Different Sensitivities to ocean acidification between Populations of the Sydney Rock Oyster: Role of Filtration and Ion-regulatory Capacities

Chair: Peter Thor

Laura S. Stapp (1)*, Laura M. Parker (2), Wayne A. O’Connor (3), Christian Bock (1), Pauline M. Ross (4), Hans O. Pörtner (1) and Gisela Lannig (1)

1 Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, 27570 Germany

2 School of Biological Sciences, Centre for the Ecological Impacts for Coastal Cities, University of Sydney, Sydney, New South Wales, 2006, Australia
3 NSW Department of Primary Industries, Port Stephens Fisheries Institute, Taylors Beach, New South Wales, 2316, Australia
4 School of Science and Health, Western Sydney University, Penrith, New South Wales, 2751, Australia

Background
Ocean acidification (OA) is projected to have detrimental effects on marine mollusks. Compared to marine ectothermic vertebrates, mollusks’ ability to compensate for OA induced disturbances of extracellular acid-base equilibria is limited. Shifted extracellular pH (pHe) values may mirror such sensitivity to OA and modulate energy budgets through effects on ion-regulatory costs. Recent findings suggest that some mollusks have the capacity to acclimate or adapt to OA. Sydney Rock oysters selectively bred for faster growth and disease resistance showed a higher resilience compared to wild oysters mirrored in a lower reduction in shell growth, reduced larval sensitivity and increased metabolic rates. We therefore examined whether higher metabolic rates under OA in selected oysters are linked to increased energy allocation to acid-base and ion-regulatory processes promoting their higher resilience.

Methods
Wild and selected oysters were exposed to ambient (500 µatm) and elevated (1100 µatm) seawater pCO2. After 6 weeks we determined their extracellular acid-base status and in vivo metabolic costs of prominent ion regulators (Na+/K+-ATPase, H+-ATPase, Na+/H+-exchange) in isolated gill and mantle tissues.

Findings
Wild oysters showed a lower pHe than selectively bred oysters. The acidosis was associated with increased hemolymph pCO2 in wild but not in selected oysters. Although we observed alterations in cellular energy allocation to ion transporters, these patterns were not tightly linked to the differing pHe values.

Conclusions
Our findings suggest that partial pHe compensation in selected oysters occurred at the systemic level due to their increased ability to eliminate metabolic CO2, possibly brought about by higher or more efficient feeding rates, which are known to be the basis for their faster growth. Thus, effective filtration and OA resilience might be positively correlated traits in oysters. Understanding the adaptive potential of marine organisms thus relies on identifying the physiological mechanisms that are modified in response to OA.