Australian Lungfish

Neoceratodus forsteri




Australian lungfish (originally described as Ceratodus forsteri by Kreffts in 1870) was once abundant across most of the Australian mainland and also parts of South America. Fossils that appear to be those of essentially the same species as today have been found from as long ago as 150 million years. Thus it can lay fame to being the longest surviving vertebrate species– a true “living fossil”. Lungfish are a group of fishes, which were abundant during the Devonian, The “Great Age of Fishes”. Today they are represented by only three genera, Protopterus in Africa, Lepidosiren in South America and Neoceratodus in Australia. Of these three living lungfish, Neoceratodus is the most primitive and its habitat is the most restricted. It occurs naturally in only a few coastal river systems in south-east Queensland, where it is usually referred to as the Queensland lungfish. It has been totally protected there for almost 100 years and is also protected by the Convention for International Trade in Endangered Species (CITES). This protection contributes to its survival but it also means that it is not as well known, as it deserves to be. Apart from the fact that it can grow, albeit slowly, to an enormous size (20 kg is not unusual), lungfish are very easy to keep but, until very recently, were not available as aquarium fish because of the protection laws. But recently Queensland Fisheries Management Authority have issued licences to a few aquarists in Queensland and at least one of these has successfully produced first generation lungfish in captivity and these are gradually appearing in the aqua pet trade, at a price!

Why are these fish so special? What makes them different from most other fish? By far the majority of fish, both in the sea and freshwater, belong to the bony fish group of actinopterygians or ray-finned fish. Almost all the remainder are cartilaginous fish, the sharks and rays. Lungfish belong to the sarcopterygians or lobe-finned fish, of which the only other living group are the coelacanths, found in deep water off the coast of Africa (primarily around the Comoro Islands) and very recently discovered also off the islands of Indonesia. Many phylogeneticists include the land vertebrates (tetrapods: amphibians, reptiles, birds and mammals) in the sarcopterygian taxon, since it is generally agreed that it was from this group of fishes that the tetrapods arose during the Devonian, some 350 million yearsago. So the lobe-finned fish were once among the most abundant and diverse of the early jawed fish groups. The evolution of the ray-finned fishes overtook them in their adaptation to life in water while the lobe-finned fish evolved towards life on land. In both these groups there are relic species with primitive characters still alive today; eg lungfish (lobefin), sturgeon, paddlefish (rayfin).

In Lungfish lab are a number of projects, all of which are exploiting the very special phylogenetic position of our lungfish as the closest living fish to the tetrapod ancestors. Having said this, the phylogenetic position of lungfish has been very controversial since they were first described as a distinct group of fish in the latter half of the 19th century, and to some extent remains so today. Most of the controversy surrounds the fossil groups of lobe-finned fishes living in the Devonian, some 400–350 million years ago. All but the lungfishes and the coelacanths became extinct over 250 million years ago. It is fairly well agreed that the closest ancestor to amphibians is one of these extinct fishes so the question is – how closely related to the lungfish is that actual ancestor? To help answer this question, I became intrigued with the idea that surviving lungfish may be neotenic; i.e. they are larvae that have grown and become reproductively mature without metamorphosing. If this can be shown to be true, it suggests that lungfishes back in the Devonian may have had life cycles that included metamorphosis (like present day amphibians, tadpoles to frogs). It has been hypothesized many times that large steps in evolution are likely to occur through larval forms, so you see
where I am coming from.

By Professor Jean Joss.