Effects of intragenerational pCO2 conditioning on metabolism, oxidative stress response, and DNA methylation of juvenile Pacific geoduck Panopea generosa

Moderate or intermittent oxidative stress is known to induce positive carry over effects and is a theorized driver of stress memory and enhanced lifespan. Stress conditioning may increase resilient phenotypes and epigenotypes advantageous for sustainable aquaculture. However, the importance of pCO₂ stress intensity remains understudied for establishment of intragenerational adaptive mechanisms to cope with subsequent exposure. To test whether pCO₂ stress elicits beneficial responses under subsequent encounters, we conditioned early life-stage Pacific geoduck Panopea generosa for 120 days in ambient and elevated pCO₂ conditions (920 µatm and 2870 µatm, respectively) before subjecting juvenile clams (~5 months old) to reciprocal exposure periods over 21 days (n = 7 d exposure^-1). We fixed whole tissue samples and measured standard metabolic rate and shell length periodically under three conditions: ambient (740 ± 40 µatm), moderate (2715 ± 70 µatm), and severe pCO₂ (4876 ± 101 µatm); this was followed by a 7-day period of ambient recovery (861 ± 45 µatm) before subsequent reciprocal 7-day exposure to two conditions: ambient (939 ± 31 µatm) and moderate pCO₂ (2983 ± 79 µatm). Clams reared in elevated pCO₂ had greater metabolic rates in response to moderate pCO₂ than clams reared in ambient conditions, suggesting an effect of preconditioning on energy homeostasis, cellular stress response, and redox regulation. Additionally, clams reared under elevated pCO₂ and subsequently exposed to acute severe pCO₂ exhibited both elevated metabolic rates upon re-exposure to moderate pCO₂ and greater shell size indicating an influence of stress duration and intensity on performance. Antioxidant assays and sequencing will shed light on the cellular and molecular mechanisms of adaptive tolerance and stress memory. Intensity-specific stress conditioning can reveal thresholds and life-stages that enhance the longevity of resilient phenotypes and epigenotypes for commercial production of this long-lived clam.