Tissue structure and mechanical properties of the exoskeleton of the huge claws of the mud crab, Scylla serrata

Abstract

The mud crab, Scylla serrata, has huge claws in comparison with its body size. The color of the claw top’s finger surface changes from white to deep-mottled blue, and this discoloration was strongly associated with the change in hardness inside the finger cross section. With special attention to the discoloration points, the tissue structure of the exoskeleton was investigated via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction (XRD), and the mechanical properties were examined using Vickers hardness and nanoindentation tests. The exocuticle in the deep blue surface exoskeleton was as thin as that in other crustaceans, and the exoskeleton was occupied by the endocuticle with a twisted plywood structure. On the other hand, in the white surface exoskeleton, the thickness of the hard and dense exocuticle accounted for 52–59% of the exoskeleton thickness. This percentage increased at the claw tip. The hardness of the exocuticle was 2.5 times that of the endocuticle, and the microstructures and mechanical properties gradually varied at the boundary between the exo- and endocuticle. The mechanical properties were almost constant in the exocuticle, but calcium (Ca) concentrations decreased from the outer surface toward the boundary in that region and magnesium (Mg) concentrations increased. The change in the unit cell volume obtained via XRD suggested that some of the Ca atoms in the calcite crystal structure in that region were replaced with Mg atoms. Changes in crustacean coloration may help us to understand the tissue structure and mechanical properties within the exoskeleton. Graphical Abstract

Mechanical Resistance and Tissue Structure of Claw Denticles of Various Sizes in the Mud Crab, Scylla serrata

“…Calcium (Ca), magnesium (Mg), phosphorus (P), carbon (C), oxygen (O), and sodium (Na) were found to be the main components, and chloride and sulfur were present in minor amounts. This result was the same as that for the exoC in the mottled, deep-blue exoskeleton of the mud crab in the previous paper [13,14]. Here, the crystalline structures of the exoskeleton of the mud crab were calcite [14].…”

“…The value, 250HV0.01, for the two large denticles is comparable to that for the exoC of the coconut crab claw [18] and the maximum value for the exoC of the American lobster claw [21]. Here, den(S) and den(M) denote small and medium denticles, and den(L1) and den(L2) denote large denticles [14].…”

“…The black spots in the endoC near the inner surface indicate pore canals (pcs). These black spots were also observed on the endoC on the outer side of the claw finger in the previous paper [14] and often can be observed on the polished surface of the exoskeleton with a TPS in crustaceans [1,18,20,23,26]. On the other hand, in Line G, shown in Figure 7, since the large denticle surface had a dense tissue structure, it was not possible to clearly observe the microstructure.…”

“…The tests were performed at an interval of 50 µm from an outer surface to an inner surface. The hardness (H IT ) and reduced elastic modulus (E r ) were analyzed from the unloading curve using the Oliver-Pharr method [24] employed in biological studies [14,15,[19][20][21].…”

“…The TPS is generally referred to as the Bouligand structure, which is found in many biological materials [1]. On the other hand, in the white exoskeleton at the fingertip, the tissue had a TPS throughout the thickness, but the rigid and dense exoC occupied more than half of the exoskeleton thickness, and the hardness changed gradually at the boundary between the exoC and endoC [14]. The proportion of the exoC in the white exoskeleton was much higher than that in the mottled, deep-blue exoskeleton and in other crustaceans, including the brown crab, Cancer pagurus [15], the Chinese mitten crab, Eriocheir sinensis [16], the Atlantic blue crab, Callinectes sapidus, the Mediterranean green crab, Carcinus aestuarii [17], the coconut crab, Birgus latro [18][19][20], and the American lobster, Homarus americanus [21].…”