The Cercozoa are a group of single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, being defined by molecular phylogenies of rRNA and actin or polyubiquitin.
The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in many other protozoa. They show a variety of forms and have proven difficult to define in terms of structural characteristics, although their unity is strongly supported by genetic studies. Cercozoa are closely related to Foraminifera and Radiolaria, amoeboids that usually have complex shells, and together with them form a supergroup called the Rhizaria.
They are sometimes grouped by whether they are "filose" or "reticulose".
The best-known Cercozoa are the euglyphids, filose amoebae with shells of siliceous scales or plates, which are commonly found in soils, nutrient-rich waters, and on aquatic plants. Some other filose amoebae produce organic shells, including the tectofilosids and Gromia. They were formerly classified with the euglyphids as the Testaceafilosia. This group is not monophyletic, but nearly all studied members fall in or near the Cercozoa, related to similarly shelled flagellates. Other notable filose cercozoans include the cercomonads, which are common soil flagellates.
Another important group placed here are the chlorarachniophytes, strange amoebae that form a reticulating net. They are set apart by the presence of chloroplasts, which apparently developed from an ingested green alga. They are bound by four membranes and still possess a vestigial nucleus, called a nucleomorph. As such, they have been of great interest to researchers studying the endosymbiotic origins of organelles.
Chlorarachniophytes are sometimes considered Filosa, rather than Endomyxa, while groups such as Gromia are considered Endomyxa. Filosa is apparently a monophyletic group, but Endomyxa is paraphyletic.
In addition, three groups that are traditionally considered heliozoans belong here: the Heliomonadida, Desmothoracida, and Gymnosphaerida, which were recently grouped into the new class of Granofilosea.
Finally, cercozoans include the Phaeodarea, marine protozoa that were previously considered radiolarians.
The exact composition and classification of the Cercozoa are still being worked out. A general scheme is:
In addition two groups of parasites, the Phytomyxea and Ascetosporea, and the shelled amoeba Gromia may be basal Cercozoa, although some trees place them closer to the Foraminifera.
The spongomonads have been included here, but more recently have been considered Amoebozoa.
Some other small groups of protozoans are considered Cercozoa but are of uncertain placement, and it is likely many obscure genera will turn out to be cercozoans with further study.
Phylogeny based on Bass et al. 2009, Howe etal. 2011 and Silar 2016.
Phylum CercozoaFamily ?GymnophrydiidaeFamily ?Gymnophryidae Mikrjukov & Mylnikov 1996Family ?Rhizoplasmidae Cavalier-Smith & Bass 2009Order ?Gymnosphaerida Poche 1913 emend. Mikrjukov 2000 [Axoplasthelida]Family Gymnosphaeridae Poche 1913 [Hedraiophryidae; Gymnidae; Wagnerellidae Poche 1913]Order Reticulosida Cavalier-Smith 2003 emend. Bass et al. 2009Family Filoretidae Cavalier-Smith & Bass 2009Class Gromiidea Cavalier-Smith 2003Order Gromiida Claparède & Lachmann 1856 s.s.Class Ascetosporea Desportes & Ginsburger-Vogel, 1977 emend. Cavalier-Smith 2009 [Aplosporidies Caullery & Mesnil, 1899; Stellatosporea; Ascetospora