New Paramecium (Ciliophora, Oligohymenophorea) congeners shape our view on its biodiversity

Paramecium is one of the best known and most intensely studied ciliate genera. It currently comprises 18 morphospecies including the P. aurelia complex of 15 sibling species. Here, we describe the new morphospecies Paramecium buetschlii sp. nov. from a freshwater pool in Norway, featuring unusual combinations of morphological characters and a high genetic diversity relative to other congeners. Three further investigated Paramecium spp. from Germany, Hungary, and Brazil are treated as cryptic species, because they are difficult to discriminate from other members of the genus relying on morphological criteria only. However, DNA-based taxonomic markers (18S-rDNA and mitochondrial COI) clearly indicate they are separate species. Due to the lack of an appropriate systematic term within the International Code of Zoological Nomenclature for distinguishing cryptic from valid biological species, we propose the provisional status Eucandidatus as a component of the taxonomic name when describing new but cryptic eukaryotes. Based on our data, we postulate that even within Europe there is a higher biodiversity within this common ciliate group that is heavily used in the classroom. By uncovering potentially distinct species that have been classified under the same species names, our molecular analyses further suggest a higher current stock diversity in Paramecium than previously thought. We also would like to emphasize that under-sampling is another major issue in estimating protist diversity. Future large-scale studies based on extensive sampling not only in exotic and remote regions, but also in less frequently sampled areas, will therefore likely improve our understanding of species richness and diversity.