Adaptive Radiation of Cichlid Fishes in Lake Tanganyika, by Kyle Taylor

Evolution is unique in that two similar scenarios can yield vastly different results. Natural selection does not work with future information, it simply selects on individuals that have traits that are currently or have previously selected them over others. An example of this is in the East African Rift Lakes, or more specifically, Lake Tanganyika. The oldest of the lakes (Brown ET. Al, 2010), Lake Tanganyika is famed for being the second largest freshwater lake in the world (N.A. 2016), with the countries of Zambia, Tanzania, DR Congo, and Burundi bordering its boundaries. Here, adaptive radiation has caused for the explosive speciation of the Cichlid Family of fishes, in which the multiple niche levels began to be filled by the family.  The causes of the increase in fish species diversity in Lake Tanganyika is likely due to external factors such as shifting lake levels, but could also be attributed to internal factors such as sexual selection and predation.

            Estimated at being nearly 9-12 million years old (Brown et. Al, 2010), nearly 60% of the animal species that inhabit the lake originated in the body of water (Sweke ET. Al, 2016). With such an extended time of geographic isolation, the lake has experienced multiple water level fluctuations throughout its history. Major lake level fluctuations could thereby explain intralacustrine allopatric speciation; low water levels in the lake would have left the Cichlid fishes geographically divided by different sub-basins (Koblmüller ET. Al, 2008). The fact that many Cichlid species are only found in small, separated sections of the lake also supports this idea (Salzburger and Meyer, 2004).  Many of the rock-dwelling Cichlids of the lake are divided due to habitat separation. Even minor lake level fluctuations can have significant impacts on fish lineages. Littoral or shoreline dwelling species could be significantly affected by minor fall in water level (Salzburger and Meyer, 2004) since their habitat would be the first to be effected by lake fluctuations.

Adaptive radiation of east African cichlid fish. 

            In this case, the extremely large size of the lake would have a greater effect on the speciation of the fish than the age of the lake would.  The emergence of multiple deep-water habitats could then act as a barrier to fish populations, while also separating the spawning sites of the Cichlid fishes, further influencing allopatric speciation (Salzburger and Meyer, 2004) . Other species have also experienced unique speciation in Lake Tanganyika as well, such as the Mastacembelid eels. While Mastacembelid eels can be found throughout Asia and Africa, a separate lineage has formed in the lake (Brown ET. Al, 2010). It is believed that low water levels in the lake nearly 7-8 million years ago began the diversification of the lineage.

            Other factors besides environmental have also played a role on the speciation of the Cichlid family as well. Female Cichlids are known for choosing their mates based on their coloration; “Fisherian runaway sexual selection”, as Meyer calls it, could describe how speciation of the lineages further occurred. Short separation of the fish could cause for gradual coloration shifts in males due to preferred selection by females. These changes, even small, could have dramatic effects on how females would select their mates, further seeding the separation of species flocks. While predation of Cichlid fishes has not be explained for their rapid speciation, it has been known to affect other organisms in the lake. For the freshwater snails of the lake, predation from the freshwater crab Potamonautes lirrangensis has affected both their shell size and shape, and has given rise to their thalassic shells (Weigand ET. Al, 2014).

Unique habitat fluctuations on the lake has led to the lacustrine diversification of fish species in Lake Tanganyika. While other factors may have also played a smaller role in shaping the diversity of aquatic species in Lake Tanganyika, evidence shows that lake levels were a major contributing factor. The lake is remarkable in that it has allowed scientists the opportunity of an isolated study environment in which the effects of allopatric speciation has been greatly induced. In more recent times, influences such as overfishing and climate change (N.A. 2016) have been affecting the lake, putting the unique diversification of species in question. Perhaps we can use these factors to study if overfishing can further induce the effects of allopatric speciation on the Cichlid fishes, but only time will tell how the formation of Lake Species will end. By studying this extraordinary ecosystem now, we can better understand the effects that allopatric speciation will have on future species and ecosystems to come.

