Sea spiders (Pycnogonida) are an abundant faunal element of the Southern Ocean (SO). Several recent phylogeographical studies focused on the remarkably diverse SO pycnogonid fauna, resulting in the identification of new species in previously ill-defined species complexes, insights into their genetic population substructures, and hypotheses on glacial refugia and recolonization events after the last ice age. However, knowledge on the life history of many SO pycnogonids is fragmentary, and early ontogenetic stages often remain poorly documented. This impedes assessing the impact of different developmental pathways on pycnogonid dispersal and distributions and also hinders pycnogonid-wide comparison of developmental features from a phylogenetic-evolutionary angle. Using scanning electron microscopy (SEM) and fluorescent nuclear staining, we studied embryonic stages and postembryonic instars of three SO representatives of the taxon Pallenopsidae (Pallenopsis villosa, P. hodgsoni, P. vanhoeffeni), the development of which being largely unknown. The eggs are large and yolk-rich, and the hatching stage is an advanced lecithotrophic instar that stays attached to the father for additional molts. The first free-living instar is deduced to possess at least three functional walking leg pairs. Despite gross morphological similarities between the congeners, each instar can be reliably assigned to a species based on body size, shape of ocular tubercle and proboscis, structure of the attachment gland processes, and seta patterns on cheliphore and walking legs. We encourage combination of SEM with fluorescent markers in developmental studies on ethanol-preserved and/or long term-stored pycnogonid material, as this reveals internal differentiation processes in addition to external morphology. Using this approach, we describe the first known cases of pallenopsid development with epimorphic tendencies, which stand in contrast to the small hatching larvae in other Pallenopsidae. Evaluation against current phylogenetic hypotheses indicates multiple gains of epimorphic development within Pycnogonida. Further, we suggest that the type of development may impact pycnogonid distribution ranges, since free-living larvae potentially have a better dispersal capability than lecithotrophic attaching instars. Finally, we discuss the bearing of pycnogonid cheliphore development on the evolution of the raptorial first limb pair in Chelicerata and support a multi-articled adult limb as the plesiomorphic state of the chelicerate crown group, arising ontogenetically via postembryonic segmentation of a three-articled embryonic limb.

中文翻译：

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首次描述南极Pallenopsidae（节肢动物门，Pycnogonida）的外形发育，并深入了解四文章海蜘蛛螯肢动物的进化

海蜘蛛 (Pycnogonida) 是南大洋 (SO) 中丰富的动物群。最近的几项系统发育地理学研究集中在极其多样化的 SO pycnogonid 动物群上，从而在以前定义不明确的物种复合体中识别出新物种，深入了解其遗传群体亚结构，并提出关于冰川避难所和最后一个冰河时代后的重新殖民事件的假设。然而，关于许多 SO pycnogonids 生活史的知识是零散的，早期个体发育阶段的记录往往很少。这阻碍了评估不同发育途径对重生殖器扩散和分布的影响，也阻碍了从系统发育-进化角度对重生殖器发育特征进行广泛比较。使用扫描电子显微镜 (SEM) 和荧光核染色，我们研究了代表苍蝇科 (Pallenopsis villosa、P. hodgsoni、P. vanhoeffeni) 的三种 SO 代表的胚胎阶段和胚胎后龄期，其发育很大程度上未知。卵很大，蛋黄丰富，孵化阶段是一个高级的卵磷脂龄期，它会附着在父亲身上以进行额外的蜕皮。据推断，第一个自由生活的幼虫拥有至少三对功能性步行腿。尽管同源体之间在形态上有相似之处，但根据体型大小、眼结节和长鼻的形状、附着腺突起的结构以及螯毛和行走腿上的刚毛模式，可以将每个龄可靠地分配给一个物种。我们鼓励在乙醇保存和/或长期保存的pycnogonid材料的发育研究中将SEM与荧光标记相结合，因为这不仅可以揭示外部形态，还可以揭示内部分化过程。使用这种方法，我们描述了第一个已知的具有外态倾向的苍蝇发育案例，这与其他苍蝇科的小型孵化幼虫形成鲜明对比。针对当前系统发育假设的评估表明，Pycnogonida 内的外态发育具有多重增益。此外，我们认为发育类型可能会影响密实分布范围，因为自由生活的幼虫可能比卵磷脂附着龄具有更好的分散能力。最后，我们讨论了螯肢动物发育对螯肢类猛禽第一肢对进化的影响，并支持多文章成年肢作为螯肢冠群的拟态状态，通过三文章胚胎的胚胎后分割而在个体发生上产生肢。