Functional diversity and ecological patterns of reef fish assemblages across depth gradients in oceanic islands of the Brazilian Province

Abstract

Environmental gradients and geography are key drivers of biodiversity and community assembly in reef ecosystems. To investigate how biogeography and depth gradients shape reef fish assemblages, we analysed two Southwestern Atlantic oceanic archipelagos: Fernando de Noronha, larger and closer to the continent, and Saint Peter and Saint Paul, smaller and more isolated. Using mixed-gas rebreather dives to depths of 120 m, we conducted visual censuses of 6,982 individuals across 95 species and categorized assemblages by depth strata: Shallow, upper, and lower mesophotic zones. We expected fish assemblages to be structured by predictions of Island Biogeography Theory and environmental gradients: Larger, nearer island, and shallower reefs, sustaining higher species and functional richness while smaller and isolated island, and deeper reefs harbouring higher functional originality and specialization. Our results revealed assemblages geographically and depth-structured, with the lower mesophotic zone more isolated. While Fernando de Noronha exhibited higher overall species richness, Saint Peter and Saint Paul unexpectedly showed greater functional richness, particularly in shallow and lower mesophotic zones, alongside higher functional originality and specialization. These findings highlight the combined evolutionary and ecological processes shaping insular reef fish assemblages and underscore the need to integrate functional diversity into conservation of deep and isolated reefs.