Fish Fauna

Species diversity By far, the most studied organisms in African lakes are Cichlid fish species. Because many are of recent origin, morphometric differences might be very subtle such that cladistic techniques are unable to separate them; hence advanced molecular genetic methods are increasingly being used. Studies of chromosome numbers and morphology have also been advanced. Chromosome number and/or chromosome arm numbers show great variation indicating that the karyotype is also species-specific. A range of chromosome numbers has been recorded (n = 15, 13, 11, 10, 9 in Aphyosemion, (n = 17, 18 in Epiplatys) and polyploidy in Barbus species (n = 148-150), indicating the great genetic variation within genera. Generally, a high chromosome number is associated with larger size, longer life, faster growth, better ecological adaptation and robustness.

Although there is still some confusion about ways of calculating Fundamental Number, which refers to long chromosome arms, comparative analysis of banding patterns for each chromosome has also been demonstrated to be species-specific. Among tilapias, whose origin is Africa, the C-heterochromatin distribution differs according to species. The diploid value in the Tilapiini tribe is relatively conservative with chromosome number of around 44; the most conservative being Oreochromis alcalicus (2n = 48) while Oreochromis karongae (2n = 38) in Lake Malawi is regarded to be of more recent origin. These results suggest a need for more research to refine characterization of stock for better formulation of conservation. Similarly, determination of variation is crucial where there is need to enrich farmed stocks as successfully demonstrated in Oreochromis niloticus.

The Siluriformes have relatively high diversity with respect to number and shape of chromosomes, exemplified by Aeuchenoglanis and Bagrus (2n = 54 or 56), Chrysichthys and Clarotes (2n = 70 or 72). At the population level, heterozygosity (H = h/r) of Chrysichthys maurus, Chrysichthys auratus, and Clarotes laticeps is high. The chromosomes number is higher in these catfishes than in Clarias gariepinus from Asia and Africa that exhibits stable karyotype. Within the major groups of cichlids, there appears to be no relationship between karyotypic diversity and phylogeny.

The greatest change of fish species diversity occurred in Lake Victoria, where more than 13 native detrivor-ous/phytoplanktivorous haplochromids became extinct and were naturally replaced by the atyid prawn, Cardina nilotica. Similarly, more than 20 zooplanktivorous haplochromis were replaced by the cyprinid Rastrineo-bola argentea. The Nile Perch Lates niloticus is reported to have predated on more than 109 species of the original stock of haplochromids. The resulting cascading trophic interactions resulted in a bizarre phenomenon where Lates niloticus resorted to eating its own young.

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