Phylum Tardigrada: an “individual” approach

Abstract

Phylum Tardigrada consists of 1000 tiny, hardy metazoan species distributed throughout terrestrial, limno-terrestrial and oceanic habitats. Their phylogenetic status has been debated, with current evidence placing them in the Ecdysozoa. Although there have been efforts to explore tardigrade phylogeny using both morphological and molecular data, limitations such as their few morphological characters and low genomic DNA concentrations have resulted in restricted taxonomic coverage. Using a protocol that allows us to identify and extract DNA from individuals, we have sequenced 18S rDNA from 343 tardigrades from across the globe. Using maximum parsimony and Bayesian analyses we have found support for dividing Order Parachela into three superfamilies and further evidence that indicates the traditional taxonomic perspective of families in the class Eutardigrada are nonmonophyletic and require re-working. It appears that conserved morphology within Tardigrada has resulted in conservative taxonomy as we have found cases of several discrete lineages grouped into single genera. Although this work substantially adds to the understanding of the evolution and taxonomy of the phylum, we highlight that inferences gained from this work are likely to be refined with the inclusion of further taxa-specifically representatives of the nine families yet to be sampled.

Genetic diversity in the parthenogenetic reproducing tardigrade Echiniscus testudo (Heterotardigrada: Echiniscoidea)

“…Very different haplotypes are present within a single sample (see for example sample 10 at locality T315 with haplotypes Et3, Et12 and Et18; Figs. 1 and 3), stressing the point that pooling of specimens in DNA extractions should be avoided, especially in population genetic studies as recently stated (Sands et al, 2008;Guil and Giribet, 2009). No general pattern of the distribution of haplotypes was evident, as some haplotypes were common at all three localities, whereas other haplotypes were numerous at one locality and scarce at other localities.…”

Integrative description of Macrobiotus canaricus sp. nov. with notes on M. recens (Eutardigrada: Macrobiotidae)

“…recens, phylogenetic trees were constructed using (1) all macrobiotid 18S rRNA sequences available from GenBank, (2) concatenated 18S rRNA+28S rRNA+ITS-2+COI sequences of macrobiotid species for which at least three of these markers were sequenced, and (3) all published M. hufelandi group COI sequences. In addition to the sequences of the hufelandi group listed in Table 3, we also used sequences of other species of the family Macrobiotidae published so far: Sands et al (2008), Guidetti et al (2009), Guil & Giribet (2012, Bertolani et al (2014), Mapalo et al (2016), Vecchi et al (2016), Zawierucha et al (2016), Mapalo et al (2017), Stec & Kristensen (2017), Stec et al (2018b). The sequences of Milnesium variefidum Morek et al, 2016aand Mi.…”

Myoanatomy of the marine tardigrade Halobiotus crispae (Eutardigrada: Hypsibiidae)

“…These difficulties seem especially pronounced in heterotardigrades belonging to the family Echiniscidae as well as in eutardigrades in general, as the demarcation of individual species is based on few ''minute characteristics'' (Kinchin, 1994). This confusion has in some instances led to the erection of ''species groups,'' or cryptic species, within Heterotardigrada as well as Eutardigrada, such as, e.g., Echiniscus testudo (see Jørgensen et al, 2007), Ramazzottius oberhaeuseri (see Faurby et al, 2008), Macrobiotus hufelandi, and Milnesium tardigradum (see Nelson and Bartels, 2007;Sands et al, 2008), in which species similar to the type species have been placed within a single ''species group,'' even though large intraspecific variations have long been recognized. Consequently, these challenges inherent to tardigrade taxonomy emphasize the importance in discovering new taxonomic characters possessing phylogenetic significance, which may aid in resolving some of the existing taxonomic issues.…”