Phylogeography and diversification of the extremophile fish genera Danakilia (Actinopterygii: Cichlidae) and Aphanius (Actinopterygii: Cyprinodontidae) of the Danakil Depression

Aim of the research is to study the phylogeography and diversification of Danakilia (Cichlidae) and Aphanius (Cyprinodontidae), two poorly known desert actinopterygian genera of the Danakil Depression of north-eastern Africa. The focus of the study is on variation of the two desert fish genera and on the interpretation of the observed variation in a geological and paleoclimatic framework. I assessed the within-genus morphological variability of the fish of the Danakil Depression by analyzing the mitochondrial genetic variation across Danakilia and Aphanius using concatenated COI and cytb mt-DNA markers and inferred the phylogeographic relationships within each genus giving an interpretation of the present distribution in the light of the geological and paleoclimatic knowledge of the Danakil Depression. I also examined variation in body and lower pharyngeal jaw of the Danakilia morphospecies using an advanced geometric morphometric approach in order to understand how body and lower pharyngeal jaw vary within and among Danakilia morphospecies and whether the observed variation in geographical space is founded on a genetic basis or on local plastic adaptation. Moreover, the analyses try to identify consistent patterns of co-variation which might point to pleiotropy or linkage as possible mechanisms accelerating diversification. Danakilia morphospecies, at concatenated COI and cytb mtDNA genes, show low differentiation, but variation is partitioned between a northern cluster containing D. dinicolai plus three undescribed riverine populations, and a southern cluster including two creek populations of D. franchettii separated by the hypersaline waters of Lake Afrera. Aphanius displayed four genetically distinct clades (A. stiassnyae in Lake Afrera; one distributed across the entire area; one in Lake Abaeded; and one in the Shukoray River), but without clear large-scale geographic structure. However, Danakil Aphanius are clearly differentiated from A. dispar sensu stricto from the Sinai Peninsula. Geological evidence suggests that after the Late Pleistocene closure of the Danakil-Red Sea connection, increased post-glacial groundwater availability caused the formation of a brackish paleo-lake flooding the entire region below the −50 m contour. Fish populations previously isolated in coastal oases during glaciation were able to mix in the paleo-lake. Subsequently, in a more arid phase starting ~7300 BP, paleo-lake regression isolated fishes in separate drainages, triggering their still ongoing diversification. The geometric morphometric approach revealed distinctiveness of D. franchettii (Afr) compared to D. dinicolai (Aba) confirming current taxonomy; a sharp separation of Afr from northern populations (Aba and three riverine populations) for both traits; a less pronounced separation among the northern populations; a deeper body shape in Afr and Zar compared with more elongated populations; slender LPJ in Afr than in the northern populations. This significant variation cannot be explained by phenotypic plasticity. The genetic analyses provided evidence of a young radiation of Danakilia in the Danakil Depression, possibly initiated at about 7300 BP. Aphanius shows a higher diversification at mt-DNA markers, with the putative presence of four species not recognizable as A. dispar sensu stricto of the Red Sea northern coast. Projects aimed at understanding aspects of the taxonomy, biology and conservation of the studied fishes are under way. In particular, a dd-RAD sequencing genetic approach will allow a better understanding of the systematic placement of Danakilia. Both genera are under study also in a classical taxonomic setting. The adaptive significance of the nuchal hump of Danakilia will be investigated across the entire family in a cause-effect statistical model. Anthropogenic threats on the Danakil Depression push to consider the desert fishes of this desert region and their fragile environment worth conservation actions.