The Rhizaria are a diverse and species-rich

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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 The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s. Except for the Chlorarachniophytes and three species in the genus ''

Paulinella ''Paulinella'' is a genus of at least eleven species including both freshwater and marine amoeboids. Like many members of euglyphids it is covered by rows of siliceous scales, and use filose pseudopods to crawl over the substrate of the benthi ...

'' in the phylum

Cercozoa 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 phylogeny, molecular phylogenies of rRNA and actin or Ubiqu ...

, they are all non-photosynthetic, but many

Foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

and

Radiolaria The Radiolaria, also called Radiozoa, are unicellular eukaryotes 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 ect ...

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 a polyphyletic assemblage of unrelated eukaryotic organisms in the Stramenopiles, Rhizaria, Discoba, Amoebozoa and Holomycota clades. Most are near-microscopic; those in the Myxogastria ...

, has been described. This group was used by

Cavalier-Smith Thomas (Tom) Cavalier-Smith, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Departme ...

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 ...

), but for the most part they are

amoeboid An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and r ...

s with

filose 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. ...

, reticulose, or

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

-supported

pseudopod A pseudopod or pseudopodium (: 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 filaments and ...

s. In the absence of an apomorphy, the group is ill-defined, and its composition has been very fluid. Some Rhizaria possess mineral exoskeletons (

thecae In biology, a theca (: thecae) is a sheath or a covering. Botany In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common are ...

or loricas), which are in different clades within Rhizaria made out of

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

(),

celestite Celestine (the IMA-accepted name) or celestite is a mineral consisting of strontium sulfate ( Sr S O). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources o ...

(), or

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

(). Certain species 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 *

– 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 unicellular eukaryotes 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 ect ...

– amoeboids with axopods, common as marine

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

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 (biology), class of parasites that are cosmopolitan, obligate biotrophic protist parasites of plants, diatoms, oomycetes and brown algae. They are divided into the orders Plasmodiophorida (International Code of Zoolog ...

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 a paraphyletic phylum 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 presence of an organic shel ...

. A noteworthy order that belongs to

is the Mikrocytida. 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 no ...

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

History

Similarities between various Rhizaria organisms have been noticed since the 19th century. In his 1861 classification of the Rhizopoda (amoebae), the zoologist William B. Carpenter proposed the order Reticularia, which consisted of

and

Gromiida Gromiida is an order of Endomyxa. 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 Delage ...

on the basis of their very similar thin, reticulose pseudopodia with granules circulating inside. However, the idea that these organisms and others such as

were all related to one another emerged rather recently, with the help of molecular phylogenetics and advanced microscopy techniques in the late 20th century.

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 eukaryote, eukaryotic organisms spanning over 400,000 species. The majority of the earth's biomass that carries out photosynthesis belongs to Diaphoretickes. In older classification systems, members of the Diaph ...

along with

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

Cryptista Cryptista is a clade of alga-like eukaryotes. It is most likely related to Archaeplastida which includes plants and many algae, within the larger group Diaphoretickes. Other characteristic features of cryptophyte mtDNAs include large syntenic ...

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 supergroup#Internal phylogeny, TSAR Supergroup (b ...

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

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

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 ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...

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

was

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. Rhizaria appear to share a common ancestor with

Stramenopiles The stramenopiles, also called heterokonts, are Protist, protists 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 sur ...

and

Alveolates The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major unranked clade or superphylum within Eukaryota. They are currently grouped with the Stramenopiles and Rhizaria among the protists with tubulocristat ...

forming part of the SAR 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 ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

Phylogeny

Rhizaria is a

monophyletic group In biology, a clade (), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy ...

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 (now synonymised with Filosa) is a phylum of ...

. Subsequently, Cercozoa and Retaria are also

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

.> The following cladogram depicts the evolutionary relationships between all rhizarian

class Class, Classes, 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 d ...

es, and is made after the works of

''et al.'' (2018), and Irwin ''et al.'' (2019).

Sexual cycle

Complete sexual life cycles have been demonstrated for two lineages (

and ''

'') and direct evidence for

karyogamy Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two cell nucleus, nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. ...

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

has been observed in five lineages (''

Euglyphida The euglyphids are a prominent group of filose amoebae that produce shells or tests that in most described species is reinforced by siliceous scales, plates, and sometimes spines, but this reinforcement is absent in other species. These eleme ...

'', ''

Thecofilosea Thecofilosea is a class of unicellular testate amoebae belonging to the phylum Cercozoa. They are amoeboflagellates, organisms with flagella and pseudopodia, distinguished from other cercozoa by their scale-lacking test composed of organic materi ...

'', ''Chlorarachniophyta'', ''Plasmodiophorida'' and ''

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 a ...

''). In particular, the Foraminifera are marine

amoeba An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...

e that are defined by a dynamic network of

pseudopodia A pseudopod or pseudopodium (: 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 filaments and ...

, and the production of intricate shells. These amoeba have complex sexual life cycles with meiosis and

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

production occurring at separate stages.

References

External links

Rhizaria
at UniEuk Taxonomy Map

Tree of Life Eukaryota
{{Taxonbar, from=Q855740 Neoproterozoic first appearances Taxa named by Thomas Cavalier-Smith Infrakingdoms