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Cercozoa are mostly heterotrophic protozoa dwelling abundantly in soil (where they are the most numerous eukaryotes) and in all freshwater and marine habitats. Some parasitize plants, invertebrate animals and other protists. A few have become algae by enslaving photosynthetic prey to form permanent cellular chimaeras. Free-living Cercozoa feed on bacteria, fungi, algae, other protozoa or even microscopic animals. They may be the most numerous predators on earth, but unlike most predators they generally lack a mouth as they can catch prey over much of their typically soft body surface.
Their ancestral body plan was a predatory, phagotrophic amoeboflagellate with one anterior and one posterior cilium. One lineage enslaved a green algal cell to become the chimaeric chlorarachnean algae, which still retain the nucleus (as a miniaturized nucleomorph), former plasma membrane (as the periplastid membrane), and chloroplast of the enslaved green alga. Cercozoa have diversified into so many distinct lineages that body plans have become extremely varied. Some have lost cilia altogether and become amoebae with thread-like (filose) or net-like (reticulose) pseudopodia for locomotion and prey entrapment. These filose or reticulose amoebae are either naked or have evolved rigid tests (like a shell), which happened independently for at least three groups (euglyphid, tectofilosid, and gromiid testate amoebae). Some have evolved radiating axopodia supported by microtubules to catch prey instead (Phaeodarea, desmothoracids) and thus mimic radiolaria or heliozoa. Others have abandoned pseudopodia and become pure flagellates, either gliding on surfaces or swimming in the plankton. Many have retained cilia (sometimes reduced to one or increased to four or more) and pseudopods. Cell surfaces of the flagellates are generally naked and quite soft, lacking the complex pellicles found in alveolates and excavates, but thaumatomonads have evolved a semi-rigid covering of complex silica scales, which still allows branching filopodia to be used for feeding and cilia to propel the cells by gliding.
Cercozoa were only recently recognised as a monophyletic group (Cavalier-Smith 1997) and were named Cercozoa in 1998 (Cavalier-Smith 1998). The name is based on the cercomonads, amoeboflagellates that are abundant in soil and freshwater, which were first discovered by Dujardin (1841) the father of protozoology, who first realised that protozoa were cells. Even earlier Dujardin (1835) had discovered the reticulose testate cercozoan amoeba Gromia. Cercomonad means ‘tailed monad’, because almost all cercomonads have an extensible pseudopodial tail that is typically drawn out along the posterior gliding cilium, sometimes obscuring it. Dujardin coined the name Rhizopoda (root feet) for organisms having filose or reticulose pseudopods. Unfortunately other authors later confusingly expanded that term to include also the unrelated Amoebozoa, which generally have broad lobose, non-root-like pseudopods. Cercozoa are now known to be one of the most diverse, speciose and ecologically important of all protozoan phyla and include the majority (not all) of eukaryotes with filose pseudopods or cilia that glide on surfaces instead of swimming (Cavalier-Smith and Chao 2003). Many lineages are currently known only from environmental DNA sequencing (Bass and Cavalier-Smith 2004; Bass et al. 2009), so cercozoan diversity must be even greater than is now appreciated.