Population divergence in growth allometry of annual killifishes across ephemeral habitats in Malawi floodplain systems

Body size is a fundamental determinant of ecological performance, and variation in length- weight relationships can provide insight into how populations respond to local environmental conditions. Annual killifishes inhabit highly seasonal wetlands characterized by substantial environmental heterogeneity, yet population-level variation in growth allometry remains poorly understood. We investigated growth allometry in 18 populations of two annual killifish species, Nothobranchius kirki and N. wattersi, distributed across ephemeral habitats in Malawi. Length- weight relationships were analyzed separately for each species using linear mixed-effects models, with locality incorporated as a random effect. Environmental variation among localities was summarized using principal component analysis of water quality variables, and population- specific allometric coefficients were subsequently related to environmental gradients. Significant population-level variation in growth trajectories was detected in both species, indicating divergence in allometric scaling among localities. Divergence was more pronounced in N. wattersi, which exhibited a broader range of population-specific allometric coefficients than N. kirki. Populations also differed significantly in relative body condition after accounting for body length and sex. Environmental gradients explained a significant proportion of variation in allometric slopes in N. kirki, whereas no significant relationship was detected in N. wattersi. Thus, the species exhibiting weaker allometric divergence showed stronger environmental associations, while the species exhibiting greater divergence showed little correspondence with measured environmental variables. These results demonstrate substantial spatial heterogeneity in growth allometry among populations of annual killifishes inhabiting seasonal wetlands. Furthermore, the contrasting environmental associations observed between species suggest that population divergence in growth trajectories may arise through different ecological and evolutionary processes, even among closely related taxa occupying similar habitats.