Structure and mineralization of the spearing mantis shrimp (Stomatopoda; Lysiosquillina maculata) body and spike cuticles

Design Strategies of the Mantis Shrimp Spike: How the Crustacean Cuticle Became a Remarkable Biological Harpoon?

“…The microstructure of the spike, observed with SEM on fractured and polished samples, features four regions with distinct fiber architecture (Figures 2 A, B and summery scheme in I). The first innermost layer, named inner helicoidal region (IHR, Figure 2C), belongs to the endocuticle [36] and displays a twisted plywood organization consisting of numerous lamellae gradually increasing in thickness from about 1.5 to 10 μm when moving outwards. Within each lamella, layered beds of chitin-protein fibers embedded into a proteinmineral matrix are stacked with regular changes in fiber directions between each bed, resulting into stepwise and rough fracture surface (Figure 2F).…”

“…One common feature of those locations is a challenging biomechanical environment, solving critical offensive or defensive tasks. Most interesting, sandwiched between the striated and the external region, there is an additional layer of the endocuticle [36] presenting a twisted plywood organization (Figure 2E), called outer helicoidal region (OHR). This location has two unusual features: it is composed of a very limited number of lamellae, spanning a width of only a few micrometers, and it appears darker under backscattered electron (BSE) imaging, indicating a lower mineral content.…”

“…Even more, when going from this outer helicoidal region to the exterior of the spike exocuticle, the helicoidal architecture is not lost immediately, but there is a transition region (TR) having high mineral content (arrows in Figure 2E) and characterized by one interface lamella. The last region of the spike cuticle identified here is the external highly mineralized region (HMR), where larger and more isolated fibers achieve a last half rotation within a heavily mineralized matrix before the twisted plywood organization is finally lost towards the spike outer surface [36] . The presence of a continuous twisted plywood at the interface between the highly mineralized region and the less mineralized outer helicoidal region is a remarkable feature of the spike that may enhance the anchoring between such dissimilar layers, as suggested by the rough fracture surface at that location (Figure 2H).…”

“…A closer examination of fracture surfaces (Figure S4 ) highlights that in this highly mineralized region, crystals form nanometer sized crystallites, in analogy with those found in the dactyl club's outer layer [35] . The remaining of the highly mineralized region presents a very low fraction of chitin-protein fibers [36] , but is still travelled by numerous pore canals, running perpendicular to the interface with the outer helicoidal region (Figure 2E). The spike cross-section is largely dominated by the striated and the inner helicoidal regions: for about two-third of the spike length, 90% of the wall thickness is only due to those two regions with the contribution of the outer helicoidal region and of the highly mineralized region being about 3% and 7%, respectively (Figure S5 ).…”

“…Micro-CT imaging of the spike cross-section reveals a thin highly mineralized cover bordering a less mineralized but much thicker layer with a central cavity (Figure 1D). This dual mineralization is a specific reinforcement feature of raptorial appendages used for impaling or smashing preys as well as for fighting with opponents [28,32,[34][35][36] . In general, the electron dense region is particularly pronounced on the impact surface and much less in other region of the cuticle [9,42] .…”