Repeat exposure to hypercapnic seawater modifies performance and oxidative status in a tolerant burrowing clam

Samuel J. Gurr¹*, Shelly A. Trigg³, Brent Vadopalas², Steven B. Roberts³, Hollie M. Putnam¹

¹University of Rhode Island, College of the Environment and Life Sciences, 120 Flagg Rd, Kingston, RI 02881 USA

²University of Washington, Washington Sea Grant, 3716 Brooklyn Ave NE, Seattle, WA 98105 USA

³University of Washington, School of Aquatic and Fishery Sciences, 1122 NE Boat St, Seattle, WA 98105 USA

Moderate oxidative stress is a hypothesized driver of enhanced adaptive capacity and lifespan. Whereas thermal stress, irradiance, and dietary restriction show evidence of dose-dependent benefits for terrestrial vertebrates, the importance of stress acclimation remains understudied for establishment of enhanced stress response in marine ectotherms. Marine invertebrates of commercial and ecological importance are threatened by elevated pCO₂ and aragonite undersaturation. Stress acclimation may drive phenotypic variation advantageous for sustainable aquaculture of long-lived mollusc species such as the Pacific geoduck Panopea generosa. To test for lifestage and stress-intensity dependence in eliciting enhanced tolerance under subsequent stress encounters, we acclimated ~30 day old pediveliger geoduck larvae for 110 days to 1) an initial exposure of ambient and moderately elevated pCO₂ (920 µatm >1 Ω and 2800 µatm 0.4 Ω, respectively). We then exposed them to 2) an acute 7-day secondary exposure to ambient, moderate, and severely elevated pCO₂ conditions (750 µatm >1 Ω, 2800 µatm 0.4 Ω, and 4900 µatm 0.2 Ω, respectively). Clams were returned to ambient pCO₂ for 7 days and then subject to 3) a 7-day tertiary exposure to ambient and moderately elevated pCO₂ conditions (970 µatm >1 Ω and 3000 µatm 0.4 Ω, respectively) in a modified-reciprocal design. Throughout each exposure, we measured metrics of physiological performance (respiration rate, shell growth, total protein and organic biomass) and cellular stress response (total antioxidant capacity). Initial exposure to moderate pCO2 stress followed by secondary and tertiary exposure to moderate and severe pCO2 stress increased respiration rate, organic biomass, and shell size suggesting a stress-intensity-dependent effect of stress acclimation on energetics. Additionally, clams acclimated to moderate pCO₂ stress during initial exposure had lower antioxidant capacity at the termination of the experiment compared to clams under ambient conditions, supporting the hypothesis that stress over postlarval-juvenile development affects energy metabolism and oxidative status later in life. Time series and stress intensity-specific approaches can reveal life-stages and magnitudes of exposure, respectively, that may lead to beneficial phenotypes for commercial production of this long-lived clam.