Diversity and endemism of hard-shelled benthic foraminifera in permanently oxygen-depleted bottom waters: An analysis from the eastern Pacific

Benthic foraminifera are single-celled organisms inhabiting all marine environments. Despite their high tolerance to oxygen depletion, the prevailing hypothesis anticipates a reduction in their diversity in permanently oxygen-depleted environments, including oxygen minimum zones. Here we re-evaluate diversity and study the endemism of benthic foraminifera in the eastern Pacific, an oceanic area hosting the largest permanently oxygen-depleted waters of the world. We focus our analysis on the oxygen-depleted bottom waters and study how they compare with well-oxygenated waters. By utilizing extensive datasets of quantitative information on benthic foraminifera assemblages obtained from morphological traits, we present evidence that challenge traditional viewpoints. Contrary to prior inferences primarily derived from regional studies, our findings reveal that the median diversity (species richness and the Shannon index) calculated on both, living and dead assemblages does not decrease in the most oxygen-depleted bottom-waters. The analysis of unique (endemic) and shared species shows a divide between the neritic-bathyal oxygen-depleted bottom waters with low number of endemic species, and the well-oxygenated abyss hosting high number of unique species. These patterns could be explained by the long-term species exchange in the upper ocean and the isolation of the lower ocean.