Hopkins Marine Station of Stanford University, Pacific Grove, CA 93950, USA
Hopkins Marine Station of Stanford University, Pacific Grove, CA 93950, USA.
J Exp Biol. 2020 May 27;223(Pt 10):jeb220277. doi: 10.1242/jeb.220277.
Mollusk shells protect against diverse environmental and predatory physical threats, from one-time impacts to chronic, low-magnitude stresses. The effectiveness of shells as armor is often quantified with a test of shell strength: increasing force is applied until catastrophic fracture. This test does not capture the potential role of fatigue, a process by which chronic or repeated, low-magnitude forces weaken and break a structure. We quantified the strength and fatigue resistance of California mussel () shells. Shells were fatigue tested until catastrophic failure by either loading a valve repeatedly to a set force (cyclic) or loading a valve under constant force (static). Valves fatigued under both cyclic and static loading, i.e. subcritical forces broke valves when applied repeatedly or for long durations. Stronger and more fatigue-resistant valves tended to be more massive, relatively wider and the right-hand valve. Furthermore, after accounting for the valves' predicted strength, fatigue resistance curves for cyclic and static loading did not differ, suggesting that fatigue fracture of mussels is more dependent on force duration than number of cycles. Contextualizing fatigue resistance with the forces mussels typically experience clarifies the range of threats for which fatigue becomes relevant. Some predators could rely on fatigue, and episodic events like large wave impacts or failed predation attempts could weaken shells across long time scales. Quantifying shell fatigue resistance when considering the ecology of shelled organisms or the evolution of shell form offers a perspective that accounts for the accumulating damage of a lifetime of threats, large and small.
软体动物的壳可以保护它们免受各种环境和捕食性物理威胁，包括一次性冲击和慢性、低幅度的压力。壳作为盔甲的有效性通常通过壳强度测试来量化：施加的力逐渐增加，直到灾难性的断裂。然而，这种测试并没有捕捉到疲劳的潜在作用，疲劳是一种慢性或反复的、低幅度的力量削弱和破坏结构的过程。我们对加利福尼亚贻贝（Mytilus californianus）的壳的强度和耐疲劳性进行了量化。通过反复加载到设定力（循环）或在恒定力下加载阀门（静态）的方式对阀门进行疲劳测试，直到灾难性失效。阀门在循环和静态加载下都会疲劳，即亚临界力会在重复施加或长时间施加时破坏阀门。强度更高、耐疲劳性更强的阀门往往更厚重，相对更宽，且为右手阀。此外，在考虑到阀门的预测强度后，循环和静态加载的疲劳阻力曲线没有差异，这表明贻贝的疲劳断裂更依赖于力的持续时间而不是循环次数。将耐疲劳性与贻贝通常经历的力联系起来，可以阐明疲劳变得相关的威胁范围。一些捕食者可能依赖于疲劳，而像大波浪冲击或捕食失败这样的偶发事件可能会在很长的时间尺度上削弱壳。在考虑有壳生物的生态学或壳形态的进化时，量化壳的耐疲劳性提供了一种视角，可以解释一生中大大小小的威胁所造成的累积损伤。
