Physiological Acclimation of an Estuarine Fish to Ocean Acidification: A Road Map for Resilience?

Ocean acidification (OA) is known to impact fish on a number of levels with implications for population and fisheries health. While the environmental change is considered too rapid for many species to adapt through genetic processes, evolutionary biologists postulate that adaptive capacity may be enhanced by phenotypic plasticity. Importantly, many of the documented impacts of OA have been linked to blood chemistry changes stemming from impaired CO₂ excretion. This study examined respiratory and acid-base plasticity after exposure to OA in the estuarine red drum (Scianops ocellatus). OA had no impacts on erythrocyte CO₂ excretion pathways; however, 14 days of exposure reduced gill diffusion distance by approximately 30% relative to controls. Furthermore, ventilation minute volume was elevated by 25% with OA exposure. While these changes are sufficient to alleviate the changes in blood chemistry that accompany acute OA exposure, there is a potential osmoregulatory cost. Consequently, branchial Na⁺, K⁺ ATPase activity doubled after 14 days of exposure and a transient up-regulation of intestinal NKCC2 expression was observed after 72 h of exposure. Nonetheless, OA acclimation had no impact on SMR, MMR, aerobic scope or U_crit, suggesting that the energetic costs associated with OA acclimation are marginal.