Depth is a strong driver of functional diversity in Caribbean reef-fish communities.

Functional diversity patterns have been thoroughly described for reef fishes across geographic gradients, but the effect of depth on these patterns remains largely unknown. Using diet, life-history, and behavioral traits, we evaluated the functional diversity of Caribbean reef-fishes from the surface to 450 m depth to test if taxonomically distinct communities along the reef slope are characterized by similar or distinct ecological traits. Core ecological traits found in shallow reef-fish communities were conserved in the upper section of deep reefs (i.e., mesophotic zone) across three taxonomically distinct assemblages. By contrast, the deepest reef communities (i.e., rariphotic zone) displayed a distinct functional structure, mainly driven by trait loss. Functional redundancy was highest in the deepest reef communities and was generated by intrageneric diversification and functional convergence across phylogenetic lineages. The decrease in functional richness and divergence combined with an increase in functional redundancy suggest that environmental filtering is the dominant evolutionary constraint shaping the functional diversity of deep-reef fishes.