Morphological and molecular analyses on Richtersius (Eutardigrada) diversity reveal its new systematic position and lead to the establishment of a new genus and a new family within Macrobiotoidea

Important contributions have been made to the systematics of Eutardigrada in recent years, but these have also revealed that several taxa are polyphyletic and that cryptic species are present. To shed light on the taxonomy and systematic position of the genus Richtersius (Eutardigrada, Macrobiotoidea), six populations attributed to Richtersius coronifer were collected and analysed from morphological (light and scanning electron microscopy) and molecular (mitochondrial cytochrome oxidase subunit 1, 18S, 28S) points of view. In particular, a new morphometric index (claw common tract: length of the common tract of the claw/total claw length 9 100) and a new morphological character (stalk system) were introduced. Our integrative study was able to unveil the ‘cryptic’ species diversity within Richtersius, showing that the genus contains more than one evolutionary lineage. A morphological peculiarity in the animals of all lineages is the dimorphism in the morphology of the cuticle. Cuticular pores are present in the newborns and are lost with the first moult; this morphological change represents a novelty in the life cycle of eutardigrades. The phylogenetic analyses carried out on Richtersius populations and other Macrobiotoidea show that Richtersius is closely related to Macrobiotus islandicus, whereas Adorybiotus granulatus is more related to Richtersius and M. islandicus than to other members of the genus Macrobiotus (type genus of Macrobiotidae); therefore, the genus Macrobiotus and the family Macrobiotidae are not monophyletic. Based on these results, the new genus Diaforobiotus (for M. islandicus) and the new family Richtersiidae (composed of Richtersius, Diaforobiotus gen. nov., and Adorybiotus) are established.