Most of the small ciliate protozoa, including Dasytricha ruminantium and Entodinium spp. living in the rumen of sheep, were found to have intracellular bacteria. These bacteria were not present in digestive vacuoles. They showed characteristic coenzyme F420 autofluorescence and they were detected with a rhodamine-labelled Archaea-specific oligonucleotide probe. The measured volume percent of autofluorescing bacteria (1%) was close to the total volume of intracellular bacteria estimated from TEM stereology. Thus it is likely that all of the bacteria living in the cytoplasm of these ciliates were endosymbiotic methanogens, using H2 evolved by the host ciliate to form methane. Intracellular methanogens appear to be much more numerous than those attached to the external cell surface of ciliates.
Summary: This is a taxonomic revision of anaerobic free-living ciliates in the genus Metopus. It includes a rationalization of all nominal species described in the literature, and the allocation of the survivors to "morphospecies". The revision is based on examination of cultured species together with an exhaustive comparison of the published descriptions of nominal species. All free-living Metopus can be allocated to one of five general morphological types. Each type contains several morphospecies (and their synonyms), each with conservative features. The seventy-six nominal species of Metopus are reduced to 22 morphospecies, and M. nivaaensis n. sp. is described.
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