Cryptic diversity constrains biogeographical inference in microscopic animals: evidence from bdelloid rotifers in Greenland

Daniel Stec, Filip Matura, Marco Antonio Jiménez Santos, Lyudmila Kamburska , Miloslav Devetter, Karel Janko, Diego Fontaneto 

Species are fundamental units of biodiversity, yet their delimitation remains challenging in many organismal groups. The increasing use of DNA data has revealed widespread cryptic diversity, in which genetically distinct lineages are morphologically indistinguishable. Consequently, many morphology-based biogeographical inferences have likely overestimated species ranges, particularly in microscopic animals traditionally considered ubiquitous. We investigated bdelloid rotifer diversity and biogeographical patterns in small aquatic habitats across Greenland, a poorly explored and geographically isolated region. Using morphological identification combined with mitochondrial COI sequencing and molecular species delimitation, we analysed Greenlandic populations in a global context. Seven morphologically defined species were recorded, all new for Greenland, and each represented a complex of cryptic lineages. We identified 24 putative species, most lacking close matches in public databases. Species richness estimates exceeded observed values, indicating extensive hidden diversity. Most putative species exhibited restricted distributions, whereas only a minority were shared with other Holarctic regions. Our results suggest that apparent ubiquity in bdelloid rotifers largely reflects unresolved cryptic diversity and limited sampling. Additional barcoding of tardigrades from the same samples revealed similar gaps in reference libraries for aquatic meiofauna. Overall, this study highlights the importance of DNA-based taxonomy for robust biogeographical inference in microscopic animals.