What tiny lakes are revealing about cichlid speciation
Tiny lakes may hold the key to the spectacular adaptive radiation of African cichlid fish, new research has shown.
Thousands of unique cichlid fishes, many of them brilliantly-coloured aquarium fish, have evolved in Lakes Malawi, Tanganyika and Victoria, making them textbook cases of evolutionary exuberance. With so many recently-evolved species to work on, cichlids have tremendous potential to yield insights into the processes of speciation and adaptive evolution. Biologists also hope that studying how cichlids evolve will help reveal why some groups of organisms diversify much more than others, at least in certain times and places.
Most researchers have looked at species that live on clear-water rocky shores. Many rocky shore cichlids are highly specialised to these patchily distributed habitats, and they lack a dispersal phase in their life histories, so they become isolated on particular islands or stretches of rocky coastline. This makes it easy to see how they could have evolved into different species in different parts of a single lake.
But, there are hundreds of less specialised species or species living offshore and in deepwater habitats that encounter no barriers to movement within the lakes. There are various theories about how these might have evolved in the distant past if the lake level had fallen enough to split the lake into smaller bodies, or even that the species might have diverged without geographic isolation, by the more controversial process of sympatric speciation. These ideas are difficult to test.
Recently, it has become clear that tiny lakes around Lake Malawi might hold the key to understanding how new cichlid species may evolve in these lakes, even if they are not confined to rocky shores. Lake Chilingali is a tiny lake about 5km long and 1km wide, about 12km to the west of Lake Malawi, near the town of Nkhotakota. In 2004, researchers now at Bangor and Bristol universities in the UK discovered that several cichlid fishes were found in this lake including two species belonging to genera unique to the Lake Malawi system. Each population showed subtle differences in body shape and ecology from the most similar forms in Lake Malawi. Live specimens of both species and their probable closest relatives from Lake Malawi were brought back to the UK. Mate choice trials carried out in research aquaria showed that the Malawi and Chilingali forms preferred to mate with their own kind, showing that they were well on the way to evolving into distinct species.
How did the Lake Malawi fishes get into Lake Chilingali? The small lake is only 30m higher than the level of Lake Malawi, but they are separated by rocky rapids on the small seasonal stream than connects them. Perhaps the fish could have swum up the stream before the rapids formed, or perhaps the water level of Lake Malawi was once 30m higher?
In 2011, Lake Malawi-type cichlids were discovered in a series of volcanic crater lakes in southern Tanzania, so this process might be happening in several places at once. These tiny lakes may hold the key to understanding cichlid speciation, but they are extremely fragile habitats.
In the 1950s, an irrigation dam was built on the stream flowing out of Lake Chilingali, making the lake bigger. But, the dam wall was never maintained and it is rumoured to have collapsed entirely in 2012. A lot of sediment is likely to have built up behind the dam, and it is not clear if there is still an open-water lake remaining. It is quite possible that the two unique Lake Chilingali cichlids are now extinct in the wild. And there have been several attempts to introduce alien Nile Tilapia into the Tanzanian crater lakes. Both Tanzanian and Malawian lakes are in need of further study and protection if we are going to make the most of the tremendous opportunities they provide to find out about the remarkable diversity of cichlid fish in the African Great Lakes.