| Summary: | Expansion and enhancement of sustainable shellfish production is necessary to prevent overexploitation of wild stock and satisfy international trade, but hatchery rearing poses a critical production bottleneck due partially to environmental stressors such as ocean acidification. Given that stress conditions exacerbated by anthropogenic activity are projected to intensify in the near-future, long-lived molluscs, such as Pacific geoduck Panopea generosa (known lifespan up to 168 years), may rely on intragenerational acclimation to buffer against rapid environmental change. While acute stressors can be detrimental, environmental stress conditioning can improve performance. For example, moderate oxidative stress (i.e. temperature, irradiance, and dietary restriction) shows evidence of dose-dependent benefits for many taxa, however stress acclimation remains understudied in marine invertebrates, despite being threatened by climate change stressors. To test the hypothesis that physiological status is altered by stress conditioning, we first subjected juvenile geoduck clams to repeated exposures of elevated pCO2 in a commercial hatchery setting followed by a period in ambient common garden. Our initial experiment found early exposure to low pH elicits compensatory carryover effects suggesting bioenergetic re-allocation facilitates growth compensation and metabolic recovery. Further, to test for life-stage and stress-intensity dependence in eliciting enhanced tolerance under subsequent stress encounters, we acclimatized post-larval geoduck for >100 days before re-exposure under two reciprocal periods of moderate and severe elevated pCO2. Stress acclimation followed by secondary and tertiary exposure to severe and moderate elevated pCO2 increased respiration rate, organic biomass, and shell size suggesting a stress-intensity-dependent effect on energetics. Moreover, stress-acclimated clams had lower antioxidant capacity compared to clams under initial ambient conditions, supporting the hypothesis that stress over postlarval-to-juvenile development affects oxidative status later in life. Transcriptomics was completed to better understand molecular underpinnings of emergent physiological phenotypes from this repeated reciprocal stress challenge. The naïve phenotype showed a high transcriptional demand involving fatty-acid degradation and glutathione components, highlighting mobilization of endogenous lipids, primarily for β-oxidation, as a favored energy source affecting somatic growth. In contrast, the transcriptome profile was more diverse and responsive to environmental changes (e.g. low pH: cellular quality control and immune defense; ambient recovery: energy metabolism and biosynthesis) and under putative control of transcriptional modifiers (e.g. histone methyltransferases and transcription factors) in the stress-acclimated phenotype, corroborating physiological traits of emergent phenotypes to propose molecular mechanisms underpinning beneficial developmental acclimation and stress resilience. Altogether, the summation of dissertation findings suggests early-life stress can trigger beneficial phenotypic variation. Thus, investigations of marine species responses to climate change should consider adaptive dose-dependent regulation and effects post-acclimation.
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