The response of two species of unionid mussels to extended exposure to elevated carbon dioxide

Changes in environmental conditions can act as stressors, with potential consequences for the health and fitness of organisms. Rising levels of carbon dioxide (CO₂) is one potential environmental stressor that is occurring more frequently in the environment and can be a stressor for aquatic organisms. In this study, the physiological responses of two species of unionid mussel, Lampsilis siliquoidea and Amblema plicata, were assessed in response to exposure to two levels of elevated partial pressure of CO₂ (pCO₂) (~ 20,000 and ~ 55,000 μatm) over a 28 d period, followed by a subsequent 14 d recovery period. Observations were consistent with responses associated with respiratory acidosis, as demonstrated by changes in hemolymph HCO₃⁻, Ca^2 +, Cl⁻, and Na^2 +. Both species exposed to elevated pCO₂ had elevated hemolymph HCO₃^– during the pCO₂ treatment period compared to control mussels, but recovered once pCO₂ was removed. Similarly, both species had elevated hemolymph Na⁺ during exposure to elevated pCO₂, and this returned to control levels for A. plicata but remained elevated for L. siliquoidea once the pCO₂ stimuli was removed. Changes in hemolymph Ca^2 + and Cl⁻ in response to elevated pCO₂ were also observed, but these changes were species-specific. Additional physiological responses to elevated pCO₂ (e.g., changes in hemolymph glucose and Mg^2 +) were consistent with a stress response in both species. This study demonstrates the importance of considering inter-specific differences in the response of organisms to stress, and also that responses to elevated pCO₂ may be transient and can recover once the stress is removed.