Biogeography and evolution of reef fishes on tropical Mid-Atlantic Ridge islands.

With their simplified and isolated communities, oceanic islands are considered 'natural laboratories' for biogeographic and evolutionary studies. Along the Mid-Atlantic Ridge (MAR), St Paul's Rocks, Ascension and St Helena islands are nearly equidistant from South America and Africa. While St Paul's Rocks is typically associated with the Brazilian Province, Ascension and St Helena are commonly treated as a unified biogeographical entity despite being separated by approximately 1300 km of open ocean. Using an expert-validated, rigorously curated dataset on Atlantic reef fishes, we demonstrate that these three islands constitute a cohesive biogeographic unit, sharing 46 species found nowhere else in the Atlantic. By integrating time-calibrated phylogenies and distributional data, we reconstructed dispersal routes of 88 MAR fishes, revealing predominant western Atlantic origins and a pronounced eastern influence among endemics. Divergence estimates for two Ascension endemics may predate the islands' emergence, highlighting the potential roles of seamounts and eustatic sea-level changes in shaping biotic persistence and diversification. Trait analysis revealed that MAR fishes are typically larger, occupy wider depth ranges and are more often pelagic spawners and rafters than MAR endemics and their coastal counterparts. Our findings underscore the complex biogeographic and evolutionary processes structuring the reef fish communities of the tropical MAR islands.