Cercozoa (now synonymised with Filosa) is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead united by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major eukaryotic group to be recognized mainly through molecular phylogenies. They are the natural predators of many species of bacteria. They are closely related to the phylum Retaria, comprising amoeboids that usually have complex shells, and together form a supergroup called Rhizaria.
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 phylogenetic studies.
Some cercozoans are grouped by whether they are "filose" or "reticulose" in the behavior of their cytoskeleton when moving:
Other important ecological groups are:
As well as being highly diverse in morphology and physiology, Cercozoa also shows high ecological diversity. The phylum Cercozoa includes many of the most abundant and ecologically significant protozoa in soil, marine and freshwater ecosystems.
Soil-dwelling cercozoans are one of the dominant groups of free-living eukaryotic microorganisms found in temperate soils, accounting for around 30% of identifiable protozoan DNA in arid or semi-arid soils and 15% in more humid soils. In transcriptomic analyses they account for 40-60% of all identifiable protozoan RNA found in forest and grassland soils. They also comprise 9-24% of all operational taxonomic units found in the ocean floor.
Some cercozoa are coprophilic or coprozoic, meaning they use feces as a source of nutrients or as transport through animal hosts. The faecal habitat is an understudied reservoir of microbial eukaryotic diversity, dominated by amoeboflagellates from the phylum Cercozoa. Strongly coprophilic examples of cercozoa are the flagellates Cercomonas, Proleptomonas and Helkesimastix, and the sorocarpic amoeba Guttulinopsis. Many new cercozoan lineages, especially among sarcomonads, have been discovered through phylogenetic sampling of feces because they appear preferentially in this medium.
Cercozoan bacterivores (i.e. predators of bacteria) are highly diverse and important in the plant phyllosphere, the leaf surfaces of plants. Particularly sarcomonads, with their ability to cyst, feed and multiply within hours, are perfectly adapted to the fluctuating environmental factors in the phyllosphere. Their predation causes shifts in the bacterial communities: they reduce populations of alphaproteobacteria and betaproteobacteria, which are less resistant to their grazing, in favour of other bacterial populations such as gammaproteobacteria.
Originally, Cercozoa contained both Filosa and Endomyxa, according to phylogenetic analyses using ribosomal RNA and tubulin. These analyses also confirmed Cercozoa as the sister group of Retaria within the supergroup Rhizaria.
However, the monophyly of the group was still uncertain. Posterior multigene phylogenetic analyses consistently found Cercozoa to be paraphyletic, because Endomyxa clustered next to Retaria instead of Filosa. Because of this, Endomyxa was excluded from Cercozoa, which became a synonym of Filosa.
More recent phylogenomic analyses with better sampling recovered a sister relationship between Filosa (=Cercozoa) and Endomyxa once again, although the modern classification of eukaryotes retains Endomyxa, Cercozoa and Retaria as separate phyla within Rhizaria.
The phylum Cercozoa previously contained both Filosa and Endomyxa, but in the latest classifications Endomyxa has been excluded, and Cercozoa is now synonymous with Filosa. It is composed of two subphyla: Monadofilosa and Reticulofilosa. According to multigene phylogenetic analyses, Monadofilosa is a robust clade, in which the deepest branching group is Metromonadea, followed by Helkesea as the second group (together forming the paraphyletic Eoglissa) before the divergence of the clade Ventrifilosa (Imbricatea, Sarcomonadea and Thecofilosea). On the other hand, Reticulofilosa is probably paraphyletic, with Granofilosea diverging earlier than Chlorarachnea, which makes Chlorarachnea the sister group of Monadofilosa.
A more recent phylogenomic analysis recovered both Monadofilosa and Reticulofilosa as monophyletic within the clade Filosa.
In addition to the known Granofilosea, Chlorarachnea and Monadofilosa, a variety of clades inside Cercozoa have been discovered in other analyses and have slowly been described and named, such as Tremulida (previously known as Novel Clade 11) and Aquavolonida (Novel Clade 10), although their specific positions among the two main cercozoan subphyla have yet to be refined. These two orders have been classified as the class Skiomonadea, within Reticulofilosa.
The classification of Cercozoa was revised in 2018: