Biogeography of Holocene coral species in the western Indian Ocean

Understanding the relationship between coral biogeography and reef development is a key topic in sedimentology and paleoecology. Although the knowledge of coral reef growth and internal facies patterns during the Holocene has increased since the 1970s, the biogeography of Holocene coral species in the Indo-Pacific regions remains poorly constrained. In this study, we examined quantitative records of coral species based on data from five cores retrieved from two Holocene reefs in the western Indian Ocean: La Pointe-au-Sable reef on Mauritius Island and Toliara reef on Madagascar. Our results indicate that the dominant reef-building corals on both reefs were Isopora palifera, Acropora robusta/abrotanoides complex. Some corals (e.g., the A. robusta/abrotanoides complex and Acropora digitifera) from 6 ka were found in Mauritius, whereas these were not identified in the cores from Madagascar before 1.5-1 ka. This delay may have been controlled by ocean currents and/or the competency period of the coral larvae. A comparison of these coral assemblages with those in other regions (Seychelles, Mayotte, Ryukyu Islands, Palau Islands, Papua New Guinea, the Great Barrier Reef, New Caledonia, and Vanuatu) showed that I. palifera was common in all of the studied regions, whereas Goniastrea retiformis probably migrated recently to Mauritius and Madagascar in response to prevailing surface ocean currents, including the South Equatorial Current and the Agulhas Current. One explanation for the late arrival of G. retiformis to the study sites could be that the competency period of G. retiformis larvae is shorter than that of I. palifera. Moreover, our quantitative analysis suggests that a given population size during the Holocene may have contributed to the abundance of species in modern reefs. This study emphasizes the need for further quantitative research on Holocene coral species in other regions to better understand the temporal and spatial patterns of coral biogeography.