Abstract
Intra- and interspecific variability, being at the very core of alpha taxonomy, has been a long-standing topic of debate among tardigrade taxonomists. Early studies tended to assume that tardigrades exhibit wide intraspecific variation. However, with more careful morphological studies, especially those incorporating molecular tools that allow for an independent verification of species identifications based on phenotypic traits, we now recognise that ranges of tardigrade intraspecific variability are narrower, and that differences between species may be more subtle than previously assumed. The taxonomic history of the genus Milnesium, and more specifically that of the nominal species, M. tardigradum described by Doyère in 1840, is a good illustration of the evolution of views on intraspecific variability in tardigrades. The assumption of wide intraspecific variability in claw morphology led Marcus (1928) to synonymise two species with different claw configurations, M. alpigenum and M. quadrifidum, with M. tardigradum. Currently claw configuration is recognised as one of the key diagnostic traits in the genus Milnesium, and the two species suppressed by Marcus have recently been suggested to be valid. In this study, we clarify the taxonomic status of M. alpigenum, a species that for nearly a century was considered invalid. We redescribe M. alpigenum, using a population collected from the locus typicus, by the means of integrative taxonomy, i.e. including light microscopy, scanning electron microscopy, ontogenetic observations, and genetic barcoding. Moreover, the redescription of M. alpigenum allowed us to verify the uncertain taxonomic status of two popular laboratory models that were originally considered to be M. tardigradum; though one was recently reidentified as M. cf. alpigenum. Our analysis showed that both laboratory strains, despite being morphologically and morphometrically nearly identical to M. alpigenum, in fact represent a new species, M. inceptum sp. nov. The two species, being disnguishable only by statistical morphometry and/or DNA sequences, are the first example of pseudocryptic species in tardigrades.
Highlights
Tardigrades, known as water bears, are a phylum of microscopic invertebrates that dwell in marine, freshwater and terrestrial ecosystems (Nelson et al 2015)
Two further species of this genus were described: M. alpigenum Ehrenberg, 1853 from Monte Rosa (Italy/Switzerland) and M. quadrifidum Nederström, 1919 from Utsjoki, Finland. Both species were differentiated from M. tardigradum based on claw morphology (Morek et al 2016a), a trait currently termed the claw configuration (CC)
Individuals isolated from samples and/or cultures were split into 3–4 analysis groups: (i) development tracking, (ii) imaging and morphometry in phase contrast light microscope (PCM), (iii) imaging in scanning electron microscopy (SEM), and (iv) DNA sequencing; see Table 1 for details
Summary
Tardigrades, known as water bears, are a phylum of microscopic invertebrates that dwell in marine, freshwater and terrestrial ecosystems (Nelson et al 2015). The first formal descriptions of tardigrade species were published in the first half of the XIX century. The species was established as the nominal taxon for the genus Milnesium Doyère, 1840, family Milnesiidae Ramazzotti, 1962, and the order Apochela Schuster et al, 1980. Two further species of this genus were described: M. alpigenum Ehrenberg, 1853 from Monte Rosa (Italy/Switzerland) and M. quadrifidum Nederström, 1919 from Utsjoki, Finland. Both species were differentiated from M. tardigradum based on claw morphology (Morek et al 2016a), a trait currently termed the claw configuration (CC).
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