The snapping shrimp preys by rapidly closing its snapping claw to generate a fast water jet, creating a shockwave that bombards the nearby prey and other shrimp. This behaviour has led to considerable interest and research. However, the structure, surface morphology and mechanical properties of the snapping claw are unreported. We used a combination of techniques including scanning electron microscopy and nanoindentation to characterise the claw. These measurements were coupled with computational fluid dynamics (CFD) to understand how the microstructure contributes to drag reduction. We found that cone-shaped micropapillae, rhombic dents and short straight stripes were hierarchically distributed on the surface of the claw. CFD simulation showed that the micropapillae units changed the interaction between the turbulent and the wall from sliding friction to rolling friction, resulting in tire-shaped vortices. This also reduced the turbulent kinetic energy in the near-wall region, thereby contributing to drag reduction. The cross section of the claw revealed four layers comprising an epicuticle, exocuticle, endocuticle and a membranous layer. The exocuticle is composed of chitin fibres arranged vertically in a lamellar fashion and the endocuticle has a Bouligand-type structure. This special structure provides the snapping shrimp with good mechanical resistance during rapid closure. Both modulus and hardness decreased from the outermost epicuticle to the innermost membranous layer. The gradient modulus and hardness may help to suppress microcracks at the interfaces between different layers. The findings improve our understanding of the unique mechanism of the snapping claw and may lead to the development of novel biomimetic materials with enhanced drag reduction, impact and crack resistance properties.
