Compressive strength of Mytilus californianus shell is time-dependent and can influence the potential foraging strategies of predators

Abstract

The calcareous exoskeletons of bivalve molluscs protect the organisms from environmental stressors, including physical loads like impacts (e.g., from wave-borne rocks) and compressions (e.g., from predators). The magnitude and time scales of physical loads can occur over a wide range of values, and little is known about how the strength of bivalve shells are affected by the speed of impact or compression. We used the intertidal mussel Mytilus californianus to test how the rate of compression affects the strength of the mussel’s shell. Shell strength was time-dependent, and shells withstood 11.5% larger loads if those loads were applied quickly than if they were applied slowly. We then tested whether the shell’s time-dependent strength could influence the strategy of predators trying to compress and fracture the shells. Between strategies with the same compression rate, using more force reduced the time and mechanical work to fracture shells. However, between strategies with the same force, compressing shells slowly reduced the time required to fracture large shells, while the opposite was true for small shells. These results indicate that shell strength is not a static property. Time-dependent strength can influence the interactions between mussels and the physical loads they experience in nature, including their interactions with predators. Studies investigating the influence of climate change on the architecture and strength of mollusc shells should incorporate a range of ecologically relevant compression rates in their measurements to better understand how climate change will affect these organisms.