Speciation in Cichlid Fishes of East African Lakes

Abstract

Prof. C. Kosswig has suggested¹ that factors which may contribute to the high evolutionary response of Cichlid fishes to the environments of the Great African Lakes are selective mating and specialized breeding habits, including monogamy at least until one brood becomes independent. A beginning has been made in the study of a group of closely related species in Lake Nyasa, the Tilapia of the T. squamipinnis group^2,3. Before 1939 these had been considered as a single species. Slight differences in pharyngeal dentition had been observed in the larger fishes among a score of specimens in the British Museum (Natural History); but these, and slight variations in proportions of head and mouth, had not been thought sufficient for the recognition of more than one species. Field-work during the fishery survey of 1939 revealed that there are in Nyasa probably four closely related species, for some of which it is still convenient to use local names, Chambo, Saka, Lidole and T. karongæ (the local name of which I do not know). These seem to be the Nyasa representatives of the T. galilæ group of the genus, but all resemble each other much more closely than any species outside the Lake. Females and non-breeding males of all are silver-grey with nearly vertical black bars. In Chambo the male becomes pale blue as the testes ripen; in the other three, ripe males are black. The four species (if they are species) cannot yet be recognized at all stages, but Lidole and Chambo can be distinguished from each other at a total length of 26 cm. and more, and it is to these two that the following remarks apply.