The Mantis Shrimp Saddle: A Biological Spring Combining Stiffness and Flexibility

Stomatopods are aggressive crustacean predators that use a pair of ultrafast raptorial appendages to strike on prey. This swift movement is driven by a power amplification system comprising components that must be able to repetitively store and release a high amount of elastic energy. An essential component of this system is the saddle structure, in which the elastic energy is stored by bending prior to striking. Here, a comprehensive study that sheds light on the microstructural and chemical designs of the stomatopod's saddle is conducted. MicroCT scans combined with electron microscopy imaging, elemental mapping, high-resolution confocal Raman microscopy, and nanomechanical mapping show that the saddle is a bilayer structure with sharp changes in chemical composition and mechanical properties between the layers. The outer layer is heavily mineralized whereas the inner layer contains a high content of chitin and proteins, leading to a spatial organization of phases which is optimized for load distribution during saddle bending. The mineralized outer layer sustains compressive stresses, whereas the inner biopolymeric layer provides tensile resistance. These findings reveal that the saddle chemical composition and microstructure have been spatially tuned to generate a stiff, yet flexible structure that is optimized for storage of elastic energy. Mantis shrimps deliver ultrafast strikes using a complex power amplification system, within which the "saddle biospring" is used to store and quickly release the elastic energy. It is demonstrated that the saddle is a bilayer material with distinct degrees of mineralization. One layer is used to maximize elastic energy storage during loading, while the other layer provides flexibility and prevents fracture.