An updated compilation of world soil ciliates (Protozoa, Ciliophora), with ecological notes, new records, and descriptions of new species

This review compiles and analyses the taxonomic and biogeographical information available on soil ciliates. Key literature (257 references) is tabulated for each species. Furthermore, about 500 new records are reported from hundreds of samples collected world-wide, and four new species are described from Kenyan soils, including the new genus Apobryophyllum; Metopus ovalis Kahl, 1927 is redescribed. 643 ciliate species were originally described or reliably recorded from about 1000 soil samples world-wide, 49 (7.6%) of them were later recognized as junior synonyms, and 78 (13.2%) have been poorly described, leaving a total of 516 well-known species. However, the studied samples contained about 700 new species, most of which (about 500) have not yet been described. Thus, the total number of soil ciliates amounts to at least 1000 species. The overwhelming portion of soil ciliates belongs to three systematic assemblages, viz. the hypotrichs (39%), gymnostomatids (26%) and colpodids (13%) if the undescribed species are taken into account. Most soil ciliates feed on bacteria (39%) or are predatory (34%) or omnivorous (20%). Some are strictly mycophagous and highly characteristic for terrestrial habitats; and a few (mainly metopids) are anaerobic. Only about one fourth of the soil ciliate species has been reliably reported from freshwater habitats and from more than three out of five main biogeographical regions, indicating a high specificity of the soil ciliate biota and a limited distribution of at least some species. This is supported by the observation that some very conspicuous species, such as Krassniggia auxiliaris and Bresslanides discoideus, have so far been found only in Gondwanan, respectively, Laurasian soils.