Tackling species boundaries of animals with variable body size yet conservative morphology—molecular analysis and morphometrics of an all-female species of Anystis (Trombidiformes: Anystidae) and sexual relatives, the whirligig mites

Anystis is one of the most ubiquitous and conspicuous genera of terrestrial mites. However, species-level identifications are nearly impossible because of vague species boundaries associated with limited taxonomic knowledge and poorly deciphered morphological variability. Using molecular markers (COI, 28S D6–D8) and morphometrics on 180 mites primarily from North America and Europe, we tested whether a single species of Anystis occupies these continents. Molecular datasets produced Bayesian trees supporting 5 putative species, which could be distinguished morphologically: a widespread Anystis ‘baccarum’ (Linnaeus, 1758) (dominant in North America and Europe), Anystis wallacei Otto, 1992 (South Africa), Anystis sp. B (Europe), Anystis sp. Q (Australia), and Anystis sp. T (Taiwan). Average COI intraspecific divergence was high (8.7% to 9.3%) for 3 species and low (0%) for A. wallacei. Maximum COI intraspecific divergences (13.4% to 14.6%) were high and overlapped with interspecific divergences (12.6% to 21.1%; average 17.3% ± 1.2). The average 28S intraspecific divergences were low (0% to 0.8%), strongly contrasting with 28S interspecific divergences (7.6% to 14.7%). Substantial intraspecific variations (30% to 80%) were observed in morphometric attributes such as the lengths of legs. Furthermore, for the all-female species tentatively identified as A. baccarum, setal lengths and other attributes are strongly correlated with body size proxies (primarily legs’ length) and are therefore of little diagnostic value. Attributes distinguishing species are primarily limited to the prodorsal shield and leg chemosensors (solenidia). Our study indicates that 28S complements the hypervariable COI barcode data and reflects meaningful morphological variation, which can be used integratively to delineate species in the morphologically conservative genus Anystis.