Lungfish is any sarcopterygian fish of the taxon Dipnoi, characterized by platelike teeth and lobed, paired fins, with modern forms typified by functional lungs and the caudal fin confluent with the dorsal and anal fins. While some consider Dipnoi to be a subclass of Sarcopterygii, Nelson (2006) considers the subclass to be Dipnotetrapodomorpha, with the dipnoians (lungfishes) comprising two superorders within this subclass, Dipterimorpha and Ceratodontimorpha, the later including the living lungfishes. The dipnoans are well represented in the fossil record, but there are only six extant species, all freshwater species. The first living lungfish to be formally described was Lepidosiren paradoxa, the South American lungfish, in 1837 (Nelson 2006).
Lungfishes are uniquely adapted to be able to survive in low oxygen water because of their ability to breath air and, in some species, even in conditions of little or no water by the ability to estivate. They trace back to the Early Devonian, and there are studies that suggest they have changed little over the years, with the Australian lungfish (Neoceratodus forsteri) seemingly not having changed in 100 million years (LFI 2004).
Lungfish, together with coelacanths, are part of the vertebrate class Sarcopterygii. The bony fish representatives of this class, known as lobe-finned fish, are characterized by lobed paired fins, which are joined to the body by a single bone (Clack 2002). Sarcopterygians also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fishes). Some taxonomists also include the tetrapods (amphibians, [[reptile]s, birds, mammals) in Sacropterygii in order to make this a monophyletic group (Nelson 2006).
The lungfish traditionally are placed in the subclass Dipnoi, while the coelacanths are placed in the subclass Coelacanthimorpha and order Coelacanthiformes. However, Nelson (2006), in his book Fishes of the World, while recognizing Coelacanthimorpha, designates Dipnotetrapodomorpha as the subclass with lungfish, and within that subclass recognizes two superorders of lungfishes, Dipterimorpha and Ceratodontimorpha. These two superorders of dipnoans are characterized by fish with platelike teeth, suitable for crushing and grinding. The living lungfishes are placed in the order Ceratodontiformes of the superorder Ceratodontimorpha. Members of Ceratodontimorpha are characterized by having a caudal fin confluent with the dorsal and anal fins, functional lungs (modified swim bladder), and the absence of premaxilla and maxilla (Nelson 2006).
Lungfish are best-known for retaining characteristics primitive within the bony fish, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton.
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 taxa, including all modern species, show a significant reduction in 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 is subdermal 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 actinopterygiians or tetrapods.
The dentition of lungfish is conspicuously 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 six extant species of lungfishes are all freshwater forms, placed in three families. There is one species of Australian lungfishes (family Ceratodontidae), one species of South American lungfishes (family Lepidosirenidae; found in Brazil and Paraguay), and four species of African lungfishes (family Protopteridae; all placed in the genus Protopterus). The air bladder (lung) of the Australian lungfishes are unpaired, but the air bladder of the other families of lungfishes are paired (Nelson 2006). Likewise, the Australian lungfishes have flipper-like pectoral and pelvic fins, large scales, and larvae without external gills, while the other species have filamentous pectoral and pelvic fins without rays, small scales, and larvae with external gills (Nelson 2006). Modern lungfish all have an elongate, snake-like body with fleshy paired pectoral and pelvic fins and a single unpaired caudal fin replacing the dorsal, caudal, and anal fin of most fishes.
All species of modern lungfishes grow large, with some African lungfishes reaching over two meters (six feet) in length (LFI 2004). The modern lungfishes have a number of larval features, which suggest paedomorphosis. They also demonstrate the largest genome among the vertebrates.
Lungfish are found today 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 cosmopolitan freshwater distribution and that the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of Pangea, Gondwana, and Laurasia. There are suggestions that the lungfish dispersed in freshwater on a united supercontinent, but some Cretaceous forms also show marine tolerance and may have dispersed without land connection (Nelson 2006).
African and South American lungfish are capable of surviving seasonal desiccation of 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 one-sixtieth 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 evolution or parallel evolution. Adults of Australian lungfish (today found in Southeast Queensland, Australia) do not estivate (Nelson 2006).
Lungfish can be extremely long-lived. The Queensland species lungfish at the Shedd Aquarium in Chicago has been part of the permanent live collection since 1933.
The relationship among lungfishes is difficult to resolve. While Devonian lungfish retain enough ossification of the endocranium 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:
,—†Family Diabolichthyidae | ,—†Family Uranolophidae | | __,—†Family Speonesydrionidae '-|-| '—†Family Dipnorhynchidae | ,—†Family Stomiahykidae '----|___ ,—†Family Chirodipteridae | '-|—†Family Holodontidae |------†Family Dipteridae | __,—†Family Fleurantiidae '-| '—†Family Rhynchodipteridae '—†Family Phaneropleuridae | ,—†Family Ctenodontidae '-| ,—†Family Sagenodontidae '-|—†Family Gnathorhizidae '—Order Ceratodontiformes |—†family Asiatoceratodontidae |—†Family Ptychoceratodontidae |—Family Ceratodontidae | '—†Genus Ceratodus | '—†Genus Metaceratodus '—Family Neoceratodontidae | '—†Genus Mioceratodus | '—Genus Neoceratodus - Queensland lungfish '—Order Lepidosireniformes '—Family Lepidosirenidae - South American lungfish '—Family Protopteridae - African lungfish
The following taxonomy is drawn from Nelson (2006).
All links retrieved August 22, 2014.
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