Rhizaria

The Rhizaria are a large and varied group of mostly single-celled eukaryotes. Most lack the ability to photosynthesize, with the exceptions being the chlorarachniophytes and three species of *Paulinella* within the phylum Cercozoa. However, many Foraminifera and Radiolaria host symbiotic unicellular algae. One known multicellular species, *Guttulinopsis vulgaris*, is a cellular slime mold. The name "Rhizaria" was formally applied to this group by Cavalier-Smith in 2002, though the term had been used earlier for smaller clades within what is now recognized as the broader taxon. Because these organisms are primarily defined by their ribosomal DNA sequences, they come in many shapes and lack a single, clear defining physical feature (synapomorphy). Most are amoeboids that use pseudopods—which can be thread-like (filose), net-like (reticulose), or supported by microtubules (axopods). Without a unique morphological trait, the group is loosely defined, and its membership has changed frequently. Some Rhizaria build mineral shells or tests (thecae or loricas), which are made of opal (silica), celestite (strontium sulfate), or calcite (calcium carbonate), depending on the lineage. Certain species can grow larger than a centimeter, and some form cylindrical colonies about 1 cm wide and over 1 meter long. They feed by capturing and engulfing prey with their pseudopodia. Those that host symbiotic algae also contribute significantly to the ocean's total primary production. **Groups**

Rhizaria consist of three main groups: - **Cercozoa** – diverse amoebae and flagellates, often with filose pseudopods, common in soil. - **Foraminifera** – amoeboids with reticulose pseudopods, common in marine sediments. - **Radiolaria** – amoeboids with axopods, common as marine plankton. A few other groups may belong within Cercozoa, but some genetic trees place them closer to Foraminifera. These include the Phytomyxea and Ascetosporea (parasites of plants and animals, respectively) and the unusual amoeba *Gromia*. The various Rhizaria groups are considered close relatives mainly due to genetic similarities, and the group was originally seen as an extension of Cercozoa. Cavalier-Smith introduced the name Rhizaria for the expanded group in 2002, also including the centrohelids and Apusozoa at that time. A notable order within Ascetosporea is the Mikrocytida, which are parasites of oysters. One species, *Mikrocytos mackini*, is the cause of Denman Island Disease; it is a small (2–3 μm diameter) protist that lacks mitochondria. **History**

Similarities among different Rhizaria organisms have been noted since the 1800s. In 1861, zoologist William B. Carpenter classified the order Reticularia within the Rhizopoda (amoebae), grouping Foraminifera and Gromiida based on their very similar thin, reticulose pseudopodia with circulating granules. However, the idea that these organisms—along with Radiolaria and others—were all related emerged only recently, thanks to molecular phylogenetics and advanced microscopy in the late 20th century. **Evolutionary relationships**

Rhizaria are part of the SAR supergroup (Stramenopiles, Alveolates, Rhizaria), a grouping hinted at in 1993 by studies of mitochondrial shapes. SAR is now placed within the Diaphoretickes, alongside Archaeplastida, Cryptista, Haptista, and several smaller clades. Historically, many rhizarians were considered animals due to their movement and feeding habits. But when the simple animal-plant split gave way to additional kingdoms, taxonomists generally placed amoebae in the kingdom Protista. By the 1970s, studies of eukaryotic evolutionary relationships showed that Protista was paraphyletic. Rhizaria appear to share a common ancestor with Stramenopiles and Alveolates, forming the SAR supergroup. Molecular phylogenetic studies support Rhizaria as a monophyletic group. The biosynthesis of 24-isopropyl cholestane precursors in various rhizaria suggests they played a relevant ecological role as far back as the Ediacaran period. **Phylogeny**

Rhizaria is a monophyletic group made up of two main branches: Cercozoa and Retaria. The cladogram below shows evolutionary relationships among major rhizarian classes, based on phylogenomic analyses from 2018 to 2026. In the most recent analysis, the classes Sarcomonadea and Imbricatea were not monophyletic. **Sexual cycle**

Complete sexual life cycles have been documented in two lineages (Foraminifera and *Gromia*), and direct evidence for karyogamy or meiosis has been observed in five others (Euglyphida, Thecofilosea, Chlorarachniophyta, Plasmodiophorida, and Phaeodarea). Foraminifera, in particular, are marine amoebae defined by a dynamic network of pseudopodia and the production of intricate shells. They have complex sexual life cycles, with meiosis and gamete production occurring at separate stages.