Understanding Catfish Colonisation and Diversification in The Great African Lakes

Why some regions or habitats contain vast, diverse communities of species, whilst others contain only relatively few species, continues to be the subject of scientific research attempting to understand the processes and conditions that allow and adaptive radiation. The Great African Lakes exist as freshwater ‘islands’, with spectacularly high levels of biodiversity and endemism. They are particularly famous for the hyperdiverse Cichlid fish, but they are also home to diverse assemblages of many other fish, such as catfish. Recent research in GEE investigated the evolution of Clatoteine catfish in Lake Tanganyika, to investigate the forces driving evolutionary radiations in the Great African Lakes. Their results suggest that evolutionary time is of key importance to catfish radiations, with recently colonised groups showing less diversity than long-standing species.

Lake Tangayika is the World’s second largest freshwater lake, covering 4 countries in the African Rift Valley (Tanzania, the Democratic Republic of the Congo, Burundi and Zambia). It is home to the highest diversity of lake-dwelling catfish on Earth, however the evolutionary history of these catfish is not fully understood. GEE academic Dr Julia Day and PhD Student Claire Peart, in collaboration with colleagues at the Natural History Museum, London and the South African Institute for Aquatic Biodiversity, investigated the evolutionary history of nocturnal Claroteine catfishes in Lake Tangayika. This group of catfish offers an excellent opportunity to investigate the influence of different factors in evolutionary diversification, as it includes multiple genera with varying range sizes and habitat types.

The Drivers of Diversification

Previous research has suggested a number of factors that are important in enabling adaptive radiations that can produce extremely high levels of biodiversity – deep lakes that experience lots of sunlight tend to favour evolutionary diversification. Diversification is also more common for species that have had a lot of evolutionary time in which to diverge and that experience high levels of sexual selection. Interestingly, although lake depth is important, the total size of the lake does not appear to be so important for diversification. A large geographical area to diversify into may influence the duration of adaptive radiations, however, with river-dwelling species showing more consistent species-production through time. This data suggests that adaptive radiations may be, to some extent, predictable, however much previous work has focussed on key model groups such as the Cichlid fish, and these hypotheses need to be generalised to other species and locations.

The authors sequenced nuclear and mitochondrial genes from 85 catfish covering 10 of the 15 species of Claroteine catfish, in order to construct an evolutionary tree for the sub-family. Estimates of the relationships between species and the evolutionary timescales of colonisation and divergence allowed the authors to distinguish between the possibilities of single or multiple colonisation events, and the processes driving diversification. The results indicated that most Claroteine catfish in Lake Tangayika originate from a single colonisation of the lake between 5 and 10 million years ago, followed by evolutionary radiations to produce the variety of species present today. One species, Chrysichthys brachynema was the exception to this rule, having independently colonised the lake around 1 – 2 million years ago. This species has not shown adaptive radiation since colonisation, probably because of the relatively short time it has been present in the lake. These results support previous work that has suggested that time is an important factor in producing highly diverse species assemblages.

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