Effect of Sea Water pH on the Maturation of Marine Mussel Plaques*

Marine mussel plaques are an exceptional model for wet adhesives. Despite understanding their protein composition, we do not know how soluble proteins are rapidly processed into load-bearing structures. Here, we examine the effects of seawater pH on the time evolution of the internal microstructures in Mytilus californianus plaques. Experimentally, plaques deposited by mussels on glass surfaces are immediately collected, placed into pH-controlled artificial seawater for varying times, and characterized using scanning electron microscopy and tensile testing. We found a pH dependent transition from a liquid-like state to a porous solid within 30 minutes for pH ≥ 6.7; these plaques are load-bearing. By contrast, samples maintained at pH 3.0 showed no porosity and no measurable strength. Interestingly, we found no differences in cuticle thickness, suggesting that cuticle mechanics may be compromised at low pH. Our results suggest that sea water infusion after deposition is critical to the rapid formation of internal structures, which in turn plays an important role in the plaques’ mechanical performance.