Heterochely and handedness in the orange mud crab Scylla olivacea: implication for future culture practice optimisation.

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.

Eyestalk Ablation to Increase Ovarian Maturation in Mud Crabs.

Mud crabs (Scylla spp.) are commercially important crustacean species that can be found throughout the Indo-West Pacific region. During culture, the induction of ovarian maturation is important to meet the consumer demand for mature mud crabs and hasten seed production. Eyestalk ablation is an effective tool to enhance ovarian maturation in mud crabs. However, there is no standard protocol for the eyestalk ablation of mud crabs. In this study, two eyestalk ablation techniques are described: cauterization (the use of hot metal to ablate the eyestalk of an anesthetized crab) and surgery (the removal of the eyestalk using surgical scissors). Before eyestalk ablation, sexually mature females (CW > 86 mm) were anesthetized using an ice bag (-20 °C) with seawater. When the water temperature reached 4 °C, the ice bag was removed from the water. Flowing seawater (ambient temperature: 28 °C) was used for recovery from the anesthesia immediately after eyestalk ablation. Mortality did not occur during or after the process of eyestalk ablation. The eyestalk ablation protocol presented here accelerated the ovarian maturation of the mud crabs.