Heterogeneity of tropical diversity and ecosystems: reefal meiofaunas in equatorial western and eastern African islands

Abstract. From an ecological perspective, oceanic islands are unique marine environments that foster endemic species and also facilitate dispersal as steppingstones, yet they are often understudied and considered missing pieces in large-scale biological patterns. In this study, we focused on ostracods and foraminifera as two representative meiobenthic groups from the São Tomé-Príncipe (STP) Archipelago in tropical east Atlantic and the Zanzibar Archipelago in west Indian Ocean. We scrutinized the diversity distribution and faunal structure of these two island regions in similar climatic and oceanographic settings in different biogeographic provinces. We found that the STP is of much lower diversity compared with species-rich Zanzibar, which is likely explained by a combination of regional, historical, and habitat factors. Within each island region, the diversity and composition of benthic assemblages vary along a habitat topographic gradient, with a primary distinction between reefal and non-reefal habitats. Furthermore, across two regions with almost completely different faunas, the ecological composition of ostracod assemblages seems to follow strong and consistent controls of benthic community in terms of the relative cover of coral, algae, and bare sand bottoms. The STP ostracod fauna shows high level of endemism within and beyond tropical east Atlantic, indicating the mid-Atlantic Barrier and Benguela Current as effective biogeographic filters. Thus, our trans-regional investigation of the exotic oceanic islands contributes to important knowledge about the general patterns and determinants of such isolated, peripheral marine ecosystems.