The centrohelids or centroheliozoa are a large group of

heliozoa Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes (protists) with stiff arms (Pseudopodia#Morphology, axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule- ...

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.

Characteristics

Individuals are unicellular and spherical, usually around 30–80 μm in diameter, and covered with long radial axopods, narrow cellular projections that capture food and allow mobile forms to move about. A few genera have no cell covering, but most have a gelatinous coat holding scales and spines, produced in special deposition vesicles. These may be organic or siliceous and come in various shapes and sizes. For instance, in ''

Raphidiophrys Raphidiophrys is a genus of 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. Characteris ...

'' the coat extends along the bases of the axopods, covering them with curved spicules that give them a pine-treeish look, and in ''Raphidiocystis'' there are both short cup-shaped spicules and long tubular spicules that are only a little shorter than the axopods. Some other common genera include ''

Heterophrys 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 ...

'', ''Actinocystis'', and ''

Oxnerella ''Oxnerella'' is a genus of 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. Characteris ...

''. The axopods of centrohelids are supported by

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 ...

s in a triangular-hexagonal array, which arise from a tripartite granule called the ''centroplast'' at the center of the cell. Axopods with a similar array occur in

gymnosphaerid The gymnosphaerids (or Gymnosphaerida) are a small group of heliozoan protists found in marine environments. They tend to be roughly spherical with radially directed axopods, supported by microtubules in a triangular-hexagonal array arising from ...

s, which have traditionally been considered centrohelids (though sometimes in a separate order from the others). This was questioned when it was found they have

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

with tubular

crista A crista (; : cristae) is a fold in the inner mitochondrial membrane, inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large a ...

e, as do other heliozoa, while in centrohelids the cristae are flat. Although this is no longer considered a very reliable character, on balance gymnosphaerids seem to be a separate group. 2023 Centrohelid

Taxonomy

History

The evolutionary position of the centrohelids is not clear. Structural comparisons with other groups are difficult, in part because no

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

occur among centrohelids, and genetic studies have been more or less inconclusive. Cavalier-Smith has suggested they may be related to the

Rhizaria The Rhizaria are a diverse and species-rich clade of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus '' Paulinella'' in the phylum Cercozoa, they are all non-photosynthetic, but many Foraminifera ...

, but for the most part they are left with uncertain relations to other groups. A 2009 paper suggests that they may be related to the cryptophytes and haptophytes (see

Cryptomonads-haptophytes assemblage The cryptomonads-haptophytes assemblage is a proposed but disputed monophyletic grouping of unicellular eukaryotes that are not included in the SAR supergroup. Several alternative names have been used for the group, including Hacrobia (derived ...

). They are currently classified as

Hacrobia The cryptomonads-haptophytes assemblage is a proposed but disputed monophyletic grouping of unicellular eukaryotes that are not included in the SAR supergroup. Several alternative names have been used for the group, including Hacrobia (derived ...

, under the

Plants+HC clade Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars fro ...

, although some research studies have found evidence against the monophyly of this group. Centrohelids were previously divided into two orders with contrasting scale morphology and ultrastructure: Pterocystida and Acanthocystida. Posterior molecular studies of 2018 have rearranged the classification of centrohelids into two taxa: Pterocystida and Panacanthocystida, which includes both Acanthocystida and the genus '' Yogsothoth''.

Classification

The modern classification of centrohelids, as of 2019: * Centroplasthelida Centrohelea ; Centroheliozoa ** Pterocystida *** Raphidista **** Choanocystidae Cavalier-Smith & von der Heyden 2007 ***** '' Choanocystis'' Penard 1904 non Cognetti 1918 ****

Raphidiophryidae Raphidiophryidae is a family of mostly freshwater centrohelids. It is the sister family of Acanthocystidae, sharing the trait of presenting silica scales and comprising the clade Chalarothoracina. Two genera, ''Raphidiophrys Raphidiophrys is a ...

Mikrjukov 1996 emend. Cavalier-Smith & von der Heyden 2007 ***** ''

'' Archer 1867 'Raphidiaphrys'' (sic) Greeff 1869*** Pterista **** Oxnerellidae Cavalier-Smith & Chao 2012 ***** ''

'' Dobell 1917 **** Pterocystidae Cavalier-Smith & von der Heyden 2007 ***** '' Chlamydaster'' Rainer 1968 ***** '' Pseudoraphidocystis'' Mikrjukov 1997 'Pseudoraphidiocystis'' (sic)***** '' Pseudoraphidiophrys'' Mikrjukov 1997 ***** '' Pterocystis'' Siemensma & Roijackers 1988 non Lohmann 1904 ***** '' Raineriophrys'' Mikrjukov 2001 'Rainierophrys'' (sic); ''Raineria'' Mikrjukov 1999 non Osswald 1928 non de Notaris 1838; ''Echinocystis">Raineria.html" ;"title