Cichlid fishes of Lake Tanganyika.  Animal Press. Source 

Works Cited

Brown, K.J., L. Rüber, R. Bills, and J.J. Day. 2010. Mastacembelid eels support Lake Tanganyika as an evolutionary hotspot of diversification. BMC Evolutionary Biology 10:188

Koblmüller, S., K.M. Sefc, and C. Sturmbauer. 2008. The Lake Tanganyika cichlid species assemblage: recent advances in molecular phylogenetics. Hydrobiologia 615: 5

N.A., 2016. FISH: Lake Tanganyika. Africa research bulletin. Economic, financial and technical series 53: 21402A-21402B

Salzburger, W. and A. Meyer. 2004. The species flocks of East African cichlid fishes: recent advances in molecular phylogenetics and population genetics. Naturwissenschaften 91: 277-290

Sweke, E.A., J.M. Assam, A. I. Chande, A.S. Mbonde, and M. Magnus. 2016. Comparing the performance of protected and unprotected areas in conserving freshwater fish abundance and biodiversity in Lake Tanganyika, Tanzania. International Journal of Ecology 2016

Weigand, A.M., and M. Plath. 2014. Prey preferences in captivity of the freshwater crab Potamonautes lirrangensis from Lake Malawi with special emphasis on molluscivory. Hydrobiologia 739: 145-153

Acoustics in Cichlid Reproductive Behavior, by Kyle Sullivan

Reproduction in fish is dependent on a large number of complex communication factors. The response of the female to male courtship behavior is largely based on the male’s identity, quality, motivation, readiness, and social status. Visual communication, the most studied and best understood mechanism, is just one of the multiple reproductive sensory mechanisms involved in reproduction. . Female choice often relies on visual traits such as size, dominance, mechanoreception, and chemoreception.  In Burton's Mouthbrooder Astatotilapia burtoni  (Günther, 1893), the male does  series of tail waggles that females use as an indicator of their dominance in the  social hierarchy.  Females also choose to reproduce with the male based on the acoustic frequency in combination with a multitude of other factors.

Soundwaves produced by a yellow dominant male during courtship (Maurska et al. 2012)

There are many factors that weigh into a female’s decision to reproduce with a male. Size is a contributor to female reception. A male displaying a certain trait that is larger (such as a larger caudal fin) may be more attractive in some species (Bischoff  et al. 1985). Some fish are considered dominant based on factors like size of territory owned or coloration. Dominant males are also more likely to reproduce (Spence 2006). Male odors can also influence a female's preference (Fisher et al. 2006). Females can also use their lateral line to either avoid males or court them (Medina et al. 2013).  The sexual response of a female Astatotilapia Burtoni is largely dependent on sounds produced by the male. Many fish use sound to deter predators and intruders, identify members of the same species, and to attract mates. Although the Astatotilapia Burtoni uses sounds in its courtship, it is important to note that not all fish, including many in the cichlidae family do not. Different sound frequencies may even influence morphological changes in some sympatric species, leading to reproductive isolation over time  (Longrie et al. 2013).

The likelihood that a female accepts a male as a mate is correlated with the frequency of the noise that he is emitting. These sounds are species specific, varying in “trill duration, number of pulses per trill, pulse period, pulse duration, and interpulse interval” (Maruska et al. 2012).  Females in the species Astatotilapia burtoni use sound to choose their mate based on frequency. This is tested in the paper written by Maruska, Ung and Fernald. The authors explain, “ we characterized the sounds and associated behaviors produced by dominant males during courtship, tested whether there were differences in hearing ability associated with female reproductive state or male social status, and then tested the hypothesis that female mate preference is influenced by male sound production.” (Maruska et al. 2012).

Astatotilpia burtoni has a brightly colored dominant male phenotype and a normal colored subordinate male phenotype. This makes them easy to distinguish and easier to use in experiments. The female’s response to courtship depends on the dominance of the male and the female’s reproductive potential at the time. Females that were mouthbrooding at the time were unlikely to respond to male courtship behavior. During courtship, males produce a low frequency sound when close to females. A female may respond to the male’s courtship differently based on the frequency of the noise produced. In the experiment from article one, the scientists used a tank with three compartments. They placed two visually and physically similar males on the outside compartments of the tank. They then introduced a female to the center compartment of the tank and played either the natural tail wag sound or the noise control from speakers in one of the outer compartments. When the natural sound was played through the speaker, the females spent a greater portion of their time on that side of the tank. When the unnatural control sound was played, the female spent equal amounts of time on either side of the tank. This suggests that females take into account acoustic signals when choosing a mate. It was determined that the tail wags are associated with courtship behavior, because “Dominant male A. burtoni produced pulsed broadband sounds during body quivers associated with courtship behaviors. Our simultaneous sound and video recordings demonstrate that these courtship sounds are produced intentionally because not all quiver behaviors were associated with sound production, suggesting that the sound is not merely a by-product of body movements, but that males have some control over when and where it is produced.” (Maruska et al. 2012).  The data show that the female prefers sounds produced by the male to be in a specific frequency threshold. More dominant males were able to produce sounds at the optimum hearing range and for a longer period of time. 

 The increase in the female sex steroid is correlated with the frequency of the sound produced by the male. According to Maruska, Ung, and Fernald, “Astatotilapia burtoni was most sensitive to low frequencies from ∼200–600 Hz, with a best frequency at 200–300 Hz, which overlaps the spectral content of the courtship sounds produced by dominant males.”  The sex steroids in the female increased when sounds in this range were played. When the steroids in the female are increased, the female is more likely to reproduce. The more dominant males are able to produce this sound while the less dominant males’ sounds are more likely to be ignored  (Maruska et al. 2012).

 

Larger males produce more sounds during courtship (Maruska et al. 2012)

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This research is a good indicator that, in combination with other traits such as size, dominance, mechanoreception and chemoreception, sound is used as a way to attract mates in the African Cichlid species Astatotilapia burtoni. During courtship, males produce courtship sounds that are conducive to increasing female sex steroids. This raises the female’s desire to mate with males who are more dominant, The experiment proved that the courtship sounds produced by Astatotilapia burtoni are both deliberate and vital to reproductive success.

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Bischoff, R. J., J. L. Gould, and D. I. Rubenstein. 1985.  Tail size and female choice in the Guppy (Poecilia reticulata).  Behavioral Ecology and Sociobiology   17(3):253-55.  doi:10.1007/bf00300143.
Fisher, H. S., B. B.m Wong, and G. G. Rosenthal. 2006. Alteration of the chemical environment disrupts communication in a freshwater fish.  Proceedings of the Royal Society B: Biological Sciences 273(1591): 1187-193. doi:10.1098/rspb.2005.3406.
Longrie, N., P. Poncin, M. Denoël, V. Gennotte, J. Delcourt, and E. Parmentier. 2013. "Behaviours Associated with Acoustic Communication in Nile Tilapia (Oreochromis niloticus)." PLoS ONE 8(4)  doi:10.1371/journal.pone.0061467.
Maruska, K. P., U. S. Ung, and R. D. Fernald.  2012.  The African Cichlid Fish Astatotilapia burtoni uses acoustic communication for reproduction: sound production, hearing, and behavioral significance." PLoS ONE 7(5):e37612  doi:10.1371/journal.pone.0037612.
Medina, L.M., C.M. Garcia, A.F. Urbina, J.Manjarrez, and A. Moyaho. 2013. Female vibration discourages male courtship behaviour in the Amarillo Fish (Girardinichthys multiradiatus).  Behavioural Processes 100:163-68. doi:10.1016/j.beproc.2013.09.007.

Spence, R. 2006.   Mating preference of female Zebrafish, Danio rerio, in relation to male dominance.  Behavioral Ecology 17(5): 779-83.  doi:10.1093/beheco/arl016.