The metamonads are a group of flagellate protozoa, including the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well. These four groups are all anaerobic, lacking mitochondria, and are mostly found as symbiotes of animals. A number of parabasalids and oxymonads are found in termite guts, and play an important role in breaking down the cellulose found in wood. Some other metamonads are important parasites.

Originally these flagellates were considered to have diverged from other eukaryotes before mitochondria appeared, along with a few other lines (pelobionts, entamoebae, and microsporidia). Cavalier-Smith proposed a separate kingdom Archezoa for such forms. However, all are now known to have lost mitochondria secondarily, and metamonads retain both organelles and nuclear genes derived from them. Mitochondrial relics include hydrogenosomes, which produce hydrogen, and small structures called mitosomes.

In addition to these groups, the genera Carpediemonas and Trimastix are now known to be close relatives of the retortamonad-diplomonad line and oxymonads, respectively. All of these belong among the excavates, a group of flagellates that have feeding grooves or are closely related to such forms. Their relationships are still being worked out, and it is possible the organisms listed here do not make up a monophyletic subgroup. However, they are linked by having flagella or at least basal bodies in characteristic groups of four, often associated with the nucleus in a structure called a karyomastigont.

References

• T. Cavalier-Smith (2003). The excavate protozoan phyla Metamonada Grassé emend. (Anaeromoadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinities and new higher taxa. International Journal of Systematic and Evoluionary Microbiology 53: 1741-1758