Kihm Ji-Hoon, Zawierucha Krzysztof, Rho Hyun Soo, Park Tae-Yoon S
Division of Glacier & Earth Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon, 21990, Korea.
Department of Animal Taxonomy and Ecology, Faculty of Biology, Adam Mickiewicz University, Uniwersytetu Poznańskiego 6, Poznań, 61-614, Poland.
Sci Rep. 2025 Jul 2;15(1):22556. doi: 10.1038/s41598-025-06766-4.
Tardigrades are microscopic panarthropods renowned for their ability to undergo cryptobiosis. While integrative taxonomy of tardigrades has been intensively applied in the description of tardigrade species over the past two decades, many details of their external morphology remain poorly recognized and under-described. This limitation is largely due to their small size and the limited morphological features useful for classical taxonomy. In particular, compared to heterotardigrades, the external sensory structures of eutardigrades are less studied. In this study, we present a detailed morphological analysis using FE-SEM of two Greenlandic apochelan species, Milnesium grandicupula sp. nov. and M. cf. longiungue. We identified several previously unobserved anatomical structures in milnesiids, including minute sensory structures in the cephalic region. Detailed microphotographs revealed that apochelan tardigrades also possess a set of external sensory structures on the head sensory fields, similar to those observed in other major evolutionary lineages within tardigrades. This finding corroborates the hypothesis of the homology of head sensory organs between eutardigrades and heterotardigrades. Notably, these small sensory structures exhibit a distinctive pattern: a relatively large central organ surrounded by smaller pores, which may be comparable to the sensory dorsal organ (SDO) of crustaceans and the cephalic median organ (CMO) of trilobites.
缓步动物是微小的泛节肢动物，以其进行隐生的能力而闻名。在过去二十年中，缓步动物的综合分类学已被广泛应用于缓步动物物种的描述，但它们外部形态的许多细节仍未得到充分认识和描述。这种局限性主要是由于它们体型小，以及对经典分类学有用的形态特征有限。特别是，与异缓步动物相比，真缓步动物的外部感觉结构研究较少。在本研究中，我们使用场发射扫描电子显微镜（FE-SEM）对两种格陵兰岛无甲缓步动物物种，即大杯米氏缓步虫（Milnesium grandicupula sp. nov.）和疑似长米氏缓步虫（M. cf. longiungue）进行了详细的形态分析。我们在米氏缓步虫中发现了几个以前未观察到的解剖结构，包括头部区域的微小感觉结构。详细的显微照片显示，无甲缓步动物在头部感觉区域也拥有一组外部感觉结构，类似于在缓步动物其他主要进化谱系中观察到的结构。这一发现证实了真缓步动物和异缓步动物头部感觉器官同源性的假设。值得注意的是，这些小的感觉结构呈现出一种独特的模式：一个相对较大的中央器官被较小的孔包围，这可能与甲壳类动物的感觉背器官（SDO）和三叶虫的头部中央器官（CMO）相当。
