Belgrandiella A. J. Wagner, 1927 is a genus comprising minute snails with conic or turriform shells that are facultative stygobionts, inhabiting subterranean waters and springs. Few specimens or even empty shells of this taxon can be found, so the taxonomy is based mostly on the shell morphology and geographic distribution. Anatomy is known for a few taxa, and its usefulness is restricted. This, coupled with a popular idea that isolation is the first factor shaping divergence and speciation of the spring/cave fauna, resulted in dozens of nominal species of Belgrandiella, known from shells at a given locality. Herein we present shells, anatomy and four molecular markers (nuclear H3, 18S and 28S and mitochondrial cytochrome oxidase subunit I) in Belgrandiella from 16 localities in Slovenia, northern Croatia, and Bosnia and Herzegovina. Five sites are type localities of Boleana umbilicata, Belgrandiella kusceri, Belgrandiella krupensis, Belgrandiella zermanica and Belgrandiella robusta, respectively. Considering the shell and geographic distribution, we have determined B. cf. robusta Belgrandiella cf. croatica, Belgrandiella cf. fontinalis, Belgrandiella cf. kuesteri, Belgrandiella cf. pageti and Belgrandiella cf. koprivnensis The shells of all, as well as penes and female reproductive organs of two putative, species have been illustrated. Molecular analysis indicates four distinct clades, most probably of species rank for B. kusceri, B. cf. kuesteri, B. cf. fontinalis and B. cf. koprivnensis. Four nominal species (B. krupensis, B. robusta, B. umbilicata and B. zermanica) and one genus (Boleana) must be synonymised with Belgrandiella kusceri, and there are no more than four valid, biological species in the studied Belgrandiella. The molecular divergence is completely unmirrored by high morphological variability. Hence the level of endemism is markedly overestimated in these snails.

The family Moitessieriidae includes minute dioecious gastropods exclusively inhabiting subterranean waters, including thermal ones. Only empty shells were collected in most species, the vast majority of them are described from their gross shell morphology alone. Several visits to a site are usually required to obtain at least some living individuals. High variability in shell morphology and the lack of diagnostic features, coupled with anticipated high levels of endemism, has resulted in a long list of nominal moitessierid species. Type specimens stored as empty shells omit unambiguous identification and delimitation of species boundaries. Due to inaccessibility of cave animals and consequent lack of material suitable for molecular analysis, the phylogenetic relationships, as well as the taxonomy of the family at genus/species level, are far from being understood. The anatomy of the family is also poorly known and provided only for a few taxa. The distinctness of the Moitessieriidae has sometimes been questioned, and their monophyly not proved. Twelve species of the Balkan Moitessieriidae are considered: two species of Paladilhiopsis, two species of Bythiospeum, six species of Iglica, Costellina turrita and Lanzaia bosnica. The shell morphology of each species, as well as the reproductive system of Paladilhiopsis and Iglica, were analysed. DNA sequences of nuclear histone H3, ribosomal 18S, ribosomal 28S and mitochondrial cytochrome oxidase subunit I (COI) were applied to infer phylogenetic relationships among the taxa. The sequences of Bythiospeum from GenBank have been used to infer relationships between Bythiospeum and Paladilhiopsis that were recently synonymized. Paladilhiopsis and Iglica are distinct, but closely related genera, as is the genus Bythiospeum, which does not occur in the Balkans. Its relationships with both former taxa remain unresolved. The Moitessieriidae are clearly distinct from all other families of the Truncatelloidea, however, their monophyly remains doubtful.

Knowledge about subterranean aquatic gastropods is still limited and species inhabiting caves are typically only known from empty shells found at the surface. Phylogenetic analyses of such fauna are extremely rare. In the present work, we analysed seven species of five genera from 13 cave localities in Bulgaria, using molecular (mitochondrial COI and three nuclear markers: 18S rRNA, 28S rRNA and H3) techniques. All analysed DNA sequences from cave specimens formed one, highly supported, monophyletic lineage, divided into six subclades. According to molecular clock analyses, this subterranean clade originated 7-6.75 Mya. Our data support the "climatic-relict" hypothesis for the speciation of these taxa, assuming that surface ancestors colonized the caves and, after extinction of the surface population, cave populations evolved in allopatry. Extinction of the surface population was most probably caused by the climatic shift associated with the Messinian Salinity Crisis (5.96–5.33 Mya). A new model of climate-driven separation promoting speciation is proposed for the origin of the cave fauna.

Since 2020, the south-western Caucasus has been recognized as a hotspot of stygobiotic Mollusca diversity after revealing a large number of new, range-restricted species within the spring snail family Hydrobiidae, subfamily Sadlerianinae sensu Szarowska. Meantime, based on extensive material collected in the south-western Caucasus during the last decades, we studied members of another spring snail subfamily Belgrandiellinae Radoman, 1983. Modern integrative taxonomic work revealed hitherto unknown diversity within this subfamily in the region and further proved the importance of the south-western Caucasus as a hotspot of stygobiotic life. In particular, the subterranean environment and springs of Georgia were known to be inhabited by the genus Tschernomorica Vinarski and Palatov, 2019 with four nominal species. Our research, based on a morpho-anatomical study and genetic investigation of COI/H3 mitochondrial/nuclear markers, revealed additionally seven species and three genera new to science—Colchiella lugella gen. et sp. nov., C. nazodelavo gen. et sp. nov., C. shiksa gen. et sp. nov., C. dadiani gen. et sp. nov., Sataplia cavernicola gen. et sp. nov., Aetis starobogatovi gen. et sp. nov., and Tschernomorica kopidophora sp. nov. —to inhabit the Georgian part of south-western Caucasus. The full taxonomic description of each new taxa, along with the review of habitat characteristics and conservation status, is provided. Molecular genetics suggests that the ancestors of Caucasian Belgrandiellinae have migrated from south-western Europe, probably more than once during the late Messinian and early Pliocene periods. Later, Plio-Pleistocene climate oscillations, particularly the repeated rise and fall of the Black Sea water level, resulted in the isolation and radiation of various lineages within the Caucasus and Crimea.