Shells of foraminifers provide indispensable data for paleoceanography, paleoecology, and paleoclimate reconstruction. This study analyzes the preservation of shell, specifically where and why bioerosion (i.e., drill holes) occur in planktonic foraminifera shells. We examined 2588 specimens from eight species and used statistical analyses and numeric models to map the distribution of these drill marks within each species. Our findings reveal that the density and location of drill holes differ between spinose and non-spinose species, with spinose species tending to have more holes. Species with thinner tests (spinose) are preferentially perforated in the earlier whorls, while those with thicker shells (non-spinose) exhibit more holes in the ultimate chambers. Foraminiferal bioeroders likely select drilling sites based on a balance between minimizing the effort required to penetrate the test and maximizing access to nutrient-rich content. In summary, our study revealed distinct bioerosion patterns across foraminiferal species, suggesting that morphological characteristics contribute to the varying vulnerability of sectors and species to bioerosion.

Despite advances in understanding planktonic foraminifera environmental interactions, their role as prey remains elusive, often inferred from indirect evidence such as drill holes. Bioerosional traces offer valuable insights into fossil assemblages, although knowledge for planktonic foraminifera remains limited compared with benthic species. This study addresses this gap by analyzing bioerosional site selectivity in late Quaternary planktonic foraminifera from the western South Atlantic. We examined 2588 specimens from eight species to map trace patterns using kernel density estimation, sector-based, and hotspot mapping approaches. Drilling traces were located, transposed to graphical representations, and transformed into x,y coordinates. We analyzed specimen frequency per trace quantity and trace frequency, sectoring them per chamber and test regions. Correspondence analysis and exact test of goodness of fit assessed groupings among the species and preferential regions. Frequencies revealed that spinose species had more multiple-drilled specimens than non-spinose ones. Bioerosional traces were prevalent in the final whorl, decreasing toward earlier chambers. However, when normalized by surface area, the penultimate whorl had higher trace frequencies, particularly for spinose species, while the ultimate whorl had higher trace density for some non-spinose ones. Spinose species are preferentially drilled in the early chambers, likely due to their thinner walls. Thus, bioeroders prioritize regions with a higher cost–benefit ratio, which is evident in the prevalence of successful–failed traces in early chambers of spinose species, but not in thicker-walled, non-spinose ones. Our study reveals distinct bioerosion patterns, highlighting strategic site selectivity and suggesting that morphological differences between spinose and non-spinose species contribute to varying vulnerability to bioerosion.