Résumé
Aim: Knowledge of the main drivers of population differentiation is crucial for understanding evolutionary processes and preserving biodiversity. While primarily studied in terrestrial habitats, the mechanisms operating in the marine realm are less well understood. This study reconstructed the phylogeographic history of a tropical seabird to identify relevant marine barriers promoting intraspecific diversity in the Western Indian Ocean.Location: Western Indian Ocean.Taxon: Three subspecies of tropical shearwater: Puffinus bailloni bailloni , P. b. nicolae, P. b. colstoni.Methods: We used restriction site-associated DNA sequencing and applied population genomics to birds from six breeding colonies to assess intraspecific diversity, population genetic structure and connectivity in the tropical shearwater. Results were complemented with data from six oceanographic variables and effective migration surfaces to evaluate the role of oceanographic factors in driving population differentiation.Results: All analyses consistently separated the birds from the northern colonies (subsp. nicolae and colstoni) from those of the southern islands (subsp. bailloni), but failed to assign the colstoni birds as a different taxon. Results revealed remarkable levels of genetic differentiation within an ocean basin in a highly vagile species and suggested higher levels of gene flow at the northern limit of the species' distribution compared to the southern range.Main Conclusions: Our study suggests that ocean surfaces and sea surface temperature may constitute an important barrier to gene flow for the tropical shearwater and potentially other marine species in the region. This study does not support the colstoni form as a different subspecies, highlighting the need for further taxonomic reassessment. Ultimately, the results allowed us to identify Europa and Aldabra as the most threatened management units and propose conservation strategies directly applicable to these most at-risk colonies.