Lungfish (Dipnoi)

The Dipnoi are distinguished by special features of the jaw and skull bones. They lack articulated premaxilla and maxilla bones in the lower jaw and the palatoquadrate (part of the upper jaw joint) is fused to the cranium part of the skull. The teeth are scattered over the palate and fuse to make sharp tooth ridges that form a crushing feding-apparatus.

There are three extant genera of lungfish, found on three continents. The Australian lungfish, Neoceratodus forsteri, is similar to the extinct Mesozoic lungfish. It is restricted to freshwater and can reach 1.5 metres in length. It swims by undulating movements but it can also crawl slowly over the bottom of a pond and even on dry land on its pectoral and pelvic appendages. They have sharp chemical sense, both smell and taste. Respiration is mostly via its gills but it has a single lung it uses in stressful situations. The behaviour of Neoceratodus is mostly unknown but they have a complex courtship and are selective about the substrate they lay their adhesive eggs on.

The South American lungfish, Lepidosiren paradoxa, is even more of a mystery as its scientific name suggests. It is more eel-like than the Australian lungfish and lives in and close to the Amazon river and some of it's tributaries. It can reach to sizes of 100 cm. The male has specialised pelvic fins made of vascularised extensions that develop during the breeding season. The fins most likely supply oxygen from the male's blood to the young in the nest cavity. They breath almost only with their lungs as the gills are degenerated. It is considered more related to the African lungfish than the Australian one.

There are four species of the African lungfish Protopterus that all look very similar. Like Lepidosiren they have weak gills and drown if prevented from using their lungs. The gills remain though as they have an important function in eliminating carbon dioxide. The African lungfish species can reach lengths up to 2 metres and they have unique filamentous appendages that are highly mobile.

Some species of African lungfish have a strange habit of aestivation. Aestivation is a type of a hibernation process induced by drying of the habitat. African lungfish are common in areas that flood in the rainy season but become extremely dry in the dry season. When the rainy season comes to an end, the lungfish dig a burrow in the mud that ends in an enlarged chamber where the lungfish remains. As the environment becomes dryer, the lungfish becomes more lethargic and breathes air from the burrow opening. When the water in the burrow completely dries up the lungfish enters the ultimate stage of estivation, curles up in a U-shape with the tail covering the eyes. Since it entered the burrow it has secreted layers of mucus that condense and form a protective layer. Only an opening remains at the mouth so it can keep on breathing. The metabolism of the lungfish does not stop during the estivation period but it slows down and muscle proteins are the only source of energy. This period is usually not longer than 6 months but in extreme cases, lungfish individuals have been revived after 4 years of estivation. As soon as the rain arrives the lungfish becomes active and feeds fiercily so in a month it has regained its previous size. Fossilised lungfish burrows with remains of estivating lungfish have been found on more than one occasion.

• Characters/ anatomy
• Fossil record
• Modern forms
• From water to land
• Literature and web links

Author: Snorri Sigurdsson (Email: ss4460@bristol.ac.uk)
Last updated: 14 November 2004
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