Ocean acidification and hypoxia (OAH) increasingly co-occur in coastal upwelling systems, yet biological thresholds that define ecological risk under combined stress exposure remain poorly constrained. The Dungeness crab (Metacarcinus magister), one of the most valuable single-species fisheries on the U.S West Coast, may be especially vulnerable to these interacting stressors in the California Current System (CCS). We exposed juvenile crabs to ecologically realistic OAH conditions and assessed responses across multiple biological levels, including survival, exoskeleton dissolution, and transcript expression. Exoskeleton dissolution and calcification gene responses occurred under moderate OAH conditions (Ωcalc ≈ 1.6; pH 7.7, DO 2.6 mg/L), preceding calcite undersaturation. Transcriptomic responses revealed broad upregulation of glycolytic, calcification, and stress-response genes, consistent with compensatory shifts in energy metabolism and shell maintenance. Mortality was primarily driven by hypoxia, but was strongly exacerbated by acidification, delineating a synergistic mortality threshold near pH 7.35 and DO 1.6 mg/L. Mapping these experimentally derived thresholds onto CCS observations and model outputs indicates that early warning conditions already occur seasonally within juvenile crab habitats. By integrating interacting stressors into an experimentally derived pH-DO boundary, this joint OAH threshold provides an experimentally informed framework for evaluating habitat vulnerability and informing ecosystem-based fisheries management and monitoring.
Bednaršek et al. (Mon,) studied this question.