| 02 Mar 2026
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.
This manuscript presents a high-quality and methodologically rigorous comparative analysis of benthic meiofaunal diversity, specifically focusing on Ostracoda and Foraminifera across the São Tomé-Príncipe (STP) (eastern Atlantic) and Zanzibar archipelagos (western Indian Ocean). Using a combination of standardized sampling and advanced statistical modeling (Hill numbers and GAMMs), the authors investigate how regional biogeography and local environmental drivers (e.g., coral cover, algae, and substrate type) influence community structure. The study highlights a stark "diversity disparity," with Zanzibar exhibiting significantly higher richness and more complex environmental partitioning compared to the relatively depauperate and homogeneous fauna of STP.
As the first comprehensive island-scale survey of ostracods for STP, the study fills a critical void in our understanding of tropical East Atlantic biodiversity and provides a holistic view of the "meiofaunal bottleneck" by comparing metazoan and protist responses to dispersal filters. The use of Hill numbers (effective number of species) is excellent for comparing datasets with varying sample sizes. The integration of Generalized Additive Mixed-effect Models (GAMM) effectively disentangles the relative contributions of environmental variables.
Zanzibar’s alpha and gamma diversity are more than twice that of STP, reflecting the broader "species-rich" nature of the Indo-Pacific vs. the isolated Atlantic. In Zanzibar, habitat heterogeneity (reefal vs. non-reefal) is the primary driver of community assembly. In STP, the fauna is largely homogeneous across different benthic covers, suggesting that isolation and regional filters override local environmental selection.
The high endemism in STP ostracods suggests that the archipelago acts more as a biogeographic "cul-de-sac" rather than a stepping stone, due to the efficiency of the Mid-Atlantic Barrier. Could you elaborate on this?
Suggestions for improvements
While the diversity indices are robust, the manuscript could benefit from a more detailed discussion on the specific functional traits of the endemic species found in STP.
The reliance on visual benthic cover (algae/coral/sand) is useful, but the absence of physical-chemical data (e.g., precise salinity) limits the ability to explain some of the "unexplained variance" in foraminiferal distributions. Are those data available?
The results show that foraminiferal evenness in Zanzibar follows a "more obscure pattern" than ostracods. The authors should hypothesize whether this is due to different dispersal capabilities or a higher sensitivity to micro-scale environmental fluctuations not captured in the current model.
While the GAMM plots are informative, a simplified schematic showing the "Environmental Filtering Model" vs. the "Biogeographic Isolation Model" would help summarize the key findings for a broader audience.
Are the species names in the appendices cross-referenced with updated WoRMS (World Register of Marine Species) databases, particularly for the endemic STP taxa.
This manuscript is a high-quality contribution to tropical marine biology and biogeography. The data is sound, and the conclusions are supported by robust statistical evidence. I recommend minor revisions to address the depth of the ecological discussion regarding foraminiferal patterns and further contextualization of the Atlantic results.