Revisiting the genus Mesobiotus (Eutardigrada, Macrobiotidae) – remarks, updated dichotomous key and an integrative description of new species from Madagascar

Abstract Species of the genus Mesobiotus are some of the most often recorded eutardigrades throughout the world. The majority of Mesobiotus species have been described based on morphology of the eggs, which is the most important character in this genus, from the taxonomic point of view. In this paper, we analysed in detail the morphology of eggs, oral cavity armature and claws in all known Mesobiotus species. Precise analysis of eggs allowed the recognition of their huge diversity and the division of Mesobiotus egg types based morphological characters of egg shell (general shape of processes, sculpture of processes, morphology of process bases and structure of egg shell). Similarly, we distinguished five different types of oral cavity armature based on number of rows of teeth that are visible under the light microscopy. We also noted the taxonomic value of claw modifications observed in some Mesobiotus species. Furthermore, we analysed other morphological traits (e.g. placoids, lunules, cuticle granulation) of adult specimens, which are often used for species differentiation in classical tardigrade taxonomy. Additionally, here we provide a diagnostic key to the currently known species of the genus and discuss in detail some problems with the species identifications. Finally, alongside with the comprehensive revision, we provide an integrative description of a new Mesobiotus species found in a moss sample collected from Madagascar – Mesobiotus fiedleri sp. nov.