There is a gap in knowledge about the biodiversity of living foraminifera (protozoa) in Brazil, particularly in coastal environments. In Sepetiba Bay (SE Brazil), a coastal system highly impacted by anthropogenic activities, several previous studies have been carried out on foraminifera based on total (living + dead) assemblages and sub-fossil records. Thus, this study intends to analyze for the first time the species richness of living foraminifera (stained with Rose Bengal) in Sepetiba Bay. Based on the morphological characteristics, 214 living species were identified in 50 samples collected in Sepetiba Bay in May 2022. The number of living specimens was counted as 6548. Living foraminifera density was < 252 specimens per gram of sediment and the species richness (S) ranged from 15 to 61 (mean 16.9 ± 15.5) in the analyzed stations. The main taxonomic classes found were Globothalamea and Tubothalamea. The main Globothalamea taxa belonged to the genus Ammonia, with Ammonia tepida, Ammonia buzasi, and Ammonia rolshauseni standing out, as well as the Bolivina, Buliminella, and Elphidium, represented mainly by Bolivina striatula, Buliminella elegantissima, and Elphidium excavatum. A non-Metric Multidimensional Scaling (nMDS) and cluster analysis based on a presence-absence matrix and Bray–Curtis similarity index allowed the identification of two main groups of stations: 1. Located in the innermost areas of Sepetiba Bay and surrounding the mainland and Marambaia Barrier Island, at shallower depths, with lower diversity; 2. an outermost group of stations with higher diversity in areas with greater oceanic influence. Compared with previous studies, the results of this work suggest an enlargement of the “innermost” zone with less diversity towards the oceanic region of Sepetiba Bay. This work shows that even using a matrix of presence/absence of living foraminifera and species richness, it is possible to distinguish environments in coastal systems. Data based on the living foraminifera is needed to understand current biodiversity to better characterize coastal environments and carry out biomonitoring studies.
