White Sea Biological Station, Zoological Institute, Russian Academy of Sciences, Saint-Petersburg, St. Petersburg, Russian Federation.
Department of Marine Science, University of Otago, Dunedin, New Zealand.
J Morphol. 2022 Mar;283(3):296-312. doi: 10.1002/jmor.21443. Epub 2022 Jan 13.
Nymphon grossipes is a common subtidal species belonging to a small and unique group of chelicerates, that is, the sea spiders. These animals have an anamorphic phase during post-embryonic development and often hatch as small, oligomeric and exotrophic larvae (protonymphons) with four postocular segments, cheliphores, and two pairs of larval legs. A common alternative to protonymphons is a large lecithotrophic larval type, where animals hatch at more advanced stages and have a foreshortened anamorphic development. Based on external morphology, N. grossipes was believed to be an intriguing intermediate between these two conditions and its hatchlings were called "lecithotrophic protonymphons." Here, we examine the anatomy and ultrastructure of instars I and II and review the variety of roles of larval appendages and associated glands in other sea spiders in order to correctly place the larva of this species among pycnogonid larval types. Compared to "typical protonymphons," N. grossipes young hatch with an advanced segmental and appendage composition: six postocular segments instead of four, buds of walking legs 1 and hidden buds of walking legs 2. This state corresponds to the instars II/III (rather than larvae) of Nymphon brevirostre and Pycnogonum litorale. Modifications of the larval appendages, chelar, and spinning glands are aligned with ecological needs of different larval types along a few typical dimensions: locomotion and feeding, dispersal, and attachment to the parent. Although the main challenge for N. grossipes young is secure attachment to the egg package while they growth, there are some discrepancies in their anatomy: N. grossipes retains an oyster basket, but an otherwise nonfunctional digestive system, and a strong silken thread for attachment, but no corresponding reduction of the larval legs. Thus, it is likely that the switch to lecithotrophy happened in the recent evolutionary history of this species.
粗手长足蛛是一种常见的亚潮间带物种，属于一小群独特的螯肢动物，即海蜘蛛。这些动物在胚胎后发育过程中具有变态阶段，通常孵化为小的、寡聚的、外营养的幼虫（原若蛛），具有四个眼后节、螯肢和两对幼虫腿。原若蛛的常见替代物是大型的卵黄营养型幼虫，其中动物在更高级的阶段孵化，具有缩短的变态发育。基于外部形态，N. grossipes 被认为是这两种情况之间的一个有趣的中间状态，其幼虫被称为“卵黄营养型原若蛛”。在这里，我们检查了若虫 I 和 II 的解剖结构和超微结构，并回顾了幼虫附肢和相关腺体在其他海蜘蛛中的各种作用，以便正确地将该物种的幼虫归入 pycnogonid 幼虫类型。与“典型的原若蛛”相比，N. grossipes 幼蛛孵化时具有更高级的节段和附肢组成：六个眼后节而不是四个，步行腿 1 的芽和隐藏的步行腿 2 的芽。这种状态对应于 Nymphon brevirostre 和 Pycnogonum litorale 的若虫 II/III（而不是幼虫）。幼虫附肢、螯肢和纺丝腺的改变与不同幼虫类型沿几个典型维度的生态需求相匹配：运动和进食、扩散和附着在亲代上。尽管 N. grossipes 幼蛛的主要挑战是在它们生长的同时安全地附着在卵包上，但它们的解剖结构存在一些差异：N. grossipes 保留了一个牡蛎篮，但没有功能的消化系统，以及用于附着的强丝，但没有相应减少幼虫腿。因此，很可能是在该物种的最近进化历史中发生了向卵黄营养的转变。
