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

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. Bacter ...

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 t ...

and three species in the genus Paulinella 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 euk ...

, 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, has also been described. This group was used by Cavalier-Smith 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 ha ...

), but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. 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 for catching food and other uses; and commonly ...

– 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 into the inner and outer portions of endoplasm and ectoplasm. The el ...

– amoeboids with axopods, common as marine

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

A few other groups may be included in the Cercozoa, but some trees appear closer to the Foraminifera. These are the Phytomyxea 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''. 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 Cavalier-Smith in 2002, who also included the centrohelids 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 ...

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 (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,

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 Hacrob ...

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

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

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 e ...

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

was

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

. 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 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 t ...

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. 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 gr ...

. 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 differently ...

es, and is made after the works of Cavalier-Smith ''et al.'' (2018), Irwin ''et al.'' (2019) and Sierra ''et al.'' (2022):

Gallery

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

: Cercomonadida File:Ebria tripartita.jpg, ''Ebria'' sp. (Cercozoa: Ebridea) File:Rhipidodendron splendidum.jpg, ''Rhipidodendron'' sp. (Cercozoa:

Spongomonadea The spongomonads are a group of flagellated protists in the phylum Cercozoa. Taxonomically, they compose the family Sarcomonadidae and order Sarcomonadida. They were originally placed among the Reticulofilosa, but were later transferred to Monad ...

) File:Euglypha sp.jpg, ''Euglypha'' sp. (Cercozoa: Euglyphida) File:Haeckel Phaeodaria 1.jpg, Phaeodarians (Cercozoa: Phaeodarea) File:Clathrulina elegans - - Print - Iconographia Zoologica - Special Collections University of Amsterdam - UBAINV0274 113 04 0030.tif, '' Clathrulina elegans'' (Cercozoa: Granofilosea: Desmothoracida) File:Chlorarachnion reptans.jpg, '' Chlorarachnion'' 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 vampyrel ...

) File:Die Gartenlaube (1890) b 083.jpg, '' Gromia'' (Cercozoa: Gromiidea) File:Powdery scab (Spongospora subterranea) of potatoes (1914) (14577838889).jpg, Powdery scab (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 P ...

) File:Foraminifères de Ngapali.jpg, Foraminiferans ( Retaria:

) File:Mikrofoto.de-Radiolarien-3.jpg, Polycystines (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 into the inner and outer portions of endoplasm and ectoplasm. The el ...

) 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