Abstract
Some decapod crustaceans have tooth-like white denticles that exist only on the pinching side of claws. We revealed the denticle microstructure in the coconut crab, Birgus latro, using optical and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and a focused ion beam (FIB)-SEM. Three-dimensional analysis and fracture surface observation were performed in order to clarify the microstructural differences in two mineralized layers—the exocuticle and the endocuticle. The denticles consist of a columnar structure normal to the surface and are covered with a very thin epicuticle and an exocuticle with a twisted plywood pattern structure. Due to abrasion, the exocuticle layer was lost in the wide area above the large denticles; conversely, these layers remained on the surface of the relatively small denticles and on the base of the denticle. The results showed that the mineralized exoskeleton of the crab’s claw is classified into three structures: a twisted plywood pattern structure stacked parallel to the surface for the exocuticle, a porous structure with many regularly arranged pores vertical to the surface for the endocuticle, and a columnar structure vertical to the surface for the denticle.
Highlights
Organisms are designed to respond and adapt well to dynamically changing environments, and they show outstanding mechanical properties
The exocuticle and endocuticle layers comprised of chitin and protein are tough and play an important role in protection from enemies
Rosen et al [19] examined the microstructure, components, and microhardness of the denticles for three brachyuran crabs and two anomuran crabs; they reported that hardness was 2.5 to 10 times higher in the denticle than in the endocuticle, that there was a decrease in the width of the pore canals that run through the cuticle and in the phosphorous content, and that there was a structure change from endocuticle to denticle
Summary
Organisms are designed to respond and adapt well to dynamically changing environments, and they show outstanding mechanical properties. The exocuticle and endocuticle layers comprised of chitin and protein are tough and play an important role in protection from enemies In crustaceans, these two layers are mineralized and very hard, and these tissues are characterized by a twisted plywood pattern structure [3,11,12,13,14]. The maximum hardness (Hmax ) and stiffness (Er (max) ) near the denticle surface indicated almost the same values, Hmax = 4 GPa and Er (max) = 70 GPa. a distinct shift in tissue structure from the denticle to the endocuticle in the coconut crab claw has been reported. The exoskeleton of claw is calcite regardless of each layer of the exocuticle, endocuticle and denticle, as shown in Figure S1 in the supporting information, the denticle microstructure is likely to have a tissue structure that is different from the other mineralized layers.
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