How many species of cichlid fishes are there in Lake Tanganyika? By Andrew G Challingsworth

Lake Tanganyika? The endemic fishes of Lake Tanganyika are a textbook examples of explosive speciation and adaptive radiation. Their study promises to yield important insight into the processes of speciation. Accurate estimates of species richness of lineages in Lake Tanganyika will be a necessary prerequisite for a comparative analysis of the intrinsic and extrinsic factors influencing rates of diversification.

Lake Tanganyika! On the basis of current estimates of species richness, number of founding lineages and age of radiations, it appears that these fishes represent the most rapid known large-scale radiation of species of any animal group in a circumscribed area.

Lake Tanganyika! Thus, we suggest that the study of cichlid fishes may offer unique opportunities to gain insight into the reason underlying the taxonomic and geographical distributions of species richness and functional diversity. Molecular phylogenetic studies have suggested that much of the radiation of species has occurred within the present basins of Lake Tanganyika

Lake Tanganyika! Molecular studies have also provided circumstantial evidence for within-lake geographical isolation of populations, confined to rocky shores of Lake Tanganyika. This suggests a possible role of intralacustrine allopatric speciation among such habitat specialist taxa.

Lake Tanganyika! Recent theoretical studies have suggested that sympatric speciation is a more plausible process than previously believed, and that it might be driven by disruptive natural or sexual selection or by a combination of both. Divergence under natural selection, or at any rate coexistence in sympatry, may be facilitated by the legendary functional plasticity of the cichlid feeding structures.

Lake Tanganyikan feeding structures! Tanganicodus irsarcae an insectivorous carnivore also awfusch feeder.

Tanganicoudus irsarcae Tanganicoudus irsarcae feeds on invertebrates found with in the algae layer found on the rocky bi cover or commonly known awfusch layer. It also has pharyngeal teeth for extracting cellulose out of the algae. Also with reduced swim bladder for surge type environment. The functional plasticity of the cichlid is a surge zone rock feeding fish.

Conspecifics that have also adapted to the surge type environment. Eretmodus cyanostictus southern variation feeds predominately on the Awfusch bi cover.

What is Awfusch Awfusch is the algae bi cover that encrusts rocks in the photic zone.

Awfusch Provides nutrients in the form of cellulose for many fish species that have pharyngeal teeth for extracting the nutrients out of the algae cell.

Pharyngeal teeth A good representation in identifying fishes is by there tooth structure, the picture below is the pharyngeal teeth of Eretmodus cyanostictus.

Conspecifics Eretmodus cyanostictus Also feeds on the awfusch bi cover. Is not an insectivore feeds solely on the Awfusch bi cover. Mimicry in its display pattern to Tanganicodus irsarcae. Also has a reduced swim bladder for surge type environment. Plasticity evolving to fill niche areas that are under utilized by other species.

Morphological traits Until the 1980s, most cichlid species were defined on the basis of morphological traits likely to be amenable to study in preserved museum specimens.

Feeding structures in Lake Tanganyika. Although what seems to dominate the feeding structure in the cichlids of Lake Tanganyika is space. The utilization of every available area of space has been utilized by the cichlids. This can be tested true due to the fluctuations of the water level of lake Tanganyika particularly the surge biotope as the water level increases the surge biotope expands and hence as the water level decreases the surge biotope contracts.

Feeding structures in Lake Tanganyika. Due to the expansion of water as the lake level rises we can see new habitats created predominately rocky algae based habitats which is then exploited by the species, often forming another new species to utilize the abundance of food resources. This ideology the expansion and contraction theory has predominated speciation in much of Lake Tanganyika’s faunal environment.

Where there seems to be allocation of space we have the representation of a species. Living in the desolated shell beds of the snail genus Neothauma is another cichlid taking advantage of the utilization of space.

Shell dwellers These are a diminutive species of cichlid that have utilized the empty snail shells of the genus Neothauma which accumulate in masses on the lake floor.

Utilization of space In this tropical lake where optimum a biotic and biotic factors such as high water temperature, calcium riched environment abundant food supply create patterns of speciation which can be viewed in respect to other working evolutionary systems representing other lineages of fish from other lakes in the Rift lake system.

Other examples of the utilization of space. Almost every example in species diversification in Lake Tanganyika can be described as the utilization of space. But what makes the utilization of space such a promising agent for speciation. It is because it is accompanied by such successful biotic and a biotic factors that make speciation so successful.

Utilization of Zooplankton! Due to the enriched sunlit waters and the high amounts of solar radiation penetrating Lake Tanganyika there is a massive production of zooplankton which specifically identifies species into certain feeding guilds one such species is Cyprichromis leptosoma.

Once again the utilization of space. It seems that where we have a food resource or even a habitat resource in Lake Tanganyika we have a species utilizing that niche to become evolutionary successful.

Cyprichromis feeding on the zooplankton! An open water swimmer utilizing the abundant zooplankton resources is Cyprichromis leptosoma which actually spaces itself out in shoals and the defined space are considered the species territory although constantly moving thus the utilization of space has been taken one step further into a moving territorial existence.

Recent surveys of Lake Tanganyika! Increasing estimates of cichlid species richness in Lake Tanganyika has risen the number of cichlids from 173 species to more than 200 species with a recent field guide listing approximately 213 species. Although to get a good rough estimate the total is most likely to be 250.

Why such a species richness! Two ancient lake wide lineages have each been subdivided into clades conforming approximately to the outlines of the three ancient basins which were split apart by documented falls in the water level.

Conclusion Species diversification has been the agents of: The utilization of space by the Cichlid species. The biotic and a biotic factors as mentioned that have influenced such a high rate of success in species speciation and diversification.

Bibliography Graduate Certificate of Aquaculture Deakin University. The Aquanet Place Research Center RO Malvern Rd Glen Iris 3146.