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

Abstract

Decapod crustaceans have tooth-like denticles on their claw fingers, which come into direct contact with predators and prey. Since the denticles are subject to more frequent and intense stress than other parts of the exoskeleton, they must be especially resistant to wear and abrasion. We clarified the mechanical resistance and tissue structure of the denticles arranged in a line on the fixed finger of the mud crab, which has huge claws. The denticles of the mud crab are small at the fingertip and become larger closer to the palm. The denticles have a twisted-plywood-pattern structure stacked parallel to the surface regardless of size, but the abrasion resistance strongly depends on the size of the denticles. Due to the dense tissue structure and calcification, the abrasion resistance increases as the denticle size increases, reaching its maximum at the denticle surface. The denticles of the mud crab have a tissue structure that prevents them from breaking when pinched. The high abrasion resistance of the large denticle surface is an essential feature for the frequent crushing of shellfish, which is the mud crab’s staple food. The characteristics and tissue structure of the claw denticles on the mud crab may provide ideas for developing stronger, tougher materials.

Mechanical Resistance of the Largest Denticle on the Movable Claw of the Mud Crab

“…Here, the results show the average weight %. Referred data (Fingertip [33]; Large denticle on the fixed finger [38]; Claw outer side [33]). As shown in Figures 5 and 6, there was an area where P was present only on the innermost surface of the denticle and the valley.…”

“…For comparison, the data for the fingertip, large denticle, and outer side of the fixed finger in the same claw [33,38] are also shown. The P concentrations of 0.8-1.0% in these areas agree with the P concentrations in the endocuticle of the fixed finger reported in previous papers [33,38]. Other inorganic components, 23.7-24.5% Ca and 1.4-2.2% Mg, also agree.…”

“…Here, the results show the average weight %. Referred data (Fingertip [33]; Large denticle on the fixed finger [38]; Claw outer side [33]). Consequently, most of the largest white cuticles, like the teeth of the movable finger, are denticles without P, and there is a layer corresponding to the endocuticle with P in the innermost part.…”

“…Figure 8 shows the relation with all data for Lines L1, L2, M1, M2, N1, and N2, as shown in Figure 2d,e. For comparison, the results for the endocuticle in the fixed finger's exoskeleton [33] and for white areas in the claws (large, medium, and small denticles on the movable finger and fingertip) [38] were also plotted. Data lying on a straight line of H 3 /E 2 , as shown in Figure 8, indicate materials with equivalent abrasion-resistance performances [39].…”