Composition and diversity of reefal ostracod assemblages from the Zanzibar Archipelago (Tanzania): Proxies for paleoenvironmental reconstructions

The tremendous biodiversity of tropical reef ecosystems fascinates ecologists and evolutionary biologists alike, yet it is threatened by increasing natural and anthropogenic disturbances worldwide. Meiobenthic biotas on coral reefs, especially ostracods, are poorly understood in terms of their diversity and compositional patterns. This study represents the first large-scale assessment of shallow-marine ostracods from three islands of the Zanzibar Archipelago (Zanzibar, Pemba, Mafia, Tanzania), where the reef ecosystem is highly diverse and productive. We characterized four ecologically distinct ostracod faunas each associated with different benthic habitats, that were deep fore reefs, shallow fringing reefs, degraded fringing reefs and intertidal algae. We identified typical ostracod taxa that showed affinities to hard corals or algae on the reef platforms, respectively. We concluded that water depth and habitat type are likely the most important environmental factors determining the diversity and faunal structure of reefal ostracod assemblages in the tropical, shallow-marine setting. Highest diversity was found on shallow fringing reefs where reefal and algal taxa exhibited maximum overlap of their distributional ranges, while the sand flats, mangrove and marginal reefs within the intertidal zone had much lower diversity with high dominance of euryhaline taxa. Furthermore, the detriment of human activities to overall reef health is evident as shown by the diversity loss and compositional changes of ostracod faunas near Zanzibar Town, where the coastal development has been the most intense during past decades. This study therefore highlights the ecological significance of reefal ostracods and indicates the usefulness of ostracods as a model proxy for environmental assessment.