Water mites (Acari, Hydrachnidia) are a significant component of freshwater ecosystems inhabiting a wide range of aquatic habitats. This study provides a first comprehensive DNA barcode library for the water mites of Montenegro. DNA barcodes were analysed from 233 specimens of water mites morphologically assigned to 86 species from 28 genera and 15 families. In the course of the study, four species, i.e. Lebertia reticulata (Koenike, 1919), Atractides inflatipalpis K.Viets, 1950, A. latipes (Szalay, 1935) and Parabrachypoda montii (Maglio, 1924) were molecularly confirmed as new for Montenegro and three species, i.e. Protzia octopora Lundblad, 1954, Piona laminata (Thor, 1901) and Unionicola ypsilophora (Bonz, 1783) are new for the Balkan Peninsula. Results are analysed using the Barcode Index Number system (BIN) and the Refined Single Linkage (RESL) of BOLD. The BIN assigned sequences to 98 clusters, while the RESL reveal 103 operational taxonomic units (OTUs). Unique BINs were revealed for 72 species (83.7%), whereas twelve species (14%) were characterised by two BINs and two species (2.3%) with three BINs. Amongst the studied taxa, 14 species were found with a high intraspecific sequence divergences (˃ 2.2%), emphasising the need for additional comprehensive morphological and molecu­lar analysis of these species.

New records of water mites from Serbia, North Macedonia and Greece revealed by DNA barcoding are presented. DNA barcodes were recovered from 63 water mite specimens morphologically assigned to 32 species. One species Lebertia algeriensis Lundblad, 1942, is reported for the first time for the Balkans; four species, i.e. Lebertia fimbriata Thor, 1899, L. rivulorum K. Viets, 1933, Atractides stankovici Pešić & Gerecke, 2010, and Wettina lacustris Pešić & Smit, 2018 are new for North Macedonia; Monatractides madritensis (K. Viets, 1930) and Mideopsis roztoczensis Biesiadka & Kowalik, 1979 are new for Serbia; and Hygrobates mediterraneus Pešić, 2020 is new for Greece. Comparing sequences of our study with the available COI sequences we recognized five possible cases of cryptic diversity involving species clusters with more than 5% divergence indicating that these species need further study.

The checklist summarizes the knowledge of the diversity of water mites of Serbia from the oldest record in 1903 to present day. The checklist is based on literature data and recent findings. The checklist includes 104 species in 18 families and 32 genera. Six species, i.e. Hydryphantus parmulatus, Lebertia glabra, Atractides glandulosus, Arrenurus bruzelii, A. virens and Hygrobates balcanicus, are added as new for the water mite fauna of Serbia. Regardless that the diversity of water mite has been addressed for over a century, the number of water mite species from Serbia is still limited and lower compared to other Balkan countries, indicating the need for further studies.

Mideopsis roztoczensis Biesiadka & Kowalik, 1979 (Acari, Hydrachnidia, Mideopsidae) is a common and often abundant component of macroinvertebrate communities in Palearctic streams. Although the external morphology and genetic structure of this species are relatively well documented, knowledge of its internal anatomy remains limited, despite its importance for an integrative understanding of morphology and evolutionary diversity. The present study utilized histological techniques to investigate and describe the internal structures of M. roztoczensis based on newly collected material from Serbia. Histological analyses revealed the organization of several internal organs, including an extensive blind sac-like excretory organ that was morphologically distinct from the midgut, while a hindgut was not identified. Additionally, other structures, including dermal glands, the midgut, salivary glands, and the synganglion, were characterized. The observed variation in internal organization, particularly the morphology of the excretory organ and the number of paired dermal glands, provides additional characteristics in comparative morphological studies of water mite. This study contributes to our current knowledge of the internal anatomy of Hydrachnidia and highlights the value of histological approaches for future integrative studies of water mites.