The Rhizaria are an ill-defined but species-rich supergroup of mostly

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s. Except for the

Chlorarachniophytes The chlorarachniophytes are a small group of exclusively marine algae widely distributed in tropical and temperate waters. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally the ...

and three species in the genus

Paulinella ''Paulinella'' is a genus of at least eleven species including both freshwater and marine amoeboids. Its most famous members are the three photosynthetic species ''P. chromatophora'', ''P. micropora'' and ''P. longichromatophora'', the first tw ...

in the phylum

Cercozoa Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major eu ...

, they are all non-photosynthethic, but many foraminifera and radiolaria have a symbiotic relationship with unicellular algae. A multicellular form, ''Guttulinopsis vulgaris'', a cellular

slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mu ...

, has also been described. This group was used by

Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford. His research has led to discov ...

in 2002, although the term "Rhizaria" had been long used for clades within the currently recognized taxon. Being described mainly from rDNA sequences, they vary considerably in form, having no clear morphological distinctive characters (

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

), but for the most part they are

amoeboid An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...

s with filose, reticulose, or microtubule-supported

pseudopod A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filamen ...

s. In the absence of an apomorphy, the group is ill-defined, and its composition has been very fluid. Some Rhizaria possess mineral exoskeleton (thecae or loricas), which is in different clades within Rhizaria made out of opal (), celestite (), or calcite (). It can attain sizes of more than a centimeter with some species being able to form cylindrical colonies approximately 1 cm in diameter and greater than 1 m in length. They feed by capturing and engulfing prey with the extensions of their pseudopodia; forms that are symbiotic with unicellular algae contribute significantly to the total primary production of the ocean.

Groups

The three main groups of Rhizaria are: *

– various amoebae and flagellates, usually with filose pseudopods and common in soil *

Foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...

– amoeboids with reticulose pseudopods, common as marine

benthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell (biology), cell into the inner and outer portions of endoplasm and Ecto ...

– amoeboids with

axopod A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filament ...

s, common as marine

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

A few other groups may be included in the Cercozoa, but some trees appear closer to the Foraminifera. These are the

Phytomyxea The Phytomyxea are a class of parasites that are cosmopolitan, obligate biotrophic protist parasites of plants, diatoms, oomycetes and brown algae. They are divided into the orders Plasmodiophorida (ICZN, or Plasmodiophoromycota, ICBN) and Pha ...

and

Ascetosporea The Ascetosporea are a group of eukaryotes that are parasites of animals, especially marine invertebrates. The two groups, the haplosporids and paramyxids, are not particularly similar morphologically, but consistently group together on molecula ...

, parasites of plants and animals, respectively, and the peculiar amoeba ''

Gromia ''Gromia'' is a genus of protists, closely related to foraminifera, which inhabit marine and freshwater environments. It is the only genus of the family Gromiidae. ''Gromia'' are ameboid, producing filose pseudopodia that extend out from the cel ...

''. The different groups of Rhizaria are considered close relatives based mainly on genetic similarities, and have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by

in 2002, who also included the

centrohelid The centrohelids or centroheliozoa are a large group of heliozoan protists. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth. Characteristics Individuals are unicellular and spher ...

s and

Apusozoa The Apusozoa are an Obazoa phylum comprising several genera of flagellate eukaryotes. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on bacteria. They are grouped together based on the prese ...

. A noteworthy order that belongs to

is the

Mikrocytida The Ascetosporea are a group of eukaryotes that are parasites of animals, especially marine invertebrates. The two groups, the haplosporids and paramyxids, are not particularly similar morphologically, but consistently group together on molecula ...

. These are parasites of

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...

s. This includes the causative agent of Denman Island Disease, ''Mikrocytos mackini'' a small (2−3 μm diameter) amitochondriate protistan.

Evolutionary relationships

Rhizaria are part of the

SAR supergroup The SAR supergroup, also just SAR or Harosa, is a clade that includes stramenopiles (heterokonts), alveolates, and Rhizaria. The name is an acronym derived from the first letters of each of these clades; it has been alternatively spelled "RAS". T ...

(Stramenopiles, Alveolates, Rhizaria), a grouping that had been presaged in 1993 through a study of mitochondrial morphologies. SAR is currently placed in the

Diaphoretickes Diaphoretickes () is a major group of eukaryotic organisms, with over 400,000 species. The majority of the earth's biomass that carries out photosynthesis belongs to Diaphoretickes. Diaphoretickes includes: * Archaeplastida (comprising red alg ...

along with

Archaeplastida The Archaeplastida (or kingdom Plantae ''sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group ...

Cryptista Cryptista is a clade of algae-like eukaryotes. It is most likely related to Archaeplastida which includes plants and many algae, within the larger group Diaphoretickes. Although it has sometimes placed along with Haptista in the group Hacrobia, ...

Haptista Haptista is a proposed group of protists made up of centrohelids and haptophytes. Phylogenomic studies indicate that Haptista, together with ''Ancoracysta twista'', forms a sister clade to the SAR+Telonemia supergroup, but it may also be sister t ...

, and several minor clades. Historically, many rhizarians were considered

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s because of their motility and

heterotrophy A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

. However, when a simple animal-plant dichotomy was superseded by a recognition of additional kingdoms, taxonomists generally placed amoebae in the kingdom

Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...

