Taxonomy, geographic distribution and molecular phylogeny of two freshwater Frontonia (Ciliophora, Peniculida), including a new cryptic species Frontonia paraleucas sp. nov.

The ciliate genus Frontonia, one of the most species-rich groups of the order Peniculida, is widely distributed across a variety of habitats. However, the definition and identification of isolates are challenging, as there are few reliable characters, and these may overlap among congeners. In the present study, we describe two easily confused species, including a new form F. paraleucas sp. nov. and F. paramagna Chen et al., 2014. Both species exhibit morphological characteristics similar to those observed in F. leucas (Ehrenberg, 1833) Ehrenberg, 1838, the type species of the genus. Furthermore, the substantial divergence of sequences among different isolates under the name of F. leucas, indicates the existence of cryptic or misidentified material and that such sequences represent a complex group rather than a single species. A meticulous examination was conducted to ascertain the reliability of the SSU rRNA gene sequences of isolates designated as F. leucas and F. paramagna in GenBank, and it was confirmed that the sequence (AM072622) represented the authentic F. leucas. The newly described species, F. paraleucas sp. nov., along with F. leucas and F. cf. leucas, constitute the F. leucas species complex. Analysis of the geographic distribution of these taxa, combined with historical records, reveals an uneven global pattern. This study emphasizes the importance of integrating molecular and morphological data to accurately identify species, and provides new insights into cryptic diversity and classification within the genus Frontonia.