Introduction to Lungfish

Six species of lungfish in three genera survive today  from a group that appeared in the Devonian and was widespread in the Paleozoic. Modern lungfish are in several ways at once specialized and simplified versions of their more completely ossified ancestors. The Australian (Queensland) Neoceratodus is the most heavily ossified of living forms, and, although there are differences in the skull and toothplates, it resembles (as its name implies) what is known of the Triassic Ceratodus. It is covered with large overlapping scales, the notochord is large, and the girdles and fin skeletons are cartilaginous. The paired fins are lobelike (sarcopterygian) contrasting with the elongate fins of the other living genera. Young Neoceratodus and Protopterus “walk” along the bottom of aquaria with their flexible paired fins, much as do urodele amphibians. Unpaired fins are lacking, but it seems that the symmetrical “caudal” may have arisen by the union of dorsal and anal fins, since there is no trace in living lungfish development of the heterocercal tail of fossil lungfish.

Lungfish eat small invertebrates and quantities of plant material, which they grind and crush with paired upper and lower tooth plates. The gut is simple and ciliated, there is no stomach nor hepatic cecum, but there is a spiral valve. Both inhalent and exhalent nasal openings to the nasal chamber lie in the roof of the mouth, and, although the exhalent opening was supposed to be the equivalent of the tetrapod internal naris, more recent detailed embryological studies have shown that it is simply the homolog of the actinopterygian posterior nasal opening, secondarily displaced into the mouth. The air bladder is septate and lung-like, paired in Protopterus and Lepidosiren, but single in Neoceratodus which normally only breathes air when very active and stressed. Like most fish groups, lungfish have electroreceptors, but studies of Neoceratodus snout have shown that interpretations of the cranial tubular systems in fossils as a complex electroreceptor system are probably wrong. The curious vascular loops in Neoceratodus skin, such as the fossil tubular systems, are likely related to dermal bone modification as the fish grows. A remarkable feature of Protopterus and Lepidosiren (but not Neoceratodus), is that if the swampy pools they live in dry up, the fish make burrows and estivate. This useful habit was also known to fossil lungfish (the burrows of Griphognathus are not uncommon) showing that air breathing was early acquired, although water breathing probably was the rule, since there is a large operculum. Estivation is not peculiar to lungfish however.

A simple valve in the truncus arteriosus of the heart allows a partial separation of the pulmonary and systemic circulations. In this, and the structure of the brain and reproductive system, lungfish resemble amphibia. They also share with amphibia ciliated skin in the embryos and early hatchlings, and the interesting feature that at these stages the skin propagates action potentials (Bone et al., 1989). Neoceratodus simply attaches its globular sticky eggs to water plants and the submerged roots of riverside trees, and then abandons them, but the other living genera make nests.