Biodiversity at the Microbial Level: The Number of Free-Living Ciliates in the Biosphere

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Abstract
For more than 200 years, ciliated protozoa have been identified and allocated species names largely on the basis of the rich morphological variety they present. We have examined the species richness of all free-living ciliate genera, described historical trends in the descriptions of new species, and estimated the number of species currently known. We have quantified the value of taxonomic revisions, and conclude that the number of known, extant, free-living species is close to 3000. We have investigated the concept of ''species'' and the meaning of ''biodiversity'' in relation to ciliates, and conclude that the biological species concept is neither appropriate nor practicable. Insofar as ciliate morphology is closely correlated with the function of the organism in nature, the morphospecies concept is as valid as any, and probably more pragmatic than any other Thus, when speaking of ''species diversity,'' or the ''biodiversity'' of ciliates, rue refer to diversity of form and function. The majority of ciliate species in the more frequently studied habitats have probably already been discovered but an accurate picture of ciliate diversity on a global scale will require substantial taxonomic revision of many long-established and crowded genera, together with the investigation and description of new forms from previously unexplored habitats.