungfishes also share an autostylic jaw suspension (where the palatoquadrate is fused to the cranium) and powerful adductor jaw muscles with the extant lungfish (Dipnoi). Like the African lungfishes, this species has an elongate, almost eel-like body. It may reach a length of 125 centimetres (4.10 ft). The pectoral 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.[14] The gills are greatly reduced and essentially non-functional in the adults.[15]
Proto-
pteridae
Marbled lungfish The Polychaos dubium and Paris japonica at 670 billion and 150 billion, respectively.[18]
Gilled African lungfish The gilled African lungfish, Protopterus amphibius is a species of lungfish found in East Africa.[19][20] It generally reaches only of 44 cm (2 ft.) long, making it the smallest extant lungfish in the world.[21] This lungfish is uniform blue, or slate grey in colour. It has small or inconspicuous black spots, and a pale grey belly.[22]
West African lungfish

The West African lungfish Protopterus annectens is a species of lungfish found in West Africa.[23][24][25] It has a prominent snout and small eyes. Its body is long and eel-like, some 9-15 times the length of the head. It has two pairs of long, filamentous fins. The pectoral fins have a basal fringe and are about three times the head length, while its pelvic fins are about twice the head length. In general, three external gills are inserted posterior to the gill slits and above the pectoral fins. It has cycloid scales embedded in the skin. There are 40-50 scales between the operculum and the anus and 36-40 around the body before the origin of the dorsal fin. It has 34-37 pairs of ribs. The dorsal side is olive or brown in color and the ventral side is lighter, with great blackish or brownish spots on the body and fins except on its belly.[26] They reach a length of about 100 cm in the wild .[27]

Spotted African lungfish

The [28]

Taxonomy

Illustration of Ceratodus by Heinrich Harder

The relationship of lungfishes to the rest of the bony fish is well understood:

Recent molecular genetic analyses strongly support a sister relationship of lungfishes and tetrapods (Rhipidistia), with Coelacanths branching slightly earlier.[31]

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

Current phylogenetic studies support the following relationships of major lungfish taxa: Class Osteichthyes, subclass Sarcopterygii, order Dipnoi.

Dipnoi

Diabolichthyidae



Uranolophidae




Speonesydrionidae


Dipnorhynchidae



Stomiahykidae


Chirodipteridae

Holodontidae



Dipteridae




Fleurantiidae


Rhynchodipteridae


Phaneropleuridae

Ctenodontidae



Sagenodontidae



Gnathorhizidae

Ceratodontiformes

Asiatoceratodontidae


Ptychoceratodontidae

Ceratodontidae

Ceratodus


Metaceratodus


Neoceratodontidae


Mioceratodus


Neoceratodus - Queensland lungfish


Lepidosireniformes

Lepidosirenidae - South American lungfish


Protopteridae - African lungfish












Timeline of genera

See also

References

  1. ^ a b page 289
  2. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  3. ^ Evolution: On the evolution of internal nostrils (choanae)
  4. ^ Chapter 24: The Respiratory System Evolution Atlas
  5. ^ LAB 2 - GNATHOSTOME FORM & FUNCTION
  6. ^
  7. ^ Electron microscopy of the intestine of the african lungfish, Protopterus aethiopicus
  8. ^
  9. ^ a b
  10. ^ a b c d
  11. ^
  12. ^ page 275
  13. ^
  14. ^ "Your Inner Fish" Neil Shubin, 2008,2009,Vintage, p.33
  15. ^
  16. ^ Fishbase.org
  17. ^
  18. ^
  19. ^ EOL.org (Retrieved Feb. 19, 2010.)
  20. ^ Fishbase.org (Retrieved Feb. 19, 2010.)
  21. ^ Primitive Fishes.com Retrieved 19 February 2010.
  22. ^ Fishbase.org (Retrieved Sep. 25, 2010.)
  23. ^ EOL.org (Retrieved May 13, 2010.)
  24. ^ Fishbase.org (Retrieved May 13, 2010.)
  25. ^
  26. ^
  27. ^ Primitivefishes.com (Retrieved May 13, 2010.)
  28. ^ a b Fishbase.org
  29. ^ 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.
  30. ^ 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.
  31. ^

Further reading

  • Ahlberg, PE, Smith, MM and Johanson, Z, (2006). Developmental plasticity and disparity in early dipnoan (lungfish) dentitions. Evolution and Development 8(4):331-349.
  • Palmer, Douglas, Ed. The Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures. A Visual Who's Who of Prehistoric Life. Pg. 45. Great Britain: Marshall Editions Developments Limited. 1999.
  • Schultze, HP, and Chorn, J., (1997). The Permo-Carboniferous genus Sagenodus and the beginning of modern lungfish. Contributions to Zoology 61(7):9-70.

External links

  • Dr Anne Kemps - Lungfish Information site
  • Lungfish information site
  • Dipnoiformes at Palaeos.com
  • Dipnoi at the University of California Museum of Paleontology
  • Tree of life illustration showing lungfish's relation to other organisms
  • Lungfish video
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