First DNA barcode reference library for water mites of the ancient Lake Ohrid reveals high diversity and lineage endemism

Lake Ohrid is the oldest lake in Europe and has been designated as a UNESCO World Heritage Site for endemism of its biota. Its biodiversity is still not fully known and some taxa are particularly challenging to identify. DNA barcoding became a common method for the identification of such taxa and has become a crucial tool in biomonitoring and biodiversity studies. Accurate species identification through barcoding relies on curated reference libraries. Water mites are a species-rich group of ecologically important and widely distributed aquatic invertebrates for which little DNA barcoding data is available. Our study aims to develop the first reference library of COI barcodes for water mites from the ancient Lake Ohrid and surrounding springs, followed by analysis of their molecular diversity. Such libraries are well-developed for many waterbodies in Europe (e.g. Alpine Lakes), but are heavily underrepresented for the Mediterranean Region and completely absent for Lake Ohrid. We provide 327 COI barcodes clustered into 34 BINs (molecular equivalents of species) of which 6 are endemic. We identified 33 species, of which 20 are new for the Ohrid basin. We also identified several cases of intraspecific diversity suggesting morphologically cryptic species or previous misidentifications, suggesting that DNA barcoding is invaluable in the taxonomic identification of water mites. We discovered a relatively low turnover of species with neighbouring Lake Skadar, which may be a result of differences in their physical and geographical characteristics. Our study represents a major advance for the future taxonomic, phylogeographic studies and biomonitoring using water mites as model taxon.