Tardigrada are a phylum of cosmopolitan invertebrates inhabiting both terrestrial and marine ecosystems. The carnivorous species are usually characterized by a wide buccal tube, which constitutes the first and rigid part of the digestive system. Among tardigrades, only the genus Milnesium is considered exclusively carnivorous and includes the largest known species in the phylum. Some members of the genus exhibit developmental variability in taxonomically important morphological traits such as the number of points on the secondary branches of claws and cuticular sculpturing. Milnesium eurystomum is one of the largest tardigrades, with body length reaching 1.2 mm, and it is equipped with an exceptionally wide buccal tube, enabling this animal to feed on various prey, including other tardigrade species. Importantly, M. eurystomum, as well as other species with wide buccal tubes, were described solely using mature females, thus their developmental variability remains unknown. Interestingly, small Milnesium specimens with remarkably wide buccal tubes have never been reported, raising a question of whether such hatchlings and juveniles (first and second life stages) are extremely rare and simply have never been reported, or their buccal apparatus morphology is different from adults and thus constitutes as an undescribed buccal tube developmental variability. To answer this question, we analysed four populations of M. eurystomum originating from Greenland, Svalbard and Scotland. Due to discrepancies in species delineation based on different types of data, we parallelly applied various methods of species delimitation, both morphological and molecular. Thanks to ontogenetic analysis, we show and describe a novel type of developmental variability in the size and shape of the buccal tube. We also provide novel morphological traits and data for this species, together with the first record and characterization of males. Finally, we discuss the consequences of our findings for the taxonomy of Milnesium and tardigrades in general.

中文翻译：

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Milnesium eurystomum Maucci, 1991 的种群间和种群内变异性分析揭示了高遗传差异和 Apochela 目中的一种新型个体遗传变异

缓步动物是生活在陆地和海洋生态系统中的世界性无脊椎动物门。肉食性物种通常以宽阔的颊管为特征，它构成了消化系统的第一个和刚性部分。在缓步动物中，只有 Milnesium 属被认为是完全肉食性的，并且是该门中已知最大的物种。该属的一些成员在分类学上重要的形态特征方面表现出发育变异性，例如爪子次级分支上的点数和表皮雕刻。Milnesium eurystomum 是最大的缓步动物之一，体长达到 1.2 毫米，它配备了异常宽的颊管，使这种动物能够捕食各种猎物，包括其他缓步动物。重要的是， M. eurystomum, 以及其他具有宽颊管的物种，仅使用成熟的雌性进行描述，因此它们的发育变异性仍然未知。有趣的是，从未报道过具有非常宽的颊管的小型 Milnesium 标本，这引发了一个问题，即这种幼体和幼体（第一和第二生命阶段）是否极其罕见且从未被报道过，或者它们的颊器官形态与成体不同因此构成了未描述的颊管发育变异性。为了回答这个问题，我们分析了来自格陵兰、斯瓦尔巴和苏格兰的四个 eurystomum 种群。由于基于不同类型数据的物种划分存在差异，我们并行应用了各种形态和分子的物种划分方法。由于个体发育分析，我们展示并描述了一种新型的颊管大小和形状的发育变异性。我们还提供了该物种的新形态特征和数据，以及雄性的首次记录和特征。最后，我们总体上讨论了我们的发现对 Milnesium 和缓步动物分类的结果。