Responses to elevated CO₂ exposure in a freshwater mussel, Fusconaia flava

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 CO₂ (pCO₂). The present study quantified the impacts of acute (6 h) and chronic (up to 32 days) exposures to elevated pCO₂ on genes associated with shell formation (chitin synthase; cs) and the stress response (heat shock protein 70; hsp70) in Fusconaia flava. Oxygen consumption (MO₂) was also assessed over the chronic CO₂ exposure period. Although mussels exhibited an increase in cs following an acute exposure to elevated pCO₂, 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 pCO₂. In response to an acute elevation in pCO₂, 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 pCO₂. A chronic elevation in pCO₂ also increased mussel MO₂. This overall increase in hsp70 mRNA levels and MO₂ in F. flava indicates that exposure to elevated pCO₂ 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 pCO₂ 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.