Monday, 3 October 2011
Aquarium Tour: Lungfish
The last large tank in the Aquarium contains a large variety of Malawi cichlids. While these are fascinating animals in their own right, I would like to write about one of the less commonly visible inhabitants of the tank, the African lungfish. There are actually four recognised species of Protopterus in Africa, with several divided into subspecies, and as they are very similar I am not sure if the two individuals we have are the same form, but they are probably P.aethiopicus. A third individual is in quarantine and will be going on show soon.
Lungfish are famous for their ability to breathe atmospheric air, in fact they will drown if prevented from reaching the water surface, but they have many other peculiarities as well. The air breathing habit is one they retain from their common ancestry with tetrapods, and indeed all fish except the sharks and rays have an internal organ which derives from an ancestral lung. In ray-finned fish the lung has been transformed into a flotation device called the swim bladder, and has lost its connection with the throat in many species. Some of these in turn have developed another air-exchange organ, but this is in addition to the swim bladder.
Lungfish are not often seen in the main lake at Malawi, although they are known from other Rift Valley lakes. This is because their preferred habitat is papyrus swamps and other areas where they can bury themselves in the mud, especially during the dry season. Malawi itself is too rocky for them.
Lungfish are fairly specialised predators, feeding mainly on snails, freshwater mussels and crustaceans, although they will prey on fish on occasion. They have specialised tooth plates, which act like nutcrackers to break shells, and have a powerful bite.
Lungfish have of course been around for a very long time. The earliest known date back well over 400 million years, and the split between them and their nearest living relatives (apart from terrestrial vertebrates), the coelacanths, must go back even further. The earliest lungfish may have been marine animals, perhaps using their lungs to survive in warm, oxygen-poor waters in back reef situations, but all the living forms are confined to freshwater. The habit of burying themselves in the mud to survive low water is also very ancient, and fossil lungfish burrows are common in the fossil record.
Lungfish breeding is keyed to their seasonal habitat. When the rains flood their burrows, they emerge, feed up quickly, then spawn in nest burrows constructed by the male, who guards the eggs fiercely. The larvae at first have gills, and remain in the nest for around 50-60 days. Once they reach around 25 mm the lungs begin to develop, and they disperse to live amongst the roots of swamp grasses. Their must be very high mortality, especially in the first year or so, before they have grown large enough to dig deep burrows to survive severe droughts. To compensate, they also appear to be extremely long lived – one in the Shedd aquarium in the US has apparently been there since the 1930’s, and was probably an adult when imported. A lifespan of 80-100 years does not seem unreasonable.
Although long lived in captivity – they are tough animals after all – as far as I am aware there has been no recorded captive breeding of this species or any other lungfish. As they grow very large – at least 1.5 m – and can be quite aggressive with each other, breeding would be a major operation. It would probably need a large, mud-bottomed, outside pool in a suitable climate which could be drained to encourage aestivation and then reflooded to replicate the habitat. As several thousand eggs can be produced, successful breeding would then also create the need to re-home vast numbers of baby lungfish – not something a responsible aquarist would like to face.
(image from wikipedia)