The staggering diversity of Africa’s terrestrial fauna is old news, but few people are aware that it also harbours the greatest freshwater fish diversity of any continent. And nowhere does this diversity reach such heights as in Lago Niassa, whose 850 described fish species – with more awaiting formal discovery – exceed the number of known freshwater species in Europe and North America combined.
Lago Niassa’s fish diversity is the product of the most dramatic incidence of explosive speciation known to evolutionists. The majority of these fish species are cichlids – pronounced ‘sicklids’ – a perch-like family of freshwater fishes called Cichlidae that ranges through the Middle East, Madagascar, Asia and South and Central America. It is in Africa’s three largest lakes, however, that this widespread family has undergone an unprecedented explosion of evolutionarily recent speciation that has resulted in it constituting an estimated 5% of the world’s vertebrate species.
The staggering diversity of Africa’s terrestrial fauna is old news, but few people are aware that it also harbours the greatest freshwater fish diversity of any continent.
The cichlids of Africa’s great lakes are generally divided into a few major groupings, often referred to by scientists by names used locally in Malawi and/or Mozambique. These include the small plankton-eating utaka, the large, pikelike and generally predatory ncheni, the bottom-feeding chisawasawa and the algae-eating mbuna.
People who have travelled in any part of Africa close to a lake will almost certainly have dined on one or other of the tilapia (or closely related oreochromis) cichlids, large ncheni that make excellent eating and are known locally as chambo. To aquarium keepers, snorkellers and scuba divers, however, the most noteworthy African cichlids are the mbuna, a spectacularly colourful group of small fish of which some 300 species are known from Lake Malawi alone.
The mbuna of Lago Niassa first attracted scientific interest in the 1950s, when they formed the subject of Dr Geoffrey Fryer’s classic study of adaptive radiation. This term is used to describe the explosion of a single stock species into a variety of closely related forms, each of which evolves specialised modifications that allow it to exploit an ecological niche quite different from that occupied by the common ancestral stock.
This phenomenon is most likely to occur when an adaptable species colonises an environment where several food sources are going unused, for instance on a newly formed volcanic island or lake. The most celebrated incidence of adaptive radiation – the one that led Charles Darwin to propose the theory of evolution through natural selection – occurred on the Galapagos Islands, where a variety of finch species evolved from one common seed-eating ancestor to fill several very different ecological niches.
With over 850 described fish species – with more awaiting formal discovery – the number of freshwater species in Lago Niassa exceeds the number of known species in Europe and North America combined.
The explosive speciation that has occurred among Africa’s cichlids is like Darwin’s finches amplified a hundredfold. The many hundreds of cichlid species in Lake Tanganyika and Lago Niassa evolved from a handful of river cichlids that entered the lakes when they formed about two to three million years ago. (No less remarkable is the probability that the 200 or so cichlids in Lake Victoria all evolved from a few common ancestors over the 10,000–15,000 years since the lake last dried up.) In all three lakes, specialised cichlid species have evolved to exploit practically every conceivable food source: algae, plankton, insects, fish, molluscs and other fishes.
Somewhat macabrely, the so-called kiss-of-death cichlids feed by sucking eggs and hatchlings from the mouths of mouth-brooding cichlids. No less striking is the diverse array in size, coloration and mating behaviour displayed across different species. In addition to being a case study in adaptive radiation, the cichlids of the great lakes are routinely cited as a classic example of parallel evolution – in other words, many similar adaptations appear to have occurred independently in all three lakes.
Why this should have occurred with the cichlids rather than any of several other fish families is a question that is likely to keep ichthyologists occupied for decades. One factor is that cichlids are exceptionally quick to mature, and thus have a rapid turnover of generations. Their anatomy also appears to be unusually genetically malleable, with skull, body, tooth and gut structures readily modifying over relatively few generations.
To aquarium keepers, snorkellers and scuba divers, however, the most noteworthy African cichlids are the mbuna, a spectacularly colourful group of small fish of which some 300 species are known from Lake Malawi alone.
This capacity to colonise new freshwater habitats is boosted by a degree of parental care rare in other fish – the mouth brooders, which include all but one of the cichlid species of Lago Niassa, hold their eggs and fry in their mouth until they are large enough to fend for themselves. Bearing in mind that the separation of breeding populations lies at the core of speciation, there is also mounting evidence to suggest that cichlids have a unique capacity to erect non-physical barriers between emergent species – possibly linked to a correlation between colour morphs and food preferences in diverging populations.
Africa’s lake cichlids are never likely to rival its terrestrial wildlife as a tourist attraction. All the same, snorkelling and diving in Lago Niassa is both thrilling in itself, and a humbling introduction to what has justifiably been described as a ‘unique evolutionary showcase’.