. When scientists began examining the evolutionary relationships among eukaryotes in the 1970's, it became clear that the kingdom

was

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

. Rhizaria appear to share a common ancestor with

Stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...

and

Alveolates The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochondr ...

forming part of the SAR (Stramenopiles+Alveolates+Rhizaria) super assemblage. Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group. Biosynthesis of 24-isopropyl cholestane precursors in various rhizaria suggests a relevant ecological role already during the

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

Phylogeny

Rhizaria is a

monophyletic group A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

composed of two sister phyla:

and

Retaria Retaria is a clade within the supergroup Rhizaria containing the Foraminifera and the Radiolaria. In 2019, the Retaria were recognized as a basal Rhizaria group, as sister of the Cercozoa Cercozoa is a phylum of diverse single-celled eukaryot ...

. Subsequently, Cercozoa and Retaria are also

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

. The following cladogram depicts the evolutionary relationships between all rhizarian

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

es, and is made after the works of

''et al.'' (2018), Irwin ''et al.'' (2019) and Sierra ''et al.'' (2022):

Gallery

File:Cercomonas sp.jpg, ''Cercomonas'' sp. (

Cercomonadida Cercomonads are small flagellates, widespread in aqueous habitats and common in soils. Characteristics The cells are generally around 10 μm in length, without any shell or covering. They produce filose pseudopods to capture bacteria, but do no ...

File:Ebria tripartita.jpg, ''Ebria'' sp. (Cercozoa:

Ebridea The Ebridea is a group of phagotrophic flagellate eukaryotes present in marine coastal plankton communities worldwide. ''Ebria tripartita'' is one of two (possibly four) described extant species in the Ebridea. Members of this group are named for ...

) File:Rhipidodendron splendidum.jpg, ''Rhipidodendron'' sp. (Cercozoa:

Spongomonadea The spongomonads are a group of flagellated protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ...

) File:Euglypha sp.jpg, ''Euglypha'' sp. (Cercozoa:

Euglyphida The euglyphids are a prominent group of filose amoebae that produce shells or tests from siliceous scales, plates, and sometimes spines. These elements are created within the cell and then assembled on its surface in a more or less regular arra ...

) File:Haeckel Phaeodaria 1.jpg, Phaeodarians (Cercozoa:

Phaeodarea Phaeodarea, or Phaeodaria, is a group of amoeboid cercozoan organisms. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa. They ...

) File:Clathrulina elegans - - Print - Iconographia Zoologica - Special Collections University of Amsterdam - UBAINV0274 113 04 0030.tif, ''

Clathrulina elegans ''Clathrulina elegans'' is a species of heliozoan eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryo ...

'' (Cercozoa: Granofilosea: Desmothoracida) File:Chlorarachnion reptans.jpg, ''

Chlorarachnion The chlorarachniophytes are a small group of exclusively marine algae widely distributed in tropical and temperate waters. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally the ...

'' sp. (Cercozoa: (

Chlorarachniophyta The chlorarachniophytes are a small group of exclusively marine algae widely distributed in tropical and temperate waters. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally the ...

) File:Vampyrella lateritia.jpg, ''Vampyrella'' sp. (Cercozoa:

Vampyrellidae The family Vampyrellidae is a subgroup of the order Aconchulinida (formerly Vampyrellida) within the phylum Cercozoa. Based on molecular sequence data, the family currently comprises the genus ''Vampyrella'', and maybe several other vampyrellid ...

) File:Die Gartenlaube (1890) b 083.jpg, ''

'' (Cercozoa:

Gromiidea Gromiida is an order of cercozoans. It is the only order in the class Gromiidea. Taxonomy Class Gromiidea Cavalier-Smith 2003 sensu Bass et al. 2009 * Order Gromiida Claparède & Lachmann 1856 s.s. ** Family Gromiidae Ruess 1862 romiina Delag ...

) File:Powdery scab (Spongospora subterranea) of potatoes (1914) (14577838889).jpg,

Powdery scab Powdery scab is a disease of potato tubers. It is caused by the cercozoan ''Spongospora subterranea'' f. sp. ''subterranea'' and is widespread in potato growing countries. Symptoms of powdery scab include small lesions in the early stages of the ...

(Cercozoa:

Plasmodiophorida The plasmodiophores (also known as plasmophorids or plasmodiophorids) are a group of obligate endoparasitic protists belonging to the subphylum Endomyxa in Cercozoa. Taxonomically, they are united under a single family Plasmodiophoridae, order ...

) File:Foraminifères de Ngapali.jpg, Foraminiferans (

) File:Mikrofoto.de-Radiolarien-3.jpg,

Polycystine The polycystines are a group of radiolarians. They include the vast majority of the fossil radiolaria, as their skeletons are abundant in marine sediments, making them one of the most common groups of microfossils. These skeletons are composed ...

s (Retaria:

Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell (biology), cell into the inner and outer portions of endoplasm and Ecto ...

) File:Haeckel Acanthometra.jpg, Acantharians (Retaria:

)

References

External links

Molecular Phylogeny of Amoeboid Protists - Tree of Rhizaria

Tree of Life Eukaryota
* https://www-nature-com. {{Taxonbar, from=Q855740 Neoproterozoic first appearances Taxa named by Thomas Cavalier-Smith Infrakingdoms