Rhizoclonium volgense sp. nov., a new species of Cladophoraceae (Ulvophyceae, Chlorophyta) from the Volga River
In 2018, an unusual green alga with a Chaetomorpha-like morphology was discovered in the upper Volga River, European Russia. Macroscopically, its thick, curled, bright green filaments were distinct from the commonly occurring Cladophora glomerata complex. Microscopic examination revealed the presence of unbranched filaments and multinucleate cells, both characteristic of Chaetomorpha. However, molecular phylogenetic analyses placed this alga within the genus Rhizoclonium, leading to its description as a new species, Rhizoclonium volgense sp. nov. Unlike many Rhizoclonium taxa, R. volgense lacks lateral rhizoids, remains unbranched, and becomes unattached in later growth stages. It was consistently found forming free-floating mats, although  the presence of primary rhizoids in sporelings indicates an initially attached developmental phase. The species is further distinguished by its exceptional high number of nuclei per cell and a comparatively larger cell diameter relative to other Rhizoclonium species. Molecular analyses suggest R. volgense is closely related to taxa from saline environments. Currently, the new species appears to be restricted to the regulated reservoirs of the upper Volga River, where fluctuating hydrological conditions may influence its distribution. Its absence from earlier studies suggests either historical oversight or a recent introduction. This study underscores the importance of integrating molecular and morphological data for accurate taxonomy in Cladophoraceae and highlights the dynamic evolutionary history of freshwater green algae.