Structural Changes and Mechanical Resistance of Claws and Denticles in Coconut Crabs of Different Sizes

Abstract

The exoskeleton of the pinching side of claws with denticles and of the outer side without them on the coconut crab, Birgus latro, which is a rare organism, were studied using a materials science approach. The mechanical resistance of three claws of different sizes was investigated along the exoskeleton thickness from the outer surface to the inner surface, and the results were compared, including the contribution of the microstructure and chemical compositions. Mechanical properties, hardness (H) and stiffness (Er), were probed through nanoindentation tests. The results showed the H, Er, microstructures, and chemical components of the exocuticle and endocuticle layers were almost the same, in a BW range of 300 g to 1650 g. At the same time, the H and Er near the surface of the denticles of a small coconut crab of 300 g were lower than those of other large coconut crabs. The microstructure of the denticles was clearly different from that of the exocuticle, but the maximum mechanical properties near their surface indicated almost the same values, Hmax = 4 GPa and Er(max) = 70 GPa, regardless of being on the pinching side or the outer side. A denticle can be regarded as a bulge of the cuticle without phosphorus and with high magnesium. The results provided novel information that expanded our knowledge about the claw microstructure of coconut crabs with different body sizes, and may be used in further studies

Columnar Structure of Claw Denticles in the Coconut Crab, Birgus latro

“…Here, in the case of a sample with large irregularities, as shown in Figure 5b,c, since it is very difficult to obtain the EDX map accurately and quantitatively due to the influence of shadowing and the problem of low statistical accuracy, a quantitative analysis of chemical compositions by the point measurements was performed. Denticle and exocuticle Ca concentrations, respectively, were in the range of 24 and 33 wt%, as reported earlier [15,20,24]. Since epicuticle is a non-mineralized substrate, the amount of Ca in the region covered with epicuticle is lower than that in the region without epicuticle.…”

“…In other denticles, the high-magnification SEM micrograph (Figure 7(e,f)) at the position where the exocuticle disappeared on the denticle surface revealed that there is a twisted plywood structure on the columnar structure and that the plywood structure disappears at the white arrow. According to the observation results so far [15,20], the Sh decreased as it approached the surface, and was less than 1 µm in the outermost layer. However, the Sh shown in Figure 7 (f) was more than 1 µm.…”

“…Besides, a small amount of exocuticle (~100 µm thick), characterized by a twisted plywood pattern structure covering the denticle, was visible only in the red king crab, Paralithodes camtschaticus. In our previous paper [20], we showed that the calcium concentration in the claw denticles of the coconut crab, Birgus latro, was slightly lower than that in the exocuticle, phosphorus concentrations were almost zero, and magnesium concentrations were high as compared to the exocuticle and endocuticle. However, curiously, the maximum hardness (H max ) and stiffness (E r(max) ) near the denticle surface indicated almost the same values, H max = 4 GPa and E r(max) = 70 GPa.…”

“…A SEM image of the exoskeleton with a denticle and a high-magnification SEM image of the denticle surface are shown in Figure 1c,d. Details of the procedures and microstructures for this sample were given earlier [20].…”

“…With this microstructure, cracks tend to progress inside, and the claws are more likely to be damaged. However, the intermediate layer between the denticle and the endocuticle is as soft [20], so only the denticles are thought to be damaged or lost before the claws are fractured. If some denticles are damaged or lost, the other denticles maintain the function of the claws, then molt, and new denticles develop.…”