Evolutionary mechanisms and diversity in a western Indian Ocean center of diversity

The biogeography and evolutionary history of the shallow marine fauna in the western Indian Ocean (WIO) is poorly known, as are the consequences of ancient and recent biodiversity-generating processes. Inwater surveys of scleractinian corals show maximum diversity in a core region centered on the northern Mozambique Channel. Maximum species richness is at ≈300 with a threshold of ≈250 that differentiates this core region from lower-diversity zones in northern Tanzania and Kenya, the Seychelles and small islands in the Mozambique channel, and the Mascarene islands. Species distributions show significant clustering of sites within this high diversity core region, distinguished from peripheral regions. A heritage of ancient Tethyan lineages is suggested by the distinctive WIO endemic fauna of monospecific genera, basal Acropora clades, and cryptic phylogenetic patterns in Stylophora and Siderastrea. Their presence in the WIO is likely associated with the closure of the Tethys Sea in the Miocene and little radiation subsequently. The dominance (90%) of IndoWest Pacific species in the extant WIO coral fauna reflects invasion during the Mio-Pliocene of the younger and more diversified fauna from the West Pacific during the same period. Various lines of evidence suggest the northern Mozambique Channel may have been a refuge for shallow tropical species throughout its history, particularly during the Cenozoic. This deep evolutionary history suggests a unique biodiversity across multiple taxonomic levels in this second peak of tropical marine biodiversity.