Cytologie und Evolutionsbiologie, Zoologisches Institut und Museum, Universität Greifswald, Greifswald, Germany.
Neuroscience Program, Wellesley College, Wellesley, Massachusetts, USA.
J Morphol. 2021 Feb;282(2):278-290. doi: 10.1002/jmor.21303. Epub 2020 Nov 30.
Individuals of the marine chelicerate lineage Pycnogonida (sea spiders) show considerable regenerative capabilities after appendage injury or loss. In their natural habitats, especially the long legs of sea spiders are commonly lost and regenerated, as is evidenced by the frequent encounter of specimens with missing or miniature legs. In contrast to this, the collection of individuals with abnormally developed appendages or trunk regions is comparably rare. Here, we studied a remarkable malformation in a postlarval instar of the species Phoxichilidium femoratum (Rathke, 1799) and describe the external morphology and internal organization of the specimen using a combination of fluorescent histochemistry and scanning electron microscopy. The individual completely lacks the last trunk segment with leg pair 4 and the normally penultimate trunk segment bears only a single aberrant appendage resembling an extension of the anteroposterior body axis. Externally, the proximal units of the articulated appendage are unpaired, but further distally a bifurcation into two equally developed leg-like branches is found. Three-dimensional reconstruction of the musculature reveals components of two regular leg muscle sets in several of the proximal articles. This confirms interpretation of the entire appendage as a malformed leg and reveals an externally hidden paired organization along its entire proximodistal axis. To explain the origin of this unique malformation, early pioneering studies on the regenerative potential of pycnogonids are evaluated and (a) an injury-induced partial fusion of the developing limb buds of leg pair 3, as well as (b) irregular leg regeneration following near complete loss of trunk segments 3 and 4 are discussed. Which of the two hypotheses is more realistic remains to be tested by dedicated experimental approaches. These will have to rely on pycnogonid species with established laboratory husbandry in order to overcome the limitations of the few short-term regeneration studies performed to date.
海洋节肢动物 Pycnogonida（海蜘蛛）的个体在附肢损伤或缺失后表现出相当大的再生能力。在其自然栖息地中，特别是海蜘蛛的长腿经常丢失和再生，这从经常遇到缺少或微型腿的标本中可以得到证明。与此相反，具有异常发育的附肢或躯干区域的个体的收集相对较少。在这里，我们研究了 Phoxichilidium femoratum（Rathke，1799）幼虫后期个体的一个显著畸形，并使用荧光组织化学和扫描电子显微镜相结合的方法描述了标本的外部形态和内部组织。个体完全缺乏第四对腿的最后一个躯干节段，正常的倒数第二个躯干节段仅带有一个单一的异常附肢，类似于前后身体轴的延伸。外部，关节附肢的近端单位是不成对的，但进一步向远端发现分叉成两个同样发达的腿状分支。肌肉的三维重建显示，几个近端文章中的两个常规腿肌组的成分。这证实了整个附肢被解释为畸形腿，并揭示了在其整个近-远轴上隐藏的外部成对组织。为了解释这种独特畸形的起源，对 Pycnogonida 再生潜力的早期开拓性研究进行了评估，并讨论了(a)发育中的第三对腿芽的部分融合，以及(b)在躯干节段 3 和 4 几乎完全缺失后不规则的腿再生。这两个假设中哪一个更现实，还有待专门的实验方法来检验。这些方法将不得不依赖于具有既定实验室饲养的 Pycnogonida 物种，以克服迄今为止进行的少数短期再生研究的局限性。
