Kihm Ji-Hoon, Zawierucha Krzysztof, Rho Hyun Soo, Park Tae-Yoon S
Division of 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.
Zoological Lett. 2023 Nov 27;9(1):22. doi: 10.1186/s40851-023-00221-w.
Phylum Tardigrada is represented by microscopic eight-legged panarthropods that inhabit terrestrial and marine environments. Although tardigrades are emerging model animals for areas of research including physiology, evolutionary biology, and astrobiology, knowledge of their external morphology remains insufficient. For instance, homologies between marine and terrestrial relatives largely remain unexplored. In the present study we provide detailed pictures of the head sensory organs in a new tardigrade, Ramazzottius groenlandensis sp. nov. Specimens were collected from a mixed moss and lichen sample on Ella Island, East Greenland. The new species differs from congeneric species in the presence of polygonal sculpturing on the dorsal cuticle, which is accentuated in the posterior region of the body, a lateral papilla on leg IV, and distinctive egg morphology. A Bayesian phylogenetic analysis (18S rRNA + 28S rRNA + COI) places the new species within the genus Ramazzottius with high confidence. Interestingly, the new species shows a full set of well-developed cephalic organs, which correspond to all sensory fields found in eutardigrades. Details on the full set of head organs were present only for heterotardigrades. The surface of these organs is covered with small pores, which presumably play a sensory role. This discovery suggests the homology of head sensory structures between heterotardigrades and eutardigrades, implying that the distinctive arrangement and positioning of sensory organs on the head is a plesiomorphic feature of tardigrades. Moreover, we find that the Ramazzottius oberhaeuseri morphotype forms a morphogroup, not a monophyletic species complex.
缓步动物门由微小的八足泛节肢动物代表，它们栖息于陆地和海洋环境。尽管缓步动物在生理学、进化生物学和天体生物学等研究领域正成为新兴的模式动物，但其外部形态学知识仍不充分。例如，海洋和陆地近缘物种之间的同源性在很大程度上仍未得到探索。在本研究中，我们提供了一种新的缓步动物——格陵兰拉马佐蒂缓步动物（Ramazzottius groenlandensis sp. nov.）头部感觉器官的详细图片。标本采集自东格陵兰埃拉岛的苔藓和地衣混合样本。该新物种与同属物种的区别在于，其背侧角质层有多边形雕刻，在身体后部更为明显，第四条腿上有一个侧乳头，以及独特的卵形态。贝叶斯系统发育分析（18S rRNA + 28S rRNA + COI）将该新物种高度可靠地置于拉马佐蒂属内。有趣的是，该新物种显示出一整套发育良好的头部器官，与真缓步动物中发现的所有感觉域相对应。关于整套头部器官的细节仅在异缓步动物中有记载。这些器官的表面覆盖着小孔，推测起到感觉作用。这一发现表明异缓步动物和真缓步动物头部感觉结构具有同源性，意味着头部感觉器官独特的排列和定位是缓步动物的一个原始特征。此外，我们发现奥氏拉马佐蒂缓步动物（Ramazzottius oberhaeuseri）形态型形成一个形态类群，而非单系物种复合体。
