Measuring and evaluating morphological asymmetry in fish: distinct lateral dimorphism in the jaws of scale‐eating cichlids

The left–right asymmetry of scale‐eating T anganyikan cichlids is described as a unilateral topographical shift of the quadratomandibular joints. This morphological laterality has a genetic basis and has therefore been used as a model for studying negative frequency‐dependent selection and the resulting oscillation in frequencies of two genotypes, lefty and righty, in a population. This study aims were to confirm this laterality in P erissodus microlepis Boulenger and P. straeleni ( P oll) and evaluate an appropriate method for measuring and testing the asymmetry. Left–right differences in the height of the mandible posterior ends ( HMPE ) and the angle between the neurocranium and vertebrae of P. microlepis and P. straeleni were measured on skeletal specimens. Snout‐bending angle was also measured using a dorsal image of the same individuals following a previous method. To define which distribution model, fluctuating asymmetry ( FA ), directional asymmetry ( DA ), or antisymmetry ( AS ), best fit to the lateral asymmetry of the traits, we provided an R package, IASD . As a result, HMPE and neurocranium–vertebrae angle of both species were best fitted to AS , suggesting that P. microlepis and P. straeleni showed a distinct dimorphism in these traits, although snout‐bending angle of P. microlepis was best fitted to FA . Measurement error was low for HMPE comparing the snout‐bending angle in P. microlepis , indicating that measuring HMPE is a more accurate method. The scale‐eating tribe Perissodini showed distinct antisymmetry in the jaw skeleton and neurocranium–vertebrae angle, and this laterality remains a valid marker for further evolutionary studies.