Stabilizing Selection Is a Common Strategy for Gastropods Against Past and Future Ocean Acidification

Understanding how organisms adapt to severe environmental crises is essential for predicting their responses to ongoing and future climate change. Previous studies have largely focused on directional selection, whether the role of stabilizing selection—a key mechanism maintaining phenotypic stability—during mass extinction events remains poorly understood. Here we present an integrated study of fossil records with laboratory simulation experiments to investigate the morphological and ecological strategies by which gastropods adapt to ocean acidification. Fossil records show that during three major ocean acidification episodes—the Permian-Triassic transition, the Early Toarcian event, and the Paleocene-Eocene transition—surviving gastropod genera consistently exhibit convergence of shell height-to-width ratios (shell aspect ratio) toward intermediate values, in line with the expectations of stabilizing selection. Controlled condition experiments on the extant gastropod Babylonia areolate further demonstrate that individuals reared in acidified seawater (pH ≤ 7.5) display the same trend in shell aspect ratio. The stabilizing selection on shell morphology is consistent with a lower energetic expenditure on calcification and a greater ability to sustain physiological homeostasis. Together, these cross-scale results suggest that phenotypic-plasticity – driven stabilizing selection driven represents a key ecological strategy by which gastropods cope with past and future ocean acidification. This finding provides a new empirical basis and testable hypothesis for understanding morphological stability during deep-time extinction events and for predicting potential response trajectories under future ocean acidification.