Tamori Masaki, Koki Jun, Motokawa Tatsuo
1 School of Life Science and Technology, W3-42, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan.
2 Center for Advanced Materials Analysis, S7-18, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan.
Zoolog Sci. 2018 Feb;35(1):92-98. doi: 10.2108/zs170109.
Sea urchins have a globiferous pedicellaria that stands from a test with a stalk on which lies a head made of three movable jaws with venom-injecting teeth. The globiferous pedicellariae of the flower sea urchin Toxopneustes pileolus, one of the most developed among sea urchins, are unique in that the jaws are provided with a jaw membrane that gives the pedicellaria an appearance of a flower when the jaws are open. We observed this membrane in an ionic liquid that does not require processes, such as fixation, dehydration, or coating with conductive materials, for observation with a scanning electron microscope. Using this technique, we discovered dumbbell-shaped ossicles, which consist of two spheres of similar size connected by a cylinder. The diameter of the sphere is 4-8 µm, and the total length of the ossicle is 10-20 µm. The jaw membrane is trimmed with an edge zone. The ossicles were found sparsely in the connective tissue of general part of the membrane, but in the edge zone their density was so high that neighboring ossicles were in close contact with each other. Some neighboring ossicles crossed their cylinders and some inserted one of their spheres to snugly fit in the gap between the spheres of neighboring ossicles. Their structural role is very likely in strengthening the jaw membrane, probably serving as fillers in the general part of the membrane; in the edge zone the interlocking of adjacent ossicles forms a loose network providing a firm frame for the head of the globiferous pedicellaria. When opened, the stiff frame prevents the membrane from sagging. When clasped, it works as a closed door, firmly keeping prey trapped.
海胆有球形叉棘，它从外壳伸出，带有一个柄，柄上有一个由三个可活动的颚组成的头部，颚上长有毒牙。花海胆（Toxopneustes pileolus）是海胆中发育最为完善的种类之一，其球形叉棘的独特之处在于，颚上有一层颚膜，当颚张开时，叉棘看起来像一朵花。我们在一种离子液体中观察到了这种膜，使用扫描电子显微镜观察时，无需进行固定、脱水或用导电材料涂层等处理。利用这项技术，我们发现了哑铃形小骨，它由两个大小相似的球体通过一个圆柱体连接而成。球体直径为4 - 8微米，小骨的总长度为10 - 20微米。颚膜边缘有一个边缘区。在膜一般部分的结缔组织中，小骨分布稀疏，但在边缘区，它们的密度很高，相邻的小骨彼此紧密接触。一些相邻的小骨交叉它们的圆柱体，还有一些将其中一个球体插入相邻小骨球体之间的缝隙中紧密贴合。它们的结构作用很可能是加强颚膜，在膜的一般部分可能起到填充物的作用；在边缘区，相邻小骨的相互连接形成一个松散的网络，为球形叉棘的头部提供一个坚固的框架。当张开时，坚硬的框架可防止膜下垂。当闭合时，它就像一扇关闭的门，牢牢困住猎物。
