Dataset for: <b>Density and social status affect body shape trade-offs</b>

We investigated how density-dependent intraspecific competition and social rank influence growth dynamics and associated morphological trade-offs in juvenile lemon damselfish, Pomacentrus moluccensis. Fish were reared for four weeks at densities of one, two, or five individuals, with dominance hierarchies identified from behavioural observations. Density and social status interacted to shape growth through a partitioned process: density regulated the absolute magnitude of linear growth in traits such as body depth and fin-free area, whereas social status determined relative body shape by influencing the proportional relationship between body depth and final length. Dominant individuals, particularly those in pairs, exhibited the fastest growth and gained the combined advantages of increased length and proportional depth, reinforcing competitive superiority. In contrast, subordinates experienced growth suppression and developed body shapes associated with enhanced maneuverability, consistent with a shift towards defensive morphologies under constant social pressure. Relationships between escape performance and body attributes were weak, suggesting that morphological divergence primarily reflects differences in growth allocation strategies rather than pressure to optimise fast-start performance. These results demonstrate how competitive intensity, mediated by density, constrains growth and generates crucial trade-offs in phenotypic plasticity during early life stages