Resistance and repair of mechanical fatigue in mussel shells
Keywords: Biology.

Rigid external armors, such as bivalve shells, have to defend against a lifetime of threats that range in frequency and magnitude from a single powerful predator strike to chronic environmental stresses like repeated insults from ocean waves. These encounters have the potential to cause accumulating, weakening, and ultimately lethal damage to shells through mechanical fatigue: a process in which low magnitude, repeated stresses weaken and break a material. Due to the cumulative nature of fatigue damage and the fact that bivalves, in some cases, can live for decades and are bound to a single shell throughout their lives, fatigue presents a necessary perspective to consider in understanding the effectiveness of bivalve shells. Using the California mussel, Mytilus californianus, as a model system, I have examined shell armor using the perspective of fatigue, considering both the threat posed by repeated encounters and the mussel's capacity to repair and respond to damage as it develops. In Chapter 1, I provided a brief introduction to the importance of considering fatigue when evaluating a biological material. Chapter 2 developed a mechanical framework to study fatigue in mussel shell. Despite the repeated nature of threats, shells' effectiveness is traditionally quantified with a simple test of one-time breaking stress, or strength, for which a shell is compressed until it breaks. This fails to take into account fatigue and the broad range of forces that shells contend with. We quantified fatigue resistance of shells of the California mussel using two tests: applying a subcritical load either constantly or cyclically until fracture. Both fatigue tests considered a broad range of subcritical forces to mimic the forces experienced by mussels in the field. Mussel shells broke when fatigued such that lower forces required longer loading periods before fracture. Furthermore, fatigue resistance curves for constant and cyclic loading did not differ significantly. Based on these curves, we showed that strong predators can fracture shells with a single impact, and low forces, like a shell clamping shut, will not cause damage on ecologically relevant timescales. Intermediate forces, though, can become a threat through fatigue; weaker predators can fatigue otherwise inaccessible prey, and failed predation attempts and episodic threats (e.g., wave-hurled debris) can weaken shells. In Chapter 3, we explored the long-term threat of fatigue damage by quantifying the capacity of mussels to repair their shells in response to fatigue. We applied 15 cycles at 67% of each shell's predicted strength. At four subsequent time points -- immediately and after 1, 2, and 4 weeks -- we compared the strength of fatigued shells with comparable non-fatigued control shells. We also stained shell growth on a subset of shells and inspected them with light and fluorescence microscopy. Despite being weakened immediately by fatigue, mussels on average repaired within one week, such that the strength of fatigued and control shells did not differ. Furthermore, within one month, mussels that had experienced more initial fatigue were stronger than mussels that had been fatigued at lower forces. Microscopy supported fracture as a mechanism of fatigue weakening but provided mixed evidence of shell deposition in response to damage. These findings indicated that a mussel would have little recourse during one predator attack, but even a few days between episodic threats could be sufficient for repair. If survived, fatiguing encounters do not cause irrecoverable lifelong damage but may in fact spur shell strengthening. In Chapter 4, we considered that fatiguing threats are encountered throughout a mussel's life, and to survive, they need to continuously repair. We tested the ability of California mussels to repair damage from chronic fatiguing forces. Every week for seven months, we compressed live mussels for 15 cycles at ~55% of a shell's predicted strength. We measured the final strength as well as the initial and final morphology of fatigued shells and a group of comparable non-fatigued control shells. Despite months of fatigue, the fatigued shells were no weaker than control shells, and they showed significant internal physical evidence of repair. Further, fatigued shells were thicker but had increased less in their width, resulting in a flatter, less domed shell. Finally, fatigued mussels had less internal soft tissue mass than control mussels and suffered increased mortality, distinct from mortality directly attributable to fatigue. These results demonstrated that mussels are able to maintain repair and change morphology, even in response to a chronic mechanical stressor. However, this response, though necessary for surviving physical encounters, comes at a cost to mussels' fitness. In Chapter 5, we discussed the fatigue-lifetime approach for quantifying fatigue resistance of biological materials. This method uses S/N curves -- plots of the number of cycles until failure vs. the breaking stress. For many materials, the log-log form of this graph is linear, and its slope describes the rate at which the forces a structure can withstand decrease with more cycles. We compiled published data on the slopes of S/N curves of a variety of biological and manufactured materials. We then used these compiled data and a few specific examples from the literature to highlight how S/N curves vary across biological structures, how this variation can be interpreted, and how S/N curves can be used to answer physiological, ecological, and evolutionary questions