The paired claws in Gazami crabs, Portunus trituberculatus, are bilaterally asymmetrical, and asymmetry is remarkable on the distal two segments of the first pereiopod, that is, the dactylus and propodus. Shells are exclusively cracked by use of the right chela, representing handedness. In Gazami crabs, handedness is reversed after autotomy of the right chela. Our study focused on the ontogeny of handedness and the mechanism of handedness reversal. Morphologically, asymmetry was first detected in megalopa larvae where the right propodus was significantly larger than the left, as was the canine at the base of the right dactylus. Presumably, the rate of chelagenesis differed between the left and right chelae. With these morphological features, the right chela functioned as a crusher. The crusher exerted a closing force two to three times that of the cutter. With loss of the right crusher, the left chela was bigger than the regenerated right chela and was converted to the crusher. In contrast, the performance of the regenerated right chela deteriorated compared to that of the original right crusher, and exertion of full closing force was inhibited by the more active left chela. Furthermore, crabs with two crusher chelae did not clearly show handedness. A decrease in size and performance of the regenerated right chela can be explained by a default program hypothesis. In conclusion, a difference in the chelagenesis rate results in bilateral asymmetry of the two chelipeds, and then handedness is generated by neural regulation in the thoracic ganglion innervating these claws. Since handedness is reversed after autotomy, the thoracic ganglion would not be lateralized in Gazami crabs. A default program hypothesis is proposed to explain the ontogeny of bilateral chela asymmetry and handedness reversal.