BIODIVERSITY AT THE MICROBIAL LEVEL - THE NUMBER OF FREE-LIVING CILIATES IN THE BIOSPHERE

For more than 200 years, ciliated protozoa have been identified and al located species names largely on the basis of the rich morphological v ariety they present. We have examined the species richness of all free -living ciliate genera, described historical trends in the description s of new species, and estimated the number of species currently known. We have quantified the value of taxonomic revisions, and conclude tha t the number of known, extant, free-living species is close to 3000. W e have investigated the concept of ''species'' and the meaning of ''bi odiversity'' in relation to ciliates, and conclude that the biological species concept is neither appropriate nor practicable. Insofar as ci liate morphology is closely correlated with the function of the organi sm in nature, the morphospecies concept is as valid as any, and probab ly more pragmatic than any other Thus, when speaking of ''species dive rsity,'' or the ''biodiversity'' of ciliates, rue refer to diversity o f form and function. The majority of ciliate species in the more frequ ently studied habitats have probably already been discovered but an ac curate picture of ciliate diversity on a global scale will require sub stantial taxonomic revision of many long-established and crowded gener a, together with the investigation and description of new forms from p reviously unexplored habitats.