Cichlid Speciation Explosion in Lake Tanganyika

Located in the heart of Africa lies the Great Rift Valley. Situated on a divergent plate boundary, this 7000 km series of trenches has fashioned a string of great lakes. The greatest of them all, Lake Tanganyika. As the second oldest lake and second largest lake by volume, Tanganyika has been an evolutionary powerhouse for the past 10 million years (Scheffel, R.L., 1980). This freshwater system has evolved to act with great resemblance to an ocean, housing a colourful assemblage of diverse species such as jellyfish, snails, sponges, crabs, turtles, and over 300 species of fish (Kelly West, 2001).

Of all the species to colonize this massive body of water, the most dominant force was a small and unassuming family of fish called Cichlidae. Over 250 species of Cichlid fish can be found in Lake Tanganyika and nearly all of them (98%) are endemic, meaning they cannot be found anywhere else in the world (Takahashi and Hori, 2012). Cichlids represent the most diverse extent of adaptive radiation observed globally for all vertebrates, making them an extraordinary case study for speciation and evolution (Meyer, Matschiner and Salzburger, 2015). This rapid diversification of species is clear by observing the community of stunning fish in Figure 1 below.

Figure 1: A diverse community of unique and colourful species of Cichlid (Fabian, 2021).

The vast adaptive radiation of these remarkable fish can be distilled down into three main stages of speciation mechanisms (see Figure 2). The first influential stage is habitat divergence, represented by the distinct rock- and sand-dwelling Cichlids (Kocher, 2004). This form of allopatric speciation, when a species separates into two separate groups, will influence the second stage which is characterized by the diversification of morphological feeding apparatuses (Danley and Kocher, 2001a). Selective pressures presented challenges for generalists which resulted in extensive mouth morphologies specialized for certain trophic adaptations. Distinguished by natural selection from trophic competition, Cichlid species diverged, becoming herbivores, detritivores, planktivores, insectivores, scale-eaters, and much more. Finally, the third major stage of speciation is characterized by colour patterns, which implies individualized sexual selection (Danley and Kocher, 2001a). It has been found that the male of most Cichlid species contribute absolutely no genes to their offspring. This has led to sexual dimorphism in many cichlid species due to the runaway evolution of male traits dictated by female mating preferences, for example, brighter coloured males (Kocher, 2004).

Figure 2: Proposed phylogenetic history of the mbuna genera of Cichlid. The common ancestor diverged during primary radiation into the sand-dwelling and rock-dwelling lineages. During secondary radiation, the rock-dwelling lineage developed diverse mouth morphologies, indicating the importance of trophic competition. The incredible richness of species arose primarily during tertiary radiation where the genus diverged in response to sexual selection, resulting in sex characteristics such as colour patterns (Danley and Kocher, 2001b). 

Cichlids are known to be incredibly social and have been observed to form complex social hierarchies. Both male and female fish strive to climb the dominance hierarchy. When given the opportunity for social status ascent, fish will engage in more aggressive behaviours such as chasing or mouth fighting another fish. If successful, social status ascension will better their chances of reproduction, particularly in males (Alonso et al., 2012). Cichlids also have a wide range of social parental care practices, including mouthbrooding by housing offspring in their mouth for extended periods of time. Once these offspring develop further, another common tactic is collaborative parental care of the free-swimming juvenile offspring for several weeks (Kocher, 2004). 

All of these factors act in confluence and are vital in creating niche differentiation to allow for more unique species to coexist within the same population. By better understanding Cichlid speciation, we gain a magnified lens into evolutionary biology and the selective mechanisms at play during adaptive evolution.

References

Alonso, F., Honji, R.M., Guimarães Moreira, R. and Pandolfi, M., 2012. Dominance hierarchies and social status ascent opportunity: anticipatory behavioral and physiological adjustments in a Neotropical cichlid fish. Physiology & Behavior, 106(5), pp.612–618. https://doi.org/10.1016/j.physbeh.2012.04.003.

Danley, P.D. and Kocher, T.D., 2001a. Speciation in rapidly diverging systems: lessons from Lake Malawi. Molecular Ecology, 10(5), pp.1075–1086. https://doi.org/10.1046/j.1365-294x.2001.01283.x.

Danley, P.D. and Kocher, T.D., 2001b. Speciation in rapidly diverging systems: lessons from Lake Malawi. Available at: <10.1046/j.1365-294x.2001.01283.x> [Accessed 14 February 2022].

Fabian, 2021. 10 Most Colorful African Cichlids. [Photograph] Available at: <https://www.aquariumnexus.com/colorful-african-cichlids/>.

Kelly West, 2001. Lake Tanganyika: Results and Experiences of the UNDP/GEF Conservation Initiative (RAF/92/G32) in Burundi, D.R. Congo,Tanzania, and Zambia. Available at: <https://iwlearn.net/resolveuid/042764c4d828970e559f86feeb1ce710> [Accessed 14 February 2022].

Kocher, T.D., 2004. Adaptive evolution and explosive speciation: the cichlid fish model. Nature Reviews Genetics, 5(4), pp.288–298. https://doi.org/10.1038/nrg1316.

Meyer, B.S., Matschiner, M. and Salzburger, W., 2015. A tribal level phylogeny of Lake Tanganyika cichlid fishes based on a genomic multi-marker approach. Molecular Phylogenetics and Evolution, 83, pp.56–71. https://doi.org/10.1016/j.ympev.2014.10.009.

Scheffel, R.L., 1980. Reader’s Digest Natural Wonders of the World. Readers Digest.

Takahashi, T. and Hori, M., 2012. Genetic and Morphological Evidence Implies Existence of Two Sympatric Species in Cyathopharynx furcifer (Teleostei: Cichlidae) from Lake Tanganyika. International Journal of Evolutionary Biology, 2012, p.e980879. https://doi.org/10.1155/2012/980879.