Fig. 1

An overview of the Lake Masoko Astatotilapia calliptera system. a The location of Lake Masoko within Africa, relative to Lake Malawi. b Bathymetric profile of Lake Masoko (data from Turner et al. 2019 [37]; data collected in 2018). Included are illustrations of males of the two ecomorphs found in Lake Masoko: the yellow littoral ecomorph (above) that dominates the habitat < 5 m; and the blue benthic ecomorph (below) that dominates the habitat > 20 m. c Dissolved O₂ concentration by depth, showing the oxycline at ~ 15 m (Delalande 2008 [38]; data collected 13/03/05) d The peak light intensity by depth for sidewelling (grey) and downwelling light (black) (data collection in summer 2018). e Principal Component Analysis (PCA) on the residuals of Procrustes coordinates on landmark data summarising the gross morphology of the lateral view of the head. Maximum and minimum morphological changes along each axis are illustrated as warped 2D meshes (n = 199). f PCA on the residuals of Procrustes coordinates from GPA on landmark data summarising the gross morphology of the ventral view of the head (n = 199). g Stable isotope analysis for ¹³C and ¹⁵N ratios from Lake Masoko cichlid muscle tissue (n = 113). Isotope data are from Malinsky et al. 2015 [31] and Carruthers et al. 2022 [33]