Resilient adults but vulnerable larvae: demographic pathways of chiton decline under ocean acidification

Ocean acidification (OA) is a major threat to marine calcifiers; however, the sensitivity across taxa and life stages remains elusive. In this study, we combined field surveys of natural CO₂ seeps with laboratory exposure, transplantation, and larval settlement experiments to assess the effect of OA on chitons, a group of calcifying grazers and bulldozers that play critical roles in the structure of rocky intertidal ecosystems. Field surveys revealed approximately 98.6% reduction in chiton (Acanthopleura loochooana, Liolophura japonica, and Acanthochitona rubrolineata) abundance at acidified habitats (pH 7.6), despite greater microalgal food availability and no detectable increase in predator abundance. Laboratory CO₂-exposure experiments showed no direct effect of OA on adult A. loochooana survival, which is consistent with the presence of protective structural features in the valves that confer resistance to dissolution. Transplant experiments revealed no evidence of increased adult A. loochooana mortality in the acidified habitats (pH 7.6). In contrast, larvae showed pronounced sensitivity to OA, with acidified seawater (pH 7.6) reducing larval settlement by approximately 81.5% compared to control conditions (pH 8.1); early life stages were the most vulnerable. These findings suggest that OA-associated decline in chiton abundance is mainly mediated by impaired recruitment rather than by direct adult mortality, predation, or food limitation. Given the role of chitons as grazers and bulldozers, their loss could substantially change intertidal community dynamics by decreasing grazing pressure and disturbing algal and microbial assemblages. Our findings underscore the criticality of considering life-stage vulnerability and ecological function when evaluating the ecosystem-level consequences of OA.