Ecomechanics of Wild and Farmed Mussels in a Warmer, Acidified Ocean

The mechanical integrity of the structures mussels produce often dictates the animal’s survival.  In nature, stretchy byssal threads prevent mussel dislodgment by water motion or predators, while thick shells deter predators and slow desiccation during emersion.  Growers suffer reduced yield when mussels develop inferior mechanical properties; weak byssus promotes sloughing (loss) from culture ropes and thin shells crack on their way from farm to table. We investigated the effect of ocean acidification (OA) and other environmental stressors on the mechanical integrity of mussels (primarily Mytilus trossulus, but also M. galloprovincialis and M. californianus). We used laboratory mesocosms to expose mussels to various ranges and combinations of OA, temperature and food supply and quantified the strength of byssus produced and the mussel’s ability to repair shell damage.  Both elevated temperature and OA reduced byssus strength, but OA did not affect shell repair.  Field observations of wild and farmed M. trossulus show byssus and shell thickness vary seasonally, weakening in summer.  Concurrent measurements of seawater conditions suggest water temperature and food supply, not ocean acidification, is the primary driver of reduced structural integrity.  Moreover, continued warming in the Pacific Northwest is likely to favor the non-native mussel M. galloprovincialis.