Africa’s Great Lakes are teeming with brightly coloured cichlids. This popular aquarium fish is arguably one of the most diverse aquatic fish groups in the world, writes Georgina Lockwood
Cichlids are a key component of Africa’s Great Lakes ecosystem and inhabit the benthic zones – shallow and deep gravel surfaces of the lakes.
They play a role in reducing mosquito populations and provide protein to lakeside communities.
Different cichlids have diversified to feed on a wide array of food sources, such as algae, sponges, invertebrates, molluscs and even fish scales.
“This diversity of diet is a result of adaptive radiation, and probably what drove the evolution of so many different species,” explains Professor Walter Salzburger from the Zoological Institute at the University of Basel, Switzerland. Adaptive radiation is an evolutionary process in which organisms diversify rapidly from an ancestral species into a multitude of new forms to fill different ecological niches.
“The cichlid adaptive radiations in the East African Great Lakes have evolved independently from single or very few ancestors that colonised these lakes,” Salzburger says.
The cichlids found in Lake Malawi, Lake Victoria and Lake Tanganyika evolved from three separate or very few ancestors as there is no gene flow between the lakes.
Cichlid predators include catfish, Nile perch, water snakes, birds, crocodiles and otters. Other threats include habitat destruction, overfishing and climate change.
Local fishermen use gill and mosquito nets to catch fish. The most destructive fishing method for shallow water cichlids is beach seining, a method of fishing using a net hanging vertically in the water from the beach. “Certain varieties are fancied by aquarium hobbyists, and can be overfished,” Salzburger adds.
Lake Tanganyika is the oldest of Africa’s Great Lakes, dating back 9–12 million years. As a result the cichlids are unique and well studied. There are 208 described and 40 undescribed species.
Some species are rarely seen because they live in deep waters, while other species are only known from old ichthyology collections.
“The Tanganyikan cichlids are the most diverse with respect to morphology, ecology and behaviour,” Salzburger explains. The cichlids in Lake Tanganyika also evolved with the presence of a natural predatory fish, the Nile perch, making them resilient to predatory invasive species.
“Unlike the fish in Lake Victoria and Lake Malawi, many of the Lake Tanganyika cichlids are substrate spawners,” Salzburger says. “They build nests or use empty snail shells or small rock crevices for shelter.
“Certain cichlids from Lake Malawi and Lake Tanganyika construct volcano-like sandcastles to lay their eggs,” says Salzburger. There is also a scale-eating cichlid that lures other fish in order to bite off their scales.
Most Tanganyikan scale-eating species have mouth dimorphisms. “Half of the cichlid population have a mouth opening to the right side and the other half to the left,” Salzburger explains. “This helps them attack their prey,” says Salzburger.
Lake Tanganyika cichlids
Brightest: Neolamprologus mustax
Unique behaviour: Neolamprologus brichardi are cooperative breeders – a breeding pair will recruit older offspring to help them take care of new babies, behaviour that is not known to fish.
Largest: Boulengerochromis microlepis is 80–90cm long, making it the largest cichlid in the world
There are an estimated 500 to 1 000 species of cichlid in Lake Malawi, of which only 250 are described, and all of which are endemic to the lake.
Unlike the fish in Lake Tanganyika, the Malawian fish are relatively understudied because of the size of the lake. They are also the most affected by future oil exploration, habitat destruction, climate change and eutrophication.
The most endangered cichlids, like Copadichromis, are those that have suffered from overfishing. Populations in the south of the lake have collapsed due to commercial trawling. It is believed that certain cichlid species have gone extinct. The lake is unfortunately too big to evaluate this.
Lake Malawi cichlids
Brightest: Metriaclima zebra
Unique behaviour: Nimbochromis cichlids imitate dead fish on the lake floor. When a fish comes to prey upon the ‘dead body’, it attacks.
It is thought there are between 500 and 700 cichlid species in Lake Victoria. The lake dried up during the last ice age, about 18 000 years ago, so its cichlid species would have had to evolve after the ice age. However, genetic testing indicates Lake Victoria’s cichlids evolved around 100 000–150 000 years ago, before the ice age.
Experts believe that the evolution of the Lake Victoria cichlids initially involved hybridisation between more distantly related cichlids that arrived via rivers.
The invasive Nile perch threatens open-water predator cichlids as both species compete to fill the same ecological niche. “The Nile perch was introduced by fisheries looking to improve the productivity of Lake Victoria,” Salzburger says.
Unfortunately, native cichlid species are small and predated on. Lake Victoria has become more turbid because of eutrophication. Deforestation has also had an effect on the lake as sediment washes into
Lake Victoria cichlids
Brightest: Pundamilia pundamilia. As Lake Victoria is relatively young geologically speaking, many of its cichlids look the same.
Unique behaviour: Some Lake Victoria species practise paedophagy – egg stealing from mouthbrooding females.
Written by Georgina Lockwood.
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