Abstract
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 both species delimitation and the estimation of species diversity and distribution. Although for over a century this species has been recorded throughout the world, recent research indicates that records to date are likely to represent a species complex rather than a single cosmopolitan species. However, in order to recognise and name species diversity within the complex, an integrative redescription of the nominal species is first needed. Here, we describe an R. coronifer population collected from Spitsbergen, i.e., one of the two localities mentioned in the original description, 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 transcriptome sequencing. We propose replacement of the neotype designated in 1981 by Maucci and Ramazzotti, as it is impossible to verify whether the existing neotype is conspecific with specimens studied by Richters in 1903 and 1904. Finally, using newly obtained cytochrome c oxidase subunit I (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 of and species diversity within the genus. Moreover, this work has the potential to be the first tardigrade redescription to provide both genetic barcodes and a transcriptome of the species in question.
Highlights
The phylum Tardigrada consists of small invertebrates which inhabit terrestrial and aquatic habitats [1], with about 1300 species discovered so far [2,3,4]
The greatest promise in unravelling tardigrade species diversity comes with the growing use of the tools of integrative taxonomy, as genetic analyses enable the detection of cryptic and pseudocryptic taxa that fly beneath the radar of classical taxonomy
Taking into consideration the discussion above, as well as our present findings that multiple morphologically very similar species exist in the genus Richtersius, a request will be prepared and submitted to the International Commission of Zoological Nomenclature asking to set aside, under its plenary power [Art. 81], the existing neotype designated by Maucci & Ramazzotti [15] and to designate a new neotype from a population collected in one of the localities mentioned in the original description of the species and examined in the present study (i.e. Billefjorden, Svalbard, Norway, see Table 1 for more details) in order to promote stability of the nomenclature for the genus Richtersius
Summary
The phylum Tardigrada consists of small invertebrates which inhabit terrestrial and aquatic habitats [1], with about 1300 species discovered so far [2,3,4]. The tardigrade taxonomy is considered as challenging due to the small number of taxonomically informative characters and microscopic size of these animals [5]. The (2020) 6:2 very similar taxa, possibly each with a limited geographic range [7,8,9,10,11]. [5, 12]) This realization has resulted in an increased number of descriptions of species based on subtler traits. Despite the increasing resolution in the detection of tardigrade species diversity, out-dated descriptions and the lack of type material for the nominal taxa remain significant obstacles to species descriptions within a given group (e.g. a genus or a species complex)
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