The causes of asymmetry in the chelae of the mud crab <i>Scylla paramamosain</i> Estampador, 1950 (Decapoda: Brachyura: Portunidae)

Abstract The asymmetry trait in brachyuran crabs is mostly expressed by the different sizes or particular details of the two chelae. The causes for this asymmetry have been rarely investigated in the economically important crab Scylla paramamosainEstampador, 1950. We found that the asymmetric chelae first appeared in the megalopa stage. The examination of four “right-handed” and two “left-handed” mothers and their 521 megalopa offspring indicated that this trait might not be influenced by dominant alleles, since 86.0% –99.0% of megalopae exhibited the right-handed phenotype in the four offspring of the right-handed mother, whereas about 93.1% and 100% megalopae exhibited the right-handed phenotype in the two left-handed-mothers’ offspring. The percentage of left-handed individuals at different stages also supports this hypothesis. We provide evidence that the amputation of the handed chela will result in a “handedness reversal,” and that the food type might influence the speed of the reversal. Crabs that were fed with fish blocks had a faster handedness reversal than those who were fed with commercial pellets feed, perhaps because of the increase in effort in individuals feeding on fish blocks.