Are we there yet? Inter- and intraspecific approaches to evaluating phenotypic optima in a range expanding East African fish, Enteromius apleurogramma (Cyprinidae).

This article investigates how the range expansion of the freshwater cyprinid fish Enteromius apleurogramma in Uganda, East Africa, driven by climate warming, is influenced by and influences adaptation to dissolved oxygen (DO) gradients, particularly hypoxia. By comparing gill traits and geometric morphometric proxies of gill size in E. apleurogramma from both historical and newly colonized ranges, and referencing both conspecific parapatric populations and sympatric heterospecific populations of Enteromius neumayeri, the study finds that range-expanding populations exhibit intermediate gill morphologies consistent with partial adaptation to hypoxic conditions. The research demonstrates that opercular area can serve as a reliable proxy for gill size and that both interspecific and intraspecific comparisons provide useful reference points for assessing adaptation targets. These findings highlight rapid phenotypic shifts likely driven by a combination of phenotypic plasticity and early-stage genetic adaptation during range expansion into novel low-oxygen habitats.