Summary Neutral Theory Cichlids in the African rift Lakes
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Cichlids in the African Rift Lakes

Comparing the three lakes

The three African rift lakes have many striking differences, but also a lot in common. The three lakes lie within the East African Rift, which is the result of the African tectonic plate splitting into two smaller tectonic plates, the "Somali Plate" and the "Nubian Plate". The Nubian Plate being the earth mass west of the three large lakes, the Somali Plate being the earth mass east of the three large lakes. The Somali plate is slowly drifting east, tearing away from the Nubian plate. The gaps it leaves behind have resulted in the African Rift lakes as we know them now.

Lake Victoria is the youngest lake (250k{750k y) (Johnson et al., 1996), and harbours about 700 cichlid species (Turner et al., 2001). Lake Malawi is about 5 million years old (Delvaux, 1995) and harbours at least 700 different cichlid species (Turner et al., 2001). Lake Tanganyika is the oldest lake, with an estimated age of 9-12 million years (Cohen et al., 1993, 1997) and contains the behaviourally most diverse cichlids (Konings, 2003, 2005). Despite being behaviourally most diverse, Lake Tanganyika contains relatively few species: only 250 different cichlid species can be found in lake Tanganyika (Turner et al., 2001).
When considering abiotic conditions, lakes Malawi and Tanganyika appear to be quite similar. Both are elongated, relatively deep lakes, that are both much older than Lake Victoria. In the following section I will compare some abiotic factors and the biodiversity of cichlid fish in lakes Malawi and Tanganyika.

Comparing Lake Tanganyika and Lake Malawi

Both lakes consist of rocky shores, with neighbouring sandy stretches. The lakes are fed by small rivers that enter the lake in swamp like environments. Apart from water plants that occur in these swampy entry zones, water plants are virtually absent in the entire lake. Length and width of the two lakes are comparable, although there is quite some difference in depth. Lake Malawi has a maximum depth of about 700m (Weyl et al., 2010), the deepest point of Lake Tanganyika however, reaches 1400m. This difference is at first sight not that important, considering that both lakes are meromictic (e.g. water layers within the lake don't mix). Only the first 100-200m have oxygen conditions that are suitable for cichlids (and other fish)(Rudd, 1980). Not only the depth, but also the bathymetry of the two lakes is hugely different. Lake Malawi is a "large hole" in the ground, whereas Lake Tanganyika consists of three large sub-basins (Scholz & Rosendahl, 1988; Scholz et al., 2003). Because these basins only exist at great depths, they are not of great importance at high water levels. But when water levels drop, and this has happened quite often in the history of the lake (Cohen et al., 1997; Nicholson, 1999; Scholz et al., 2003), the differences in bathymetry might start to play a role.

Behavioural groups

Not only are the total numbers of species different between the two lakes (250 vs 700 species), behavioural diversity also appears to differ a lot between the lakes.
Morphological analyses first divided the cichlids of Lake Tanganyika into 12 different tribes (Poll, 1986). Further molecular analysis confirmed these 12 tribes, and added another 4 (Koblmuller et al., 2008; Salzburger et al., 2002; Takahashi, 2003). First of all there is a large group of cichlids that feed on algae and small organisms that live between the algae (Aufwuchs): the Tropheini and Eretmodini. Lake Tanganyika also harbours a whole range of sand dwelling species; the Boulengerochromini, Limnochromini and Ectodini. Deep water and pelagic species are found in the tribes of Trematocarini, Bathybatini, Cyphotilapiini, Hemibatini, Cyprichromini and Benthochromini. The largest species group is the Lamprologini (>81 species), substrate breeding cichlids that can also be found to breed within abandoned snailshells. The Lamprologini also harbour several species that perform cooperative breeding (see e.g. Witsenburg et al. 2010). The Tilapiini, a group of cichlids that is endemic to Africa and has undergone adaptive radiation in other lakes, only has one representative in Lake Tanganyika: Oreochromis tanganicae. The Tylochromini have only recently invaded Lake Tanganyika and have probably entered the lake from rivers (Koch et al., 2007). Last but not least is a spectacular species group: the Perissodini. The Perissodini have specialized in preying on scales of other cichlids. The Perissodini are well known for frequency dependent selection on their "beakedness" (being right beaked, or left beaked (see Hori, 1993)). Lake Malawi cichlids can roughly be divided into two functional groups: the Mbuna and the sand dwellers (Danley & Kocher, 2001; Konings, 2003; Kocher, 2004; Sturmbauer, 2008). The Mbuna are a trophic group that grazes on algae, primarily feeding on Aufwuchs, that have undergone morphological changes to optimize uptake of vegetal matter, such as an elongated intestinal tract. The Mbuna are closely related to the Tropheini from lake Tanganyika. The sand dwellers on the other hand live on sandy stretches were they sift the sand for small organisms living in it.
The cichlid species fl ock of Lake Malawi is monophyletic. As a result, all cichlid species in lake Malawi are mouth breeders. Although the cichlids of lake Malawi are not commonly divided into different tribes, there is considerable diversity in trophic adaptations. Presumably the high competition for food has driven the cichlids in lake Malawi to adapt to less common food resources, causing the sometimes extraordinary trophic adaptations we see nowadays. Trophic adaptations can range from mimicry to come close to the prey (scale eaters and some piscivores), modified beaks to suck prey from crevices and a protusible mouth that can generate suction to capture zoo plankton (Ribbink et al., 1983; Konings, 2005; Oliver, 2010).


Web Site (C) Thijs Janzen 2011-2014
Lake Tanganyika pictures courtesy Jen Reynolds