Hidden lineages of Monatractides madritensis (K. Viets, 1930) across Europe and Türkiye: evidence from COI Barcoding

Monatractides madritensis (K. Viets, 1930) is a common water mite species traditionally considered to be widely distributed across Europe and Asia Minor. In the recent water mite literature, this taxon includes at least seven junior synonyms. In this study, we conducted COI-based phylogenetic analyses, which recovered three well-supported clades corresponding to distinct Barcode Index Numbers (BINs). The first clade (BOLD:AED3803) comprises specimens ranging from Portugal to Serbia and can be regarded as the nominotypical lineage. The second clade includes specimens from eastern Serbia and Romania (the Carpathian clade), while the third clade encompasses specimens from eastern Türkiye (the Anatolia clade). Genetic distances among these clades range from 4.8% to 7.5% (K2P). The co-occurrence of the nominotypical and Carpathian lineages at the same locality, the Crnovrška River in eastern Serbia, suggests that they are likely reproductively isolated. The taxonomic status of the Carpathian and Anatolia lineages remains unresolved, and at present they are treated as a single species exhibiting pronounced phylogeographic structure. We discuss the possibility that the Carpathian lineage is conspecific with Monatractides carpaticus (Șoarec, 1939), a species originally described from the Carpathian region of Romania and later regarded as a junior synonym of M. madritensis. However, confirmation of this hypothesis, given the loss of the type material and the limited original description, will require detailed morphological and molecular analyses of specimens collected from the type locality of M. carpaticus. The challenges associated with resurrecting junior synonyms are briefly discussed.