Chela asymmetry in a durophagous crab: predominance of right handedness, and handedness reversal linked with chela size and closing force

Abstract

Portunus trituberculatus is a durophagous brachyuran. Right-handed crabs are predominant, but left-handed crabs are also found in nature. Left-handedness may arise from loss of the right crusher. We examined whether heterochely (morphology) was correlated with differences in closing force (physical property) and handedness (behaviour). The closing force was stronger in the larger chela and larger apodeme height, and handedness resided in the chela with stronger closing force. With loss of the right chela (autotomy), handedness was transitioned from the right to left chela, and all crabs were left-handed thereafter. Reversed handedness was accompanied with reduction of size and closing force in the regenerated right chela, and growth of the original left chela. After handedness reversal, dentition on the left dactylus of the newly-converted crusher was close to that of the original right crusher, but did not attain the same shape even 10 times of molt. So left-handed crabs were significantly worse at crushing hard-shelled preys than right-handed crabs. Chela formation was symmetric in the zoea; and heterochely and right handedness started in the megalopa, regardless of maternal handedness. Since the left chela is capable of being the crusher, heterochely may be caused by differences in morphogenetic velocity between the right and left chelae, under a signal discriminating right from left. Right handedness is an attribute of P. trituberculatus, and would be inheritable across generation. It is probable that right handedness was employed in the earliest durophagous crabs, and this trend has been succeeded to extant species.