The about 1,000 species of tintinnid ciliates are identified and classified almost exclusively based on their lorica features, although the shortcomings of this structure are well‐known, e.g. causing uncertain species limitations and nonmonophyletic taxa. Hence, the present redescription of Tintinnopsis everta Kofoid and Campbell, 1929 considers not only the lorica characteristics, but focuses on cell and genetic features. The species is redescribed from the North Atlantic and adjacent sea areas, namely the east coast of the USA, using live observation, protargol‐stained material, scanning electron microscopy, and genetic analyses. The main stages of cell division are described, and the species’ phylogenetic relationships are inferred from morphological data and the small subunit ribosomal RNA gene sequence. The estimates of its biogeographical distribution and autecology are based on a literature survey. The species is characterised by a complex somatic ciliary pattern with a unique position of the posterior kinety and a conspicuously large distance between the somatic ciliary fields and the collar membranelles. The phylogenetic relationships of Tintinnopsis everta vary in the molecular trees depending on the algorithms used and are, therefore, regarded as unresolved. Nevertheless, the new kind of complex somatic ciliary pattern distinctly contributes to a better understanding of the tintinnid biodiversity and evolution and provides features for a future split of the nonmonophyletic genus Tintinnopsis.

Molecular phylogenies of Oligotrichea currently do not contain all genera and families and display topologies which are often incongruent with morphological findings. In ciliates, the somatic kinetids are rather conserved, i.e., their ultrastructures, particularly the fibrillar associates, often characterise the main groups, except for the choreotrichids. Four different kinetid types are found in protargol-stained choreotrichids and used for reconstructing the taxon’s evolution (the “Kinetid Transformation Hypothesis”). Proof for this hypothesis requires transmission electron microscopic studies, which are very rare in the choreotrichids and oligotrichids. Such an approach provides insights into the ultrastructural variability of somatic kinetids in spirotrichs and may also detect apomorphies characterising certain choreotrichid families. In the model tintinnid Schmidingerella meunieri, the ultrastructure of the three kinetid types in the somatic ciliature is studied in cryofixed cells. The data support the “Kinetid Transformation Hypothesis” regarding tintinnids with a ventral kinety. This first detailed study on kinetids in tintinnids and choreotrichids in general reveals totally new kinetid types in ciliates: beyond the three common associates, they are characterised by two or three conspicuous microtubular ribbons extending on the kinetids’ left sides. These extraordinary ribbons form together with the overlapping postciliary ribbons a unique network in the cortex of the anterior cell portion. The evolutionary constrains which might have fostered the development of such structures are discussed for the Oligotrichea, the choreotrichids, and tintinnids as their first occurrence is currently uncertain. Additionally, the kinetids in tintinnids, aloricate choreotrichids, oligotrichids, hypotrichs, and euplotids are compared.

Species of the ciliate genus Urotricha are key players in freshwater plankton communities. In the pelagial of lakes, about 20 urotrich species occur throughout an annual cycle, some of which play a pivotal role in aquatic food webs. For example, during the phytoplankton spring bloom, they consume a remarkable proportion of the algal production. In ecological studies, urotrich ciliates are usually merely identified to genus rank and grouped into size classes. This is unsatisfying considering the distinct autecological properties of individual species and their specific spatial and temporal distribution patterns. As a basis for future research, we characterized in detail four common urotrich morphotypes, i.e., specimens identified as U. furcata and tentatively as U. agilis, U. pseudofurcata, and U. castalia, using state-of-the-art methods. We used an integrative polyphasic approach, in which morphological studies (in vivo observation, silver staining methods, scanning electron microscopy) were linked with a molecular approach exploiting four different gene fragments as taxonomic DNA barcodes with different resolution potential (SSU rDNA, ITS-1, ITS-2, hypervariable V4 and V9 regions of the SSU rDNA). We shed light on the diversity of urotrich ciliates as well as on their global distribution patterns, and annual cycles. Additionally, we coupled individual species occurrences and environmental parameters, and subsequently modeled the distribution and occurrence, using logistic regressions. Furthermore, for one strain putatively identified as U. castalia, we ascertained the optimal cultivation media and food preferences. Thereby, our comprehensive view on these important freshwater ciliates that frequently occur in environmental high throughput sequencing datasets worldwide will allow future studies to better exploit protistan plankton data from lakes.

Ciliates of the genus Urotricha are widely distributed and occur in almost any freshwater body. Thus far, almost all species have been described from morphology only. Here, we applied an integrative approach on the morphology, molecular phylogeny and biogeography of two species isolated from high mountain lakes in the Central Alps, Austria. As these remote lakes are known to have water temperatures <15 °C, our hypothesis was that these urotrichs might prefer ‘cold’ environments. We studied the morphological details from living and silver-stained individuals, and their molecular sequences (ribosomal operon, ITS), and screened available datasets for their biogeography. The two Urotricha species resembled morphological features of several congeners. An accurate species assignment was difficult due to several overlapping characteristics. However, we tentatively attributed the investigated species to Urotricha nais and Urotricha globosa. The biogeographic analyses revealed their occurrence in Europe, Africa and Asia, and no correlations to (cold) temperatures were found. Our findings suggest that these two urotrichs, originating from two cold and remote habitats, are probably cryptic species well adapted to their harsh environment.

The ultrastructure of the oral apparatus is supposed to be significant for elucidating more recent common ancestry and might thus provide support for particular groupings of oligotrichean ciliates. The transmission electron microscopical study on mainly cryofixed Schmidingerella meunieri specimens provides the first detailed data for tintinnids and Oligotrichea in general. Ten new characters are included into the cladistic analysis. These features together with the very limited body of literature suggest that substantial changes in the oral ultrastructure correlate only with the formation of a circular adoral zone in choreotrichids. Despite homoplasious morphological and ontogenetic adaptations to the planktonic lifestyle in halteriid hypotrichs and oligotrichids, their oral apparatuses generally retain the plesiomorphic ultrastructure of the Perilemmaphora. The highly complex ultrastructure of the adoral zone is thus able to accomplish an extension in the zone's functionality without obvious changes; only the position of the adoral zone at the apical cell portion together with a globular to obconical cell shape are apparently crucial. Merely, minute apomorphies characterise the Oligotrichea and tintinnids, respectively. Tintinnids with derived somatic ciliary patterns possess distinct microtubular bundles connecting the oral apparatus with the myoneme in the peduncle.