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New stands of Paramecium dodecaurelia were found in Russia, Ukraine, Kazahstan, Poland, and Tenerife. Molecular studies (RAPD and sequencing of LSU rRNA and COI mtDNA gene fragments) of P. dodecaurelia strains depicted intraspecific polymorphism within the species, revealed as distinct differences between strains from the USA in comparison with strains from different regions of the Palearctic.

This is the first report on the presence of P. biaurelia in Tasmania, an island that has probably never been investigated before for the occurrence of the P. aurelia species. P. tetraurelia was recorded in Brazil, another very poorly investigated country in terms of this species complex. New stands of P. biaurelia and P. tetraurelia were also recorded in Japan. We present data concerning the occurrence and distribution of the P. aurelia species on different continents as a background for the newly described stands of P. aurelia spp.

The geographic distribution and temporal occurrence of ciliates are still little known. In the present article, the occurrence of the Paramecium aurelia species complex in a natural pond situated in Kraków (Opatkowice) was investigated in different seasons in two following years. A sequence of species occurrence of the P. aurelia complex was observed. Always, paramecia were found only in some sampling points among six points sampled each time and not necessarily in the same ones. Paramecia appearing in one habitat (water body) might occupy different niches characterized by various environmental features suitable for paramecia. The following species were found in the pond: P. biaurelia, P. triaurelia, P. tetraurelia, P. pentaurelia, and P. dodecaurelia. The occurrence of some rare species (P. tetraurelia, P. pentaurelia, and P. dodecaurelia) may be connected with migrating birds which can transport paramecia with drops of water from other water bodies. If a species was observed in successive seasons or years, the possible genetic variation was investigated by analysis of sequences of LSU rDNA and mitochondrial cytb gene fragments. Among the studied species (P. biaurelia, P. triaurelia, P. pentaurelia, and P. dodecaurelia) only P. dodecaurelia showed haplotype variation in different seasons and sampling points, probably caused by the colonization of the pond by different populations of paramecia.