The difficulties in obtaining species-level abundance estimates of marine larvae have hindered comparisons of diversity across life stages, severely limiting our knowledge of how adult diversity is maintained. To explore factors shaping diversity across life stages, we surveyed adult coral reef fishes, compiled data on their ecological and life history traits and paired these with a unique dataset of species-level larval abundances. Relative larval abundance was more even compared to adults and matched random expectations, whereas the adult community was markedly uneven and less functionally diverse, suggesting species filtering effects. While adult abundance was positively linked to larval abundance, species size and diet altered this association, with larger and non-planktivorous adults being less abundant than expected from their larval supply. Our results illustrate that while larval supply is important in determining adult taxonomic and functional diversity, post-larval processes increase the numerical dominance of particular species, thus reducing overall diversity.