Asymmetric body traits in bilateral organisms are common and serve a range of different functions. In crustaceans, specifically among brachyuran crabs, heterochely and handedness in some species are known to aid in behavioural responses such as food acquisition, and sexual and territorial displays. However, the heterochely of the intertidal mud crab genus Scylla is still poorly understood. This study investigated the cheliped morphometric characteristics of orange mud crab Scylla olivacea and the relation of heterochely and handedness to sex. Scylla olivacea is heterochelous, with predominant right-handed (70.2%). Three morphometric variables, i.e., propodus length (PL), propodus depth (PD), and propodus width (PW) were significantly larger in the right cheliped and the estimated handedness based on these three variables were consistent with the presence of molariform teeth. The effect of sex had no influence on the occurrence of heterochely or handedness. The frequency of left-handedness increased with size, especially in males. We postulate that handedness reversal, a phenomenon seen in other crab species when the dominant hand is lost, also occurs in S. olivacea, thereby resulting in a change in left-handedness frequency. The use of chelipeds by males in mate and territorial defenses might provide an explanation for the higher risk of losing a dominant cheliped and thus, higher left-handedness frequency compared to females. Future behavioural research could shed light on the selective forces that affect the handedness distribution in mud crabs. Knowledge on heterochely and handedness of mud crabs could be useful for future development of less aggressive crab populations by claw reversal and the optimisation of limb autotomy techniques.

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Male and female mud crabs may look alike at first glance, but clear differences in physical characteristics exist. These variations between sexes are known as sexual dimorphism. Sexual dimorphism may help species to survive in their environments. We found that three mud crab species (orange, green, and purple mud crabs) showed similar patterns of sexual dimorphism. Males generally have larger body and claw sizes, whereas females have larger abdomens. Orange mud crabs also have a unique body shape variation compared to green and purple mud crabs. All mud crabs have spines along the top edge of their shells, and scientists have numbered these spines to identify them. Unlike the green and purple species, orange mud crabs are wider at spine 8 than spine 9. Sexual dimorphism in crabs might be linked to how they reproduce and where they live.