Elevated seawater temperature, not pCO₂, negatively affects post-spawning adult mussels (Mytilus edulis) under food limitation

Pre-spawning blue mussels (Mytilus edulis) appear sensitive to elevated temperature and robust to elevated pCO₂; however, the effects of these stressors soon after investing energy into spawning remain unknown. Furthermore, while studies suggest that elevated pCO₂ affects the byssal attachment strength of Mytilus trossulus from southern latitudes, pCO₂ and temperature impacts on the byssus strength of other species at higher latitudes remain undocumented. In a 90 day laboratory experiment, we exposed post-spawning adult blue mussels (M. edulis) from Atlantic Canada to three pCO₂ levels (pCO₂ ~625, 1295 and 2440 μatm) at two different temperatures (16°C and 22°C) and assessed energetic reserves on Day 90, byssal attachment strength on Days 30 and 60, and condition index and mortality on Days 30, 60 and 90. Results indicated that glycogen content was negatively affected under elevated temperature, but protein, lipid, and overall energy content were unaffected. Reduced glycogen content under elevated temperature was associated with reduced condition index, reduced byssal thread attachment strength, and increased mortality; elevated pCO₂ had no effects. Overall, these results suggest that the glycogen reserves of post-spawning adult M. edulis are sensitive to elevated temperature, and can result in reduced health and byssal attachment strength, leading to increased mortality. These results are similar to those reported for pre-spawning mussels and suggest that post-spawning blue mussels are tolerant to elevated pCO₂ and sensitive to elevated temperature. In contrast to previous studies, however, elevated pCO₂ did not affect byssus strength, suggesting that negative effects of elevated pCO₂ on byssus strength are not universal.