Sprague 1979]Order Claustrosporida Cavalier-Smith 2003Order Paradiniida Cavalier-Smith 2009Order Mikrocytida Hartikainen et al. 2014Order Paramyxida Chatton 1911 [Paramyxea Chatton 1911; Paramyxidea Chatton 1911; Paramyxa]Order Haplosporida Caullery & Mesnil 1899 [Balanosporida Sprague, 1979; Haplosporidia Hall, 1953; Haplosporea Caullery 1953; Haplospora Margulis & Schwartz, 1998; Haplosporidiidea Poche, 1913; Haplosporidies Caullery and Mesnil, 1905]Class Phytomyxea Engler & Prantl 1897 em. Cavalier-Smith 1993 [Phytomyxinae Engler & Prantl, 1897; Phytomyxinea Poche, 1913]Order Phagomyxida Cavalier-Smith 1993Order Plasmodiophorida Cook 1928 em. Cavalier-Smith 1993Class Vampyrellidea [Cristivesiculatia Page 1987 stat. nov. Cavalier-Smith 1993]Order Vampyrellida West 1901 emend. Hess et al. 2012 [Aconchulinida de Saedeleer 1934 emend. Bass et al. 2009; Cristivesiculatida Page 1987]Subphylum Filosa Leidy 1879 emend. Cavalier-Smith 2003Class SkiomonadeaOrder Tremulida Cavalier-Smith & Howe 2011Class Chlorarachnea Hibberd & Norris 1984 [Chlorarachniophyceae Hibberd & Norris 1984; Chlorarachniophyta Hibberd & Norris 1984; Chlorarachnia Cavalier-Smith 1993]Order Chlorarachniida Hibberd & Norris 1984 [Chlorarachniales; Chlorarachniophida]Class Granofilosea Cavalier-Smith & Bass 2009Order ?Axomonadida [Exonucleoaxoplastiales]Order Desmothoracida Honigberg et al. 1964 [Cryptaxohelida; Desmothoraca Hertwig & Lesser 1874]Order Cryptofilida Cavalier-Smith & Bass 2009Order Limnofilida Cavalier-Smith & Bass 2009Order Leucodictyida Cavalier-Smith 1993 emend. 2003Infraphylum Monadofilosa Cavalier-Smith 1997 [Eoglissa Cavalier-Smiths 2012]Family ?Katabiidae Cavalier-Smith 2012Order ?PseudodimorphidaOrder Cercomonadida Poche 1913 emend. Karpov et al. 2006Class Metromonadea Cavalier-Smith 2007 s.s.Order Metromonadida Bass & Cavalier-Smith 2004Order Metopiida Cavalier-Smith 2003Clade Glissomonadida-SainouridaOrder Glissomonadida Howe & Cavalier-Smith 2009 emend. Hess et al. 2013Order Pseudosporida Hibberd 1983 sensu Cavalier-Smith 1993Order Pansomonadida Vickerman 2005Order Sainourida [Sainouroidea Cavalier-Smith et al. 2009]Order Dimorphida Siemensma 1991 [Endonucleoplastiales]Class Silicofilosea Adl et al. 2005 emend. Adl et al. 2012 [Imbricatea Cavalier-Smith 2011]Order Perlofilida Cavalier-Smith 2012Order Spongomonadida Hibberd 1983 emend. Karpov 1990Order Zoelucasida Cavalier-Smith 2014Order Rotosphaerida Rainer 1968Order Thaumatomonadida Shirkina 1987Order Discocelida Cavalier-Smith 1997Order DiscomonadidaOrder VariglissidaOrder Marimonadida Cavalier-Smith & Bass 2011 [Auranticordida]Order Euglyphida Copeland 1956 emend. Cavalier-Smith 1997 [Euglyphina Bovee 1985]Class Thecofilosea Cavalier-Smith 2003 emend. Cavalier-Smith 2011 [Tripylea Hertwig 1879]Order Hemimastigida Foissner, Blatterer & Foissner 1988 [Hemimastigophora Foissner, Blatterer & Foissner 1988 sensu Cavalier-Smith 1993; Hemimastigea Foissner, Blatterer & Foissner 1988; Hemimastigidea]}Order Ventricleftida Cavalier-Smith 2011Order Matazida Cavalier-Smith 2012Order Ebriida Poche [Ebriidea Lemmermann 1901 emend. Deflandre 1936; Ebiophyta; Ebriacea Lemmermann 1901; Ebriophyceae Loeblich 1970; Ebriales Honigsberg 1964; Stereotestales]Order Cryomonadida Cavalier-Smith 1993 [Cryothecomonadea; Cryothecomonadida]Order TectofilosidaOrder Eodarida Cavalier-Smith 2012Order Opaloconchida Cavalier-Smith 2012