Hydrobioids, known earlier as the family Hydrobiidae, represent a set of truncatelloidean families whose members are minute, world-wide distributed snails inhabiting mostly springs and interstitial waters.

Heleobia dobrogica is the only gastropod species living in the Movile Cave in Dobrogea, Romania. In the cave there is little oxygen but large amounts of carbon dioxide and methane in the atmosphere, and a large amount of hydrogen sulphide in the water. All the non-predatory animals feed on the chemoautotrophic microorganisms that draw energy from the sulphide hot springs beneath the cave. Five COI mtDNA sequences were used for maximum likelihood phylogeny reconstruction together with eight sequences of cochliopids and two outgroup rissooids from GenBank., Salenthydrobia ferrerii and Peringia ulvae were used as an outgroup for the calibration of the molecular clock. The estimated time of divergence between the two species was 2.172±0.171 Mya. This coincides with the period marking the beginning of the fall in temperature and precipitation that initiated the glacial period in Europe, predating the Pleistocene. Most probably at that time Heleobia dobrogica found a safe shelter within a warm cave. Our results suggest that H. dobrogica is closely related to H. dalmatica, and both species may be congeneric with Heleobops docimus.

Horatia Bourguignat, 1887 was the first genus established for hydrobiid snails with valvatoid shell, and numerous valvatoid-shelled hydrobioids were classified as Horatia. The genus was the type one for some tribe/family-rank taxa. Thus it is one of the crucial hydrobiid genera. Horatia seems to inhabit only Croatia and Macedonia, and its type species: H. klecakiana Bourguignat, 1887, inhabits the springs in the Cetina River Valley. In the present paper the shell, operculum, soft part pigmentation, protoconch SEM microsculpture, female reproductive organs, and penis of H. klecakiana from the spring Studenci, N of Kučiće, in the valley of the Cetina River, Croatia, are described, to confirm the identity of the studied specimens with this species. Mitochondrial cytochrome oxidase subunit I (COI) and nuclear 18S ribosomal RNA gene sequences are used to infer phylogenetic relationships of Horatia, especially with Radomaniola and the sequence of Horatia from GenBank. The results suggest close relationships of the genus with Sadleriana, not with Radomaniola.