Masunari Nobufumi, Sekiné Kazuki, Kang Bong Jung, Takada Yoshitake, Hatakeyama Masatsugu, Saigusa Masayuki
Biol Bull. 2020 Feb;238(1):25-40. doi: 10.1086/707648. Epub 2020 Feb 21.
The paired claws in Gazami crabs, , 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.
梭子蟹的一对螯足呈双侧不对称，且在第一对步足的远端两节，即指节和掌节上不对称尤为明显。蟹壳仅通过使用右螯来敲裂，这体现了用螯的偏好。在梭子蟹中，右螯自切后用螯偏好会发生反转。我们的研究聚焦于用螯偏好的个体发育以及用螯偏好反转的机制。从形态学上看，在大眼幼体中首次检测到不对称，此时右掌节明显大于左掌节，右指节基部的齿也是如此。据推测，左右螯的螯形成速率不同。基于这些形态特征，右螯起到碾碎器的作用。碾碎器施加的闭合力是切割器的两到三倍。随着右碾碎器的缺失，左螯比再生的右螯更大，并转变为碾碎器。相比之下，再生右螯的性能相较于原来的右碾碎器有所下降，且左螯更活跃抑制了其完全闭合力的发挥。此外，拥有两个碾碎螯的螃蟹并没有明显表现出用螯偏好。再生右螯尺寸和性能的下降可以用默认程序假说来解释。总之，螯形成速率的差异导致了两个螯足的双侧不对称，然后通过支配这些螯的胸神经节中的神经调节产生了用螯偏好。由于自切后用螯偏好会反转，梭子蟹的胸神经节不会出现侧化。我们提出了一个默认程序假说来解释双侧螯不对称的个体发育和用螯偏好反转。
