Ontogenetic ecological shifts drive level-dependent patterns of phenotypic disparity in reef fishes.

Ecological shifts are major drivers of phenotypic diversification across the Tree of Life. Yet in many organisms, these transitions occur not only across evolutionary time but within individual life histories, complicating expectations for how ecological shifts shape morphology. In such cases, developmental remodelling may mediate the translation of ecological opportunity into phenotypic variation, raising the question of whether ontogenetic habitat transitions consistently promote morphological disparity, or if patterns instead reflect lineage-specific developmental constraints. This question is especially salient in coral reef fishes, where most lineages possess a biphasic life cycle: larvae disperse through the pelagic ocean before undergoing thyroid hormone-mediated metamorphosis to settle across a diverse range of suitable reef habitats. Whether this ecological shift to a wider range of ecological zones coincides with an expansion in phenotypic space between adult and larval communities remains inconclusive. Here, we quantified body shape disparity at settlement and adult stages across diverse reef fish lineages using various morphometric analyses. We found that while adult communities generally exhibited greater morphological disparity than larval cohorts, this pattern was far from universal. Instead, ontogenetic dynamics of disparity varied widely among clades, with several lineages showing morphological variation that is reduced, increased or equivalent to that of the adult forms. These findings challenge the assumption that ecological complexity uniformly drives phenotypic diversification and highlight how lineage-specific developmental trajectories and historical contingencies constrain or facilitate the expansion of phenotypic space during ontogenetic ecological transitions.