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Multiple environmental stressors may interact in complex ways to exceed or diminish the impacts of individual stressors. In the present study, the interactive effects of two ecologically relevant stressors [increased temperature and partial pressure of carbon dioxide (Pco 2 )] were assessed for freshwater mussels, a group of organisms that are among the most sensitive and rapidly declining worldwide. The individual and combined effects of elevated temperature (22°C-34°C) and Pco 2 (~230, 58,000 µatm) on juvenile Lampsilis siliquoidea were quantified over a 5- or 14-day period, during which physiological and whole animal responses were measured. Exposure to elevated temperature induced a series of physiological responses, including an increase in oxygen consumption rates following 5 days of exposure at 31°C and an increase in carbonic anhydrase ( ca) and heat shock protein 70 mRNA levels following 14 days of exposure at 28°C and 34°C, respectively. Treatment with elevated Pco 2 activated acid-base regulatory responses including increases in CA and Na + -K + -ATPase activity and a novel mechanism for acid-base regulation during Pco 2 exposure in freshwater mussels was proposed. Thermal and CO 2 stressors also interacted such that responses to the thermal stressor were diminished in mussels exposed to elevated Pco 2 , resulting in the greatest level of mortality. Additionally, larger mussels were more likely to survive treatment with elevated Pco 2 and/or temperature. Together, exposure to elevated Pco 2 may compromise the ability of juvenile freshwater mussels to respond to additional stressors, such as increased temperatures, highlighting the importance of considering not only the individual but also the interactive effects of multiple environmental stressors.

Freshwater mussels may be exposed to elevations in mean partial pressure of carbon dioxide (pCO 2 ) caused by both natural and anthropogenic factors. The goal of the present study was to assess the effects of a 28-d elevation in pCO 2 at 15 000 and 50 000 μatm on processes associated with biomineralization, ion regulation, and cellular stress in adult Lampsilis siliquoidea (Barnes, 1823). In addition, the capacity for mussels to compensate for acid-base disturbances experienced after exposure to elevated pCO 2 was assessed over a 14-d recovery period. Overall, exposure to 50 000 μatm pCO 2 had more pronounced physiological consequences compared with 15 000 μatm pCO 2 . Over the first 7 d of exposure to 50 000 μatm pCO 2 , the mRNA abundance of chitin synthase (cs), calmodulin (cam), and calmodulin-like protein (calp) were significantly affected, suggesting that shell formation and integrity may be altered during pCO 2 exposure. After the removal of the pCO 2 treatment, mussels may compensate for the acid-base and ion disturbances experienced during pCO 2 exposure, and transcript levels of some regulators of biomineralization (carbonic anhydrase [ca], cs, cam, calp) as well as ion regulation (na + -k + -ATPase [nka]) were modulated. Effects of elevated pCO 2 on heat shock protein 70 (hsp70) were limited in the present study. Overall, adult L. siliquoidea appeared to regulate factors associated with the control of biomineralization and ion regulation during and/or after the removal of pCO 2 exposure. Environ Toxicol Chem 2018;37:538-550. © 2017 SETAC., (© 2017 SETAC.)

Freshwater mussels are at-risk taxa and may be exposed to high levels of carbon dioxide (CO 2 ) because of the potential use of CO 2 to control the movement of invasive aquatic fish species. One potential behavioral response to a change in the partial pressure of CO 2 (pCO 2 ) may be altered valve movement. In this study, three species of mussels were fitted with modified sensors and exposed to two regimes of pCO 2 to define thresholds of impaired valve movement. The first experiment demonstrated that Pyganodon grandis were much more tolerant to rising pCO 2 relative to Lampsilis siliquoidea (acute closure at ∼200,000 μatm in comparison to ∼80,000 μatm). The second experiment consisted of monitoring mussels for 6 days and exposing them to elevated pCO 2 (∼70,000 μatm) over a 2-day period. During exposure to high pCO 2 , Lampsilis cardium were open for nearly the entire high pCO 2 period. Conversely, P. grandis were closed for most of the period following exposure to high pCO 2 . For L. siliquoidea, the number of closures decreased nearly 40-fold during high pCO 2 . The valve movement responses observed suggest species differences, and exposure to elevated pCO 2 requires a reactive response.

Freshwater mussels are some of the most imperiled species in North America and are particularly susceptible to environmental change. One environmental disturbance that mussels may encounter that remains understudied is an increase in the partial pressure of CO 2 (pCO 2 ). The present study quantified the impacts of acute (6 h) and chronic (up to 32 days) exposures to elevated pCO 2 on genes associated with shell formation (chitin synthase; cs) and the stress response (heat shock protein 70; hsp70) in Fusconaia flava. Oxygen consumption (MO 2 ) was also assessed over the chronic CO 2 exposure period. Although mussels exhibited an increase in cs following an acute exposure to elevated pCO 2 , long-term exposure resulted in a decrease in cs mRNA abundance, suggesting that mussels may invest less in shell formation during chronic exposure to elevated pCO 2 . In response to an acute elevation in pCO 2 , mussels increased hsp70 mRNA abundance in mantle and adductor muscle and a similar increase was observed in the gill and adductor muscle in response to a chronic elevation in pCO 2 . A chronic elevation in pCO 2 also increased mussel MO 2 . This overall increase in hsp70 mRNA levels and MO 2 in F. flava indicates that exposure to elevated pCO 2 initiates activation of the general stress response and an increased energy demand. Together, the results of the present study suggest that freshwater mussels respond to elevated pCO 2 by increasing processes necessary to 'deal with' the stressor and, over the long-term, may reduce their investment in non-essential processes such as shell growth.