Throughout the Caucasus, in localized, often hidden caves and springs, dwell miniature mollusks of the Hydrobiidae family. These snails represent one of the least studied groups of freshwater invertebrates in this region. These tiny groundwater dwellers are rarely encountered by researchers: many species are known only from isolated finds, and their evolutionary history has remained a mystery.
An international team of scientists, including specialists from the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE RAS), the Biology Department of M.V. Lomonosov Moscow State University, and St. Petersburg State University, conducted the first extensive molecular phylogenetic analysis of this group of mollusks. The researchers included a significantly larger sample size and used four genetic markers, complementing the genetic analysis with morphological features.
A total of 13 genera of stygobiont hydrobiids have been identified in the Caucasus, five of which are new, and at least 37 species, approximately 20 of which are potentially new to science. This represents an extremely high level of cryptic biodiversity for a relatively small region.
Molecular phylogenetic analysis revealed that hydrobiids in Caucasian groundwater form three distinct lineages, corresponding to subfamily levels:
- Caucasopsiinae – a new lineage to science at the subfamily level, uniting the majority of endemic Caucasian hydrobiid genera.
- Belgrandiellinae – includes the stygobiont genus Sitnikovia and several crenobiont genera.
- Islamiinae – is represented by a single monotypic genus.
A morphological study revealed that the structures of the female reproductive system are the most taxonomically significant. In contrast, a traditional taxonomic trait—shell shape—demonstrates a high degree of variability both within and between genera. Therefore, its use for diagnosing generic affiliation without correlation with anatomical data can lead to erroneous taxonomic conclusions.
The photograph shows the diverse shell shapes of Caucasian hydrobiids. Take a guess: how many different species and how many genera are there? The answer is far from as obvious as the shell shape suggests.
The discovered diversity and profound phylogenetic divergence indicate the long-term evolution of subterranean hydrobiids of the Caucasus in conditions of relative isolation. Unique endemic lineages, ranging from species to subfamilies, have developed here, not found in other regions. The Caucasus functions as an independent centre of speciation for cave fauna, demonstrating exceptionally high levels of endemism at all taxonomic levels.
"These results demonstrate how little we know about the life of the Caucasus's underground ecosystems. Each new study uncovers dozens of hidden forms, many of which are narrowly endemic and potentially vulnerable," notes co-author Dmitry Palatov, PhD in biology.
This work was supported by a Russian Science Foundation project (No. 25-24-00006).
The article was published in the journal Molecular Phylogenetics and Evolution: Elizaveta Chertoprud, Irina Ekimova, Dmitry Palatov, Jozef Grego, Levan Mumladze, and Maxim Vinarski, "Little treasures hidden in the darkness: diversity and phylogeny of stygobiotic hydrobiidae (Mollusca: Gastropoda) of the Caucasus," Volume 213, December 2025, 108439.