THE GLOBAL DIVERSITY OF PROTOZOA AND OTHER SMALL SPECIES

It is widely believed that the number of species of micro-organisms in the world is extremely large. Here, we offer the contrasting view-tha t the number may be quite modest. Most of the work reviewed refers to the ciliated protozoa. As with all microbial groups, we must define ou r concept of ''species'', and for ciliates, the ''morphospecies'' conc ept appears to be at least as robust as any other. Critical examinatio n of published descriptions of ciliates provides a ''best estimate'' o f 3744 for the global number of free-living morphospecies. Of these, 7 93 are associated with marine sediments, and 1370 with freshwater sedi ments. In an independent analysis based on extrapolation (assuming the ubiquity of species) from ecological datasets, we estimate the number s of species in marine and freshwater sediments as 597 and 732, respec tively (i.e. within a factor of two of the figures obtained from taxon omic analysis). This apparent convergence of independent estimates wil l strengthen if, as is likely, the number of nominal species is furthe r reduced by taxonomic revision. These relatively low numbers of speci es are consistent with (a) the vast amount of published information in dicating typically cosmopolitan distributions for ciliates and other m icrobes, and (b) recent experimental evidence that most free-living ci liates are rare or cryptic-seldom detectable, but present, and ''waiti ng'' for suitable conditions to arrive. In summary, most ciliates (and other micro-organisms) are probably ubiquitous, endemics are rare, gl obal species richness is relatively low, and, at least in the case of the ciliates, most species have already been discovered. (C) 1998 Aust ralian Society for parasitology. Published by Elsevier Science Ltd.