Lungfish

Lungfish (also known as salamanderfish ) 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, they 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 that advanced lungfish had a widespread freshwater distribution and that the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of Pangaea, Gondwana and Laurasia.

Anatomy and morphology
All lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition. The lungfish is a true carnivore. Basal 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. It is thought these fins 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 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 fin of most fishes.

Lungs


All lungfish have two lungs, with the exception of the Australian lungfish, which only has one. The lung(s) 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 '.

While other species of fish can breathe air via modified, vascularized gas bladders, these bladders are usually simple sacs, devoid of complex internal structure. In contrast, the lungs of lungfish are subdivided into numerous smaller air sacs, maximising 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 of lungfish, 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 by-pass arterioles of the 3rd and 4th gill arches (which do not actually have gills) are shut, the 2nd, 5th and 6th gill arch arterioles are open, the ductus arteriosus branching off the 6th arteriole is open and the pulmonary arteries are closed. As the water passes through the gills, the lungfish uses a buccal pump. Flow though 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 3rd and 4th gill arches open, the 2nd and 5th gill arches close (minimizing the possible loss of oxygen that was obtained in the lungs through the gills), the 6th arteriole's ductus arteriosus is closed, and the pulmonary arteries open. It is important to note that during air breathing, the 6th 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
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 the lungfish 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 as well as that gnathorhizids are directly ancestral to modern Lepidosireniformes, but it is possible that the similarity is simply due to convergent or parallel evolution.

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

Taxonomy
The relationship of lungfishes to the rest of the bony fish is well-understood:
 * Lungfishes are most closely related to Powichthyes, and then to the Porolepiformes.
 * Together, these taxa form the Dipnomorpha, the sister group to the Tetrapodomorpha.
 * Together, these form the Rhipidistia, the sister group to the Coelacanths.

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 that have been identified may not be monophyletic. Current phylogenetic studies support the following relationships of major lungfish taxa:

Class Osteichthyes

Subclass Sarcopterygii

Order Dipnoi

,--Family Diabolichthyidae (extinct) | ,--Family Uranolophidae (extinct) | | __,--Family Speonesydrionidae (extinct) '-|-| '--Family Dipnorhynchidae (extinct) |    ,--Family Stomiahykidae (extinct) '|___ ,--Family Chirodipteridae (extinct) |     '-|--Family Holodontidae (extinct) |--Family Dipteridae (extinct) | __,--Family Fleurantiidae (extinct) '-| '--Family Rhynchodipteridae (extinct) '--Family Phaneropleuridae (extinct) | ,--Family Ctenodontidae (extinct) '-| ,--Family Sagenodontidae (extinct) '-|--Family Gnathorhizidae (extinct) '--Order Ceratodontiformes |--family Asiatoceratodontidae (extinct) |--Family Ptychoceratodontidae (extinct) |--Family Ceratodontidae | '--Genus Ceratodus (extinct) | '--Genus Metaceratodus (extinct) '--Family Neoceratodontidae | '--Genus Mioceratodus (extinct) | '--Genus Neoceratodus - Queensland lungfish '--Order Lepidosireniformes '--Family Lepidosirenidae - South American lungfish '--Family Protopteridae - African lungfish