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.