Mussel Attachment in Changing Climates: An Ecomaterial Approach

Mussels dominate temperate rocky shores worldwide by forming dense aggregations that are firmly tethered by byssal threads, extracellular collagenous secretions of the mussel foot.  Strong byssal attachment ensures mussels can withstand heavy wave action, resist mobile predators and overgrow competitors for limited space.  Field studies with Mytilus spp. have shown byssus strength, or tenacity, follows a strong seasonal cycle, rendering both wild and farmed populations prone to “fall-off” in late summer/early fall when increased storm activity coincides with weak attachment.  These seasonal cycles in tenacity are not due to changes in rate of thread production, but rather reflect environmentally-induced changes in the material properties of the individual byssal threads mussels produce; threads produced in summer are significantly weaker and less extensible than those produced in spring.  The underlying mechanism by which byssal threads weaken seasonally is unknown. Using custom laboratory mesocosms, we quantified the effects of two common environmental stressors, elevated temperature and pCO₂ (= acidification), on the mechanical performance of byssal threads secreted by the bay mussel M. trossulus.  Both stressors caused thread weakening and loss of extensibility, but targeted different regions of a thread.  Specifically, high water temperature (~25C) weakened the proximal region, while high pCO₂ (> 1200 μatm pCO₂) reduced the strength of the plaque.  Both stressors reduced overall thread extensibility as well.  These results suggest multiple environmental stressors, including ocean acidification and warming, can combine to critically compromise the structural integrity of mussels.  Understanding the underlying mechanism by which environment alters the manufacturing process of mussel byssus (e.g. direct physiochemical  v. indirect physiologicial effects) will provide insight into developing strategies that promote the manufacture of strong byssus material, in industry and in nature.