Paramecium (Ciliophora) is an ideal model organism to study the biogeography of protists. However, many regions of the world, such as Central America, are still neglected in understanding Paramecium diversity. We combined morphological and molecular approaches to identify paramecia isolated from more than 130 samples collected from different waterbodies in several states of Mexico. We found representatives of six Paramecium morphospecies, including the rare species Paramecium jenningsi, and Paramecium putrinum, which is the first report of this species in tropical regions. We also retrieved five species of the Paramecium aurelia complex, and describe one new member of the complex, Paramecium quindecaurelia n. sp., which appears to be a sister species of Paramecium biaurelia. We discuss criteria currently applied for differentiating between sibling species in Paramecium. Additionally, we detected diverse bacterial symbionts in some of the collected ciliates.

Paramecium (Ciliophora) systematics is well studied, and about twenty morphological species have been described. The morphological species may include several genetic species. However, molecular phylogenetic analyses revealed that the species diversity within Paramecium could be even higher and has raised a problem of cryptic species whose statuses remain uncertain. In the present study, we provide the morphological and molecular characterization of two novel Paramecium species. While Paramecium lynni n. sp., although morphologically similar to P. multimicronucleatum, is phylogenetically well separated from all other Paramecium species, Paramecium fokini n. sp. appears to be a cryptic sister species to P. multimicronucleatum. The latter two species can be distinguished only by molecular methods. The number and structure of micronuclei, traditionally utilized to discriminate species in Paramecium, vary not only between but also within each of the three studied species and, thus, cannot be considered a reliable feature for species identification. The geographic distribution of the P. multimicronucleatum and P. fokini n. sp. strains do not show defined patterns, still leaving space for a role of the geographic factor in initial speciation in Paramecium. Future findings of new Paramecium species can be predicted from the molecular data, while morphological characteristics appear to be unstable and overlapping at least in some species.

The genera Lagenophrys Stein, 1852, Paralagenophrys Clamp, 1987, Clistolagenophrys Clamp, 1991, and Operculigera Kane, 1969 make up the family Lagenophryidae and, together, contain more than 80 species worldwide that live mainly as ectosymbionts of crustaceans. Lagenophryids are characterized by possession of a lorica, but the lorica aperture and the associated parts of the body differ between genera. Despite their widespread distribution, relatively few papers have been published on lagenophryids in recent years, and the present paper is intended to promote research on lagenophryids by providing an annotated checklist of all known species and a brief review of the family, including data of their geographic distribution and their hosts.

Records of species of the peritrich ciliate genus Lagenophrys, obligate epibionts of crustaceans, were analysed using panbiogeographical track analysis to detect its worldwide distribution patterns. Track analysis of 28 species of this genus found five generalized tracks: (1) Euroamerican; (2) American; (3) Caribbean; (4) Tasmanian; and (5) South American. Two panbiogeographic nodes were found at the intersection of generalized tracks 1 and 2, and 2 and 5. The distributions of the Lagenophrys species show complex patterns that could be related to trophic opportunities, habitat and distribution of hosts, and other factors. The distribution patterns suggest that a number of allopatric events have caused regionalization of the taxa.