The Dipnoi, commonly known as lung fish, are considered to be specialized or degenerate descendants of the more primitive lobe fins which they closely resemble. They were once supposed to be the ancestors of the amphibians, a view no longer held. They are large, bizarre fishes represented only by three living genera and furnishing an example of discontinuous distribution. In all three, the air bladder serves functionally as a lung by which they can breathe air when necessity arises. The endangered monotypic Australian lungfish, Neoceratodus (-Epiceratodus) forsteri, restricted to fresh waters, attains a length of 1.5 meters or more.
It has a single lung (Monopneumona) and cannot survive out of water. The monotypic South American lungfish is Lepidosiren paradoxa, while the African lungfish, Protopterus, has four or so related species. These two genera have weekly developed gills so that they will drown if prevented from using their paired lungs (Dipneumona). They live on mudy swamps or marshes.
During drought, when the marshes dry up, they aestivate by secreting mucous cocoons in mud, in which they become dormant and breathe atmospheric air through burrow openings. These 1 to 2 meters long, elongated, eel-like or snake-like forms have unique filamentous but highly mobile paired appendages Lungfishces are mostly cartilaginous. They have specialized crushing tooth plates to feed effectively upon shell fish.
Their embryogeny is more primitive than that of other living bony fishes. Cleavage of eggs is holoblastic and gastrulation occurs by invagination. Their tadpole like larva has external gills as accessory respiratory structures
General Organisation Of Dipnoi
The three genera of living Dipnoi are allied in all the essential features of their structure. Therefore it is only necessary to mention here the principal points in which Neoceratodus and Lepidosiren differ from Protopterus. Protop terus and Lepidosiren resemble one another more closely than either resembles Neoceratodus and are placed in the same family, the Lepidosirenida.
The limbs of Lepidosiren are similar to those of Protopterus, but in Neo ceralodus they are broad and leaf-like, with a correspondingly modified skeleton. The tapering central axis is composed of a stout basal cartilage and a number of short, cartilaginous segments, with serially arranged rows of jointed, cartilag inous radialia projecting on either side. The skull of Neoceratodus differs some what from the lepidosirenid type, being an undivided mass, devoid of fontan elles. In many respects it resembles the skull of the fossil dipnoan Ceratodus, and is more primitive than that of lepidosirenids. Furthermore, true splenials and vestigial dentaries are present.
There are two lungs in Protopterus and Lepidosiren, but only one in Neoccratodus. The monopneumonous condition in Neoceratodus is possibly due to the suppression of the original left lobe (Goodrich). The internal structure of the lung, too, is simpler in this species. In Neoceratodus only four pairs of gills are developed, but each gill arch carries a double row of filaments. The hyoidean gill is a true pseudobranch. A peculiarity in this genus is that the branchial lamellae of cach gill arch extends on to the dorsal and ventral walls of the gill cleft, so that the hemibranchs of each cleft are continuous, both dorsally and ventrally. The fifth arch lacks gills.
Differences in the branchial systems of Protoplerus, on the one hand, and Neoceratodus, on the other, are probably correlated with the more aquatic habitus of the latter. Neoceratodus does not æstivate, and is said to inhabit only permanent waters. It resorts to aerial respiration when the water becomes foul during the dry season. Although able to live in stagnant water that is toxic to other fishes, Neoceratodus dies if removed from water. It is not asphyxiated by prolonged immersion.
External gills are developed in the larvae of Lepidosiren, but not in Neoceratodus, Vestiges of the external gills persist in adult Protopterus annectens and P. amphibius. During the breeding season vascular filaments develop on the pelvic fins of male Lepidosiren. The function of these filaments is unknown, but it is thought that they may serve as accessory respiratory organs.
The vascular system of Lepidosiren closely resembles that of Protopterus. In Neoceratodus, however, there are several points of difference. The heart in this genus is less specialised, in that the longitudinal septa of the conus are incomplete and represented by longitudinally arranged valves. Consequently there is more mixing of oxygenated and deoxygenated blood supplied to the anterior afferent vessels. Since there is a hyoidean pseudo branch (as opposed to a hemibranch in Protopterus and Lepidosiren), the anterior afferent vascular system is different. The pseudobranch is supplied by a vessel from the efferent branchial artery of the first gill-arch. Anterior and posterior cerebral arteries originate from the pseudobranch.
The venous system in Protopterus and Lepidosiren is very similar, and represents in general a more advanced evolutionary stage than that of Neo ceratodus. An abdominal vein, derived from the union of the pelvic veins, is present in Neoceratodus, but not in the other genera.
The brain of Neoceratodus differs from that of Protopterus (and Lepidosiren) in having paired optic lobes and a clearly demarcated bulbus and lobus in the olfactory tract. In cach genus the alimentary canal is similar in most respects except for differences in dentition. In the urinogenital system the kidneys are relatively more elongate in Protopterus and Lepidosiren than in Neoceratodus. In Lepidosiren the posterior aspects are unfused.
The testes of Protopterues and Lepidosiren are elongate, and divisible into an anterior, longer spermatogenic part and a posterior vesicula seminalis. In Neoceralodas and Lepidosiren about six vasa ellerentia rise from the posterior region and enter the mesonephric tubule, the spermatozoa then passing through the mesonephric ducts. This arrangement shows considerable affinity with the Amphibia
In Protopterus it has been demonstrated that the homologue of the adrenal medulla occurs in the walls of the intercostal branches of the dorsal aorta (Giacomini) and its chromaffin cells are innervated from the sympathetic chains (Holmes). It has been further suggested by Holmes that the cortical homo- logue may be widely distributed as diffuse specialised tissue kdentifiable in various organs throughout the body cavity.
The origin and evolution of Dipnoi remains problematic due to diverse opinions. They combine characteristics in which they resemble almost all the other groups of fishes as well as Amphibia. Fossil primitive Dipnoi (e.g. Diptems), shows geater similarity with fossil crossopterygians (e.g. Osteolepis), than do their living members. During the course of their evolution, the modern Dipnoi have undergone several changes or specializations such as :
- Anterior dorsal fin was reduced and eventually lost.
- Remaining median fins elongated and fused so that originally heterocercal tail became symmetrically diphycercal.
- Reduction in number of dermal bones of skull and operculum.
- Thick bony cosmoid scales modified into thin cycloid scales.
- Extensive sheet of cosmine covering head and body was lost.
- Fusion of conical teeth into crushing tooth plates to feed effectively upon shell fish.
- Air bladders became functional lungs not unlike those of higher vertebrate
Evidence indicates that Dipnoi are degenerate descendants of Crossopterygii which early dipnoans closely resembled. Romer (1945) thought that Dipnoi and rhipidistian Crossopterygii had a common ancestor. On the other hand close similarity between Dipnoi and Amphibia led early workers to conclude that dipnoans gave rise to amphibians, a view no longer held now.
However, it is universally accepted that Amphibia have originated either directly from rhipidistian Crossopterygii or from some common ancestor