Meeting Abstract
Bivalve shells provide protection from a variety of potentially lethal predatory and environmental threats. These threats range in frequency and magnitude from a single powerful predator strike to repeated insults from waves and tightly-packed neighbors. Shells’ effectiveness at defending from such forces is traditionally quantified with a simple test of strength: a shell is rapidly compressed until it breaks. However, this technique cannot test the alternative possibility that low magnitude, repeated stresses can break a shell through the process of fatigue. We explored the threat of different realistic sources of shell damage by quantifying and contextualizing the strength and fatigue resistance of the California mussel (Mytilus californianus). We repeated the classic strength test by applying an increasing compressive force until fracture. Additionally, we used two distinct tests of fatigue resistance: a subcritical load was either applied constantly (i.e., static loading) or cyclically until fracture. Both fatigue tests considered a broad range of subcritical forces to mimic the forces experienced by mussels in the field. When stripped of living tissue, shells fatigued and broke under both static and cyclic loading conditions; lower forces required more cycles or longer static loading periods to result in shell fracture. This relationship demonstrates how the seemingly insignificant forces imposed by clamping shell valves or packing in a mussel bed could ultimately generate lethal damage, and invites the question of whether living tissue can somehow counteract the process of fatigue. These findings highlight how a range of accumulated threats might underlie variation in shell morphology and microstructure, and provide inspiration for future considerations of the evolution of shell form.
