Alexeeva Nina, Tamberg Yuta, Shunatova Natalia
Department of Invertebrate Zoology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russian Federation.
Department of Marine Science, University of Otago, 310 Castle Street, Dunedin, 9016, New Zealand.
Arthropod Struct Dev. 2018 May;47(3):299-317. doi: 10.1016/j.asd.2018.03.002. Epub 2018 Mar 21.
Sea spiders form a small, enigmatic group of recent chelicerates, with an unusual bodyplan, oligosegmented larvae and a postembryonic development that is punctuated by many moults. To date, only a few papers examined the anatomical and ultrastructural modifications of the larvae and various instars. Here we traced both internal and external events of the whole postembryonic development in Nymphon brevirostre HODGE 1863 using histology, SEM, TEM and confocal microscopy. During postembryonic development, larvae of this species undergo massive reorganization: spinning apparatus and chelar glands disappear; larval legs redifferentiate; three new segments and the abdomen are formed with their corresponding internal organs and appendages; circulatory and reproductive systems develop anew and the digestive and the nervous systems change dramatically. The body cavity remains schizocoelic throughout development, and no traces of even transitory coeloms were found in any instar. In Nymphon brevirostre, just like in Artemia salina LINNAEUS 1758 the heart arises through differentiation of the already existing schizocoel, and thus the circulatory systems of arthropods and annelids are not homologous. We found that classical chelicerate tagmata, prosoma and opisthosoma, are inapplicable to adult pycnogonids, with the most striking difference being the fate and structure of the seventh appendage-bearing segment.
海蜘蛛是螯肢动物中一个小型且神秘的类群，具有独特的身体结构、寡节幼虫以及由多次蜕皮构成的胚后发育过程。迄今为止，仅有少数论文研究了幼虫和不同龄期的解剖学及超微结构变化。在此，我们运用组织学、扫描电子显微镜（SEM）、透射电子显微镜（TEM）和共聚焦显微镜，追踪了1863年霍奇命名的短吻海蛛整个胚后发育过程中的内部和外部变化。在胚后发育过程中，该物种的幼虫经历了大规模的重组：纺器和螯腺消失；幼虫的腿重新分化；形成了三个新的体节以及腹部，并伴有相应的内部器官和附肢；循环系统和生殖系统重新发育，消化系统和神经系统也发生了显著变化。在整个发育过程中，体腔始终为裂体腔，在任何龄期都未发现哪怕是短暂的真体腔痕迹。在短吻海蛛中，就像在1758年林奈命名的卤虫中一样，心脏是通过已有裂体腔的分化产生的，因此节肢动物和环节动物的循环系统并非同源。我们发现，经典的螯肢动物体区，即前体部和后体部，并不适用于成年海蜘蛛，最显著的差异在于第七个具附肢体节的命运和结构。
