Replacing the neotype of Richtersius coronifer, supported by transcriptome, provides resources for describing concealed species diversity within the monotypic genus Richtersius (Tardigrada)

Richtersius coronifer, the nominal species for the family Richtersiidae and a popular laboratory model, exemplifies a common problem in modern tardigrade taxonomy. Despite undeniable progress in the field, many old and incomplete descriptions of taxa hinder species delimitation as well as estimation of species diversity and distribution. Although for over a century this species was recorded throughout the world, recent research indicates that these records likely represent a species complex rather than a single cosmopolitan species. However, in order to recognise and name species diversity within the complex, first, an integrative redescription of the nominal species is needed. Here, we describe an R. coronifer population collected from the original locus typicus in Spitsbergen, with detailed morphological and morphometric data associated with standard DNA sequences of four standard genetic markers (18S rRNA, 28S rRNA, ITS-2 and COI) and supported by whole genome sequencing. We also propose to replace the neotype designated in 1981 by Maucci and Ramazzotti because it is impossible to verify whether the existing neotype is conspecific with specimens from the original type locality. Finally, using newly obtained COI sequences of populations from Spitsbergen, Italy, Poland and Greece together with sequences deposited in GenBank (China, Greenland, Italy, Mongolia), we performed genetic species delimitation which indicated seven distinct potential species within the genus Richtersius, in addition to the nominal taxon. This study marks a starting point for further research on the taxonomy and species diversity within the genus. Moreover, this work has a potential to be the first tardigrade redescription that not only does provide genetic barcodes but also a transcriptome of the species in question.