Larval developmental histories shape phenotypes and stage‐specific fitness of a reef fish

Abstract Developmental histories shape individual fitness and demographic heterogeneity in many systems; yet, the consequences of this variation across complex life histories remain relatively underexplored. Here, we (1) characterized variability in larval developmental histories of a reef fish and (2) evaluated patterns and sources of variation in post‐settlement selection that reshaped trait distributions in older life stages. More specifically, we sampled cohorts of the common triplefin ( Forsterygion lapillum ) at successive life stages (larva, juvenile, and adult) across six populations, evaluated microchemical profiles within their otoliths, and identified three distinct “larval developmental histories” indicative of time spent in different nearshore coastal environments. Developmental histories showed contrasting survival advantages across different life stage transitions. Specifically, selection across the settlement transition favored a developmental history that was indicative of nearshore larval development, suggesting that larval retention (or early entrainment) in a coastal boundary layer enhances juvenile recruitment. In contrast, selection across the maturation transition favored a developmental history that was indicative of offshore larval development, suggesting trade‐offs in fitness across life stages. Competitor density and habitat structure were significantly associated with per capita mortality rates, further shaping survival outcomes for favored developmental histories. Per capita mortality rates of the favored developmental history increased with conspecific density across both the settlement transition and maturation transition. This study highlights the importance of stage‐specific selection and demographic heterogeneity (e.g., local patterns of phenotypic composition) on the dynamics of ecological systems.