Many protistan morphospecies have been demonstrated to harbour a high degree of genetic diversity. Whilst the morphotype itself is assumed to have been conserved as an adaptive peak, the ecological significance of the cryptic genetic diversity within a morphospecies is poorly understood. \ud \ud In this study, ribosomal DNA (rDNA) sequence variation within the common and ubiquitous ciliate morphospecies Cyclidium glaucoma has been investigated. A total of 54 sequences from both clonal cultures and environmental samples (using a specific primer) have been obtained from 47 locations over seven continents. Sequence comparisons reveal three lineages which correlate with the salinities of the habitats from which the sequences were obtained. Ribotypes isolated from hyperhaline habitats formed a discrete sub-cluster. But there is no detectable geographic pattern in the distribution of C. glaucoma ribotypes, for example, sequences originating from Argentina, Peru, Japan, Morocco, Russia and Ukraine were identical, as were those from Denmark and Australia. The significance of these results will be discussed; do the rDNA sequence clusters represent cosmopolitan ecotypes? \u
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