Exoskeletons of mud crabs, Scylla serrata, of different sizes: Body weight, surface morphology, internal tissue structure, and mechanical resistance

• The carapace surface of mud crab is composed of a bimodal bulge structure, with the diameters of 264.5 μm and 100.0 μm. • Surface morphology of exoskeleton, exocuticle thickness, and mechanical properties in endocuticle are weight-independent. • Endocuticle thickens with weight, suggesting molting-stage influence. Few studies have simultaneously examined shell size and changes in the surface and interior of the exoskeleton across a wide range of body weights in mud crabs. This study first analyzed the relationship between the sex, weight, and shell size of 131 mud crabs caught in Okinawa, and compared the findings with data from seven regions. Next, using laser microscopy and nanoindentation, the study examined how surface morphology, internal structure, exoskeleton thickness, hardness, and Young’s modulus change with body weight ( BW ) in 18 male crabs, weighing 249 g to 1920 g. The growth coefficients of mud crabs in Okinawa were 3.52 for males and 2.91 for females, and these values were the largest among those reported so far in various countries. The exoskeleton surface in mud crabs consisted of a bimodal bulge structure with a large bulge 264.5 μm in diameter and a small bulge 100 μm in diameter. Such surface morphology and thickness of the hard exocuticle of 150 μm were independent of BW . On the other hand, the endocuticle thickness increased with BW . Even if the exoskeleton feels hard, the endocuticle may be thin because it is not fully developed by the effect of molting.