A diversified kettle of fish: phenotypic variation in the endemic cichlid genus Danakilia of the Danakil Depression of northeastern Africa

The Danakil Depression of northeastern Africa is among the harshest environments on Earth. Yet, despite extreme aridity, this desert region hosts the endemic cichlid genus Danakilia. As currently recognized, the genus includes at least two populations of Danakilia franchettii from groundwater springs feeding Lake Afrera (Ethiopia), one population of Danakilia dinicolai from volcanic Lake Abaeded, and three recently discovered riverine populations (Eritrea) of uncertain taxonomic status. Here, we analyse the variation in shape of the body and lower pharyngeal jaw of all known populations, using a geometric morphometrics approach. We investigate whether body and lower pharyngeal jaw shape, two evolutionarily important traits in cichlids, vary within and between Danakilia populations; whether patterns of variation in these traits are consistent with local adaptation (genetic or plastic) or a neutral model of variation in geographical space; and whether these traits show consistent patterns of covariation that might result from pleiotropy or linkage. We found that shape variation between the Afrera and the Abaeded samples confirms current taxonomy. We observed a sharp separation of the southern Afrera populations from all northern populations for both traits, with a less pronounced separation among the northern populations, and a deeper body in some populations compared with the more elongated shape of other riverine populations. Significant variation between all populations is not easily explained by phenotypic plasticity, suggesting that populations might be on independent, possibly neutral or quasineutral, evolutionary trajectories and constitute separate, highly vulnerable units deserving of conservation efforts.