Molecular, behavioral, and performance responses of juvenile largemouth bass acclimated to an elevated carbon dioxide environment

Aquatic hypercarbia, either naturally occurring or anthropogenically induced, can have extensive impacts on aquatic environments and resident organisms. While the impact of acute hypercarbia exposure on the behavior and physiology of fishes has been well studied, relatively little work has examined the physiological impact and acclimation capacity of fishes to chronic hypercarbia. To better understand the impacts of prolonged hypercarbia exposure, largemouth bass were held at ambient CO₂ (13 mg L⁻¹) and elevated CO₂ (31 mg L⁻¹; ≈21,000 µatm) for 58 days. Following this acclimation period, fish were subjected to three separate, yet complementary, experiments: (1) acute hypercarbia challenge of 120 mg L⁻¹ CO₂ for 1 h to quantify physiological and molecular responses; (2) hypercarbia avoidance challenge to compare CO₂ agitation and avoidance responses; and (3) swim performance challenge to quantify burst swimming performance. Acclimation to 31 mg L⁻¹ CO₂ resulted in a significant constitutive upregulation of c-fos expression in erythrocytes, combined with significant constitutive expression of hsp70 in both gill and erythrocytes, relative to controls. Largemouth bass acclimated to elevated CO₂ also had a reduced glucose response (relative to controls) following an acute CO₂ exposure, indicating a reduced stress response to CO₂ stressors. In addition, largemouth bass acclimated to elevated CO₂ conditions required 50 % higher CO₂ concentrations to illicit agitation behaviors and displayed prolonged burst swimming abilities in high CO₂ environments relative to controls. Together, results demonstrate that largemouth bass exposed to chronic hypercarbia may possess a physiological advantage during periods of elevated CO₂ relative to naïve fish, which may permit increased performance in hypercarbia.