Saigusa M, Terajima M
Faculty of Science, Okayama University, Japan.
J Exp Zool. 2000 Dec 1;287(7):510-23.
Hatching of decapod crustaceans is characterized by the sudden rupture of the egg case. This study focused on the following two issues regarding the hatching mechanism of the estuarine terrestrial crab Sesarma haematocheir: (1) dissolution of the egg case, and (2) the site where the egg case breaks. The egg case comprises three layers: the outer two (E1 and E2) layers and the inner (E3) thin layer (0.2 microm in thickness). The outer layers showed no morphological changes upon hatching, but the inner layer (E3) was markedly digested. The digestion of this layer would enable the embryo to absorb ambient water via reverse peristalsis of the intestine, resulting in an increase of the volume. The egg case always ruptured perpendicular to the longitudinal axis of the embryo. In addition, breakage of the egg case occurred at the dorsal thorax of the embryo. The three major organs positioned at this area were (1) a sharp projection (dorsal spine), (2) an assemblage of muscles, and (3) a pair of secretory glands, each of which was about 30 microm in diameter. The dorsal projection is soft before hatching, and it is clear that the egg case does not break with the posterior expansion of this projection. The rupture instead appears to be caused by the expansion of the muscles arranged perpendicular to the body axis. In addition, some (unknown) factor might weaken the egg case just before hatching. The secretory glands may be a kind of rosette gland, but the role that this gland plays at hatching is not known. As a duct comes out from the center and enters the dorsal projection, some active substance may be released at the tip of this projection. However, immunochemical studies are not consistent with this substance being an ovigerous hair stripping substance (OHSS).
十足目甲壳动物的孵化以卵壳的突然破裂为特征。本研究聚焦于河口陆生蟹红螯相手蟹孵化机制的以下两个问题：（1）卵壳的溶解，以及（2）卵壳破裂的部位。卵壳由三层组成：外层的两层（E1和E2）以及内层（E3）的薄层（厚度为0.2微米）。孵化时外层未显示出形态变化，但内层（E3）被显著消化。这一层的消化会使胚胎通过肠道的反向蠕动吸收周围的水，从而导致体积增加。卵壳总是垂直于胚胎的纵轴破裂。此外，卵壳的破裂发生在胚胎的背胸部。位于该区域的三个主要器官分别是：（1）一个尖锐的突起（背棘），（2）一组肌肉，以及（3）一对分泌腺，每个分泌腺的直径约为30微米。孵化前背棘是柔软 的，很明显卵壳不会随着这个突起的向后扩张而破裂。相反，破裂似乎是由垂直于身体轴线排列的肌肉的扩张引起的。此外，在孵化前某个（未知）因素可能会使卵壳变弱。分泌腺可能是一种玫瑰状腺体，但该腺体在孵化时所起的作用尚不清楚。由于有一条导管从中心伸出并进入背棘，一些活性物质可能会在这个突起的尖端释放。然而，免疫化学研究并不支持这种物质是抱卵刚毛剥离物质（OHSS）的观点。
