Near-future levels of ocean temperature weaken the byssus production and performance of the mussel Mytilus coruscus

Marine mussels are key ecological engineers that form dense aggregations to maintain the vital habitat in benthic systems. It is essential to understand the consequences of mussel byssus attachment in elevated temperatures associated with ocean warming. We evaluated byssus production and the mechanical performance of threads in the mussel Mytilus coruscus at 21° (control), 27 °C (average temperature in the M. coruscus habitat during the summer season) and 31 °C (4 °C raised) for 72 h. We quantified byssus secretion and shedding number, measured byssal breaking force, byssal polyphenol oxidase (PPO) activity, byssal thread length and diameter. Expression of byssus foot protein genes was analyzed by quantitative real-time PCR in foot tissue. High seawater temperature decreased the number of newly secreted byssus and the diameter of byssal threads, leading to the reduction of byssal breaking force and the alteration of the weakest part of the thread. Increased breakpoints in the upper part of the thread (proximal region) were higher at 27 °C than at 21 °C. High-temperature stress significantly reduced the PPO activity in byssus at 31 °C in comparison to 21 °C. The expression of mussel foot protein genes was affected by elevated temperature. The increased gene expression of byssus collagen-like protein 2 (Mccol2) at 31 °C conflicted with the number of byssuses produced. Suggesting the reduction of mussel foot protein abundance is not the cause of decreased byssus production at 31 °C. These results show that byssus, as an extracellular structure of mussels, may be highly susceptible to the adverse effects of ocean warming.