Identification of the Cichlid Fishes of Lake Malawi/Nyasa Part 2: Pseudotropheina (the ‘mbuna’ sub-radiation).

George Francis Turner

With an estimated 900-1000 species, the cichlid fishes of Lake Malawi represent the largest known adaptive radiation of vertebrates from a single common ancestor in a limited geographical area, in this case a single lake. They provide an outstanding opportunity to study the rapid diversification of form and function on a limited genetic background and to attempt understand why lineages vary so much in their propensity for diversification and how this may be influenced by their environment. However, they present formidable difficulties in terms of the reconstruction of phylogenetic relationships, not only because of their low degree of genetic differentiation, exacerbated by issues of incomplete lineage sorting and introgression among non-sister taxa, but also because of the tremendous difficulties in species identification. This is likely partly a result of taxonomic neglect (at least half of plausible species remain undescribed) but also because of the tendency of species to attain a high degree of reproductive isolation on the basis of minimal morphological differentiation, often through divergence of signal systems including (but probably not limited to) differences in male courtship colours and display structure (bower) form. To this end, a major programme of genome sequencing is in progress, covering the entire radiation. However, a major challenge has been encountered in accurate identification of the specimens sampled. The present work reports on progress in the identification of these specimens, assessing evidence from examination of newly collected specimens and photographs in conjunction with studies of type material and literature. Here, I focus on the subtribe Pseudotropheina (or Mbuna), of which 350+ known species are considered: this will probably prove to be a considerable underestimate. Many taxa within this group are tightly bound to patchily-distributed rocky habitats, leading to substantial within-lake geographic isolation. This is associated with sharp discontinuities in patterns of male breeding dress, which may merit recognition as narrowly-distributed endemic species or geographic colour variants, according to the model of species definition being applied. But, some populations with very similar phenotype appear to have evolved independently and may show assortative mating in laboratory trials, suggesting that they too should be regarded as distinct species. It is intended that this work should be available as a free-standing pre-print which can be modified as further sequences are obtained, identifications are improved and species formally described.