Natural hybrids of the African cichlid fishes Tilapia spilurus nigra and T. leucosticta: a case of hybrid introgression

Hybridization is documented between two species of African cichlid fishes, Tilapia spilurus nigra and T. leucosticta. First discovered in a rice padi at Tebere in eastern Kenya (ostensibly stocked only with T. s. nigra), similar hybrids were later found in Lake Naivasha, Kenya (stocked inter alia with T. s. nigra and T. leucosticta). Hybrid characters are examined individually and by summation as a hybrid index. Pond and laboratory crosses indicate that hybridization occurs readily and produces at least partially fertile hybrids. Hybrid introgression appears to be occurring in the Tilapia populations of Lake Naivasha. Ecological features of the natural populations of both parental species and of the hybrids are examined with particular reference to habitat preferences and sex ratios.