Structural and mechanical properties facilitate shock wave damping by helmet-like orbital hoods in snapping shrimp

Abstract: Snapping shrimp damp the shock waves they produce and use as weapons with a helmet-like structure termed the orbital hood. Here, we ask how structural and material properties contribute to shock wave damping by orbital hoods in Alpheus heterochaelis. Using tensile mechanical testing, we find orbital hoods are approximately half as stiff as carapace and have twice the capacity for viscous energy dissipation. Microstructural features probably contribute to tissue-specific mechanical properties: the endocuticles of orbital hoods have almost twice as many lamellae as those of carapaces despite being half as thick, suggesting a mechanism for enhanced material mobility underlying viscous behaviour. Using material properties from mechanical testing and geometric data from micro-computed tomography, we developed finite element simulations of interactions between shock waves and orbital hoods. These simulations predict orbital hoods reduce shock wave-induced strain and stress in the neural tissues of shrimp by 28% and 22%, respectively. Orbital hoods appear optimized for shock wave damping: simulated increases or decreases in their material properties reduce their protective capabilities. We conclude that structural and viscoelastic properties contribute to shock wave damping by orbital hoods, a promising step towards bio-inspired improvements to contemporary armour systems that currently underperform in preventing blast-induced neurotrauma in humans.