Microstructure
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
protein-mineral matrix are stacked with regular changes in fiber
directions between each bed, resulting into stepwise and rough fracture
surface (Figure 2F). Pore canals, pervading the cuticle up to the
epicuticle, are visible at high magnification SEM and have a
characteristic twisted-ribbon shape [44] with
unmineralized dark lumens (Figure 2C). They serve for the secretion of
the cuticle and perhaps provide supplementary “waterproof” capability,
as inferred by their lipid content [36]. In the
second layer, referred as the striated region of the endocuticle (STR),
the helicoidal pattern is replaced by highly aligned fibers parallel to
the long axis of the spike as highlighted by a longitudinally fractured
sample showing fiber sheets (oriented vertically in Figure 2G). In this
region, the pore canals lose their typical twisted-ribbon aspect and
cross perpendicularly the horizontal fiber beds, giving a characteristic
striated pattern typically observed in polished (transverse) sections,
where the canal lumen appears dark and the mineralized fibrous sheath
appears bright (Figure 2D). While the twisted plywood motif is universal
in arthropods cuticle, the parallel-fiber organization is very unusual[45]. In stomatopods, striated regions with
parallel longitudinal fibers have been found only in specific locations
including the dactyl club of the smasher [32], the
impact area of the dactyl in the appendage carrying the spikes of the
spearer [28] and the back defensive spikes located
at the uropods (posterior structures found in crustaceans)[42]. 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 ).