In the Athi river, Kenya, an upper river population of cichlid fishes has been described as Tilapia nigra, closely resembling T. mossambica in the lower Athi, but distinguished by a higher anal spine count (four to six as against three, rarely four, spines). Field studies indicated a steady increase in anal spine numbers with altitude, and a slight overlap is found in morphometric and meristic characters between the two species. T. nigra is however geographically isolated from T. mossambica in the Athi river by the Lugards Falls, and the consequent reproductive isolation, coupled with meristic and colour differences, is here considered indicative, if not of complete, at least of incipient speciation.
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