Global biogeographic structuring of tropical shallow-water brittle stars

Aim Biogeographic barriers emerged in the tropical oceans as continental masses moved with plate tectonics, and as the tropics contracted to lower latitudes from the late Eocene. These barriers have shaped tropical marine biodiversity. We characterize large-scale diversity patterns for tropical brittle stars and investigate the effect of biogeographic barriers on these in space and time. Location Shallow-water (<200 m) tropical oceans. Taxon Tropical shallow-water brittle stars (class Ophiuroidea). Methods We integrate phylogenetic and biogeographic modelling to test and quantify the biogeographic structuring across the major ocean basins for five families of brittle stars. These are well-sampled in our phylogenies (173 species) and represent an important component of the brittle star fauna of tropical shallow waters. We define major bioregions based on patterns of compositional and phylogenetic beta diversity. Results We find congruence between patterns of shared ancestry of regions and inferred biogeographic histories. Biogeographic reconstructions show that faunal patterns reflect the emergence of biogeographic barriers in the tropical world, with evidence of vicariant events driven by the opening of the Atlantic Ocean, the narrowing of the Tethyan Seaway and the rise of the Isthmus of Panama. Main conclusions Biogeographic barriers almost completely isolated regional faunas. However, divergence age estimates predate the onset of the different barriers, suggesting that changes associated with the gradual emergence of the barriers had a strong effect on the evolutionary history of tropical shallow-water brittle stars. Limited, very recent, bi-directional dispersal was detected across the East Pacific Barrier, which is otherwise an important barrier for dispersal of brittle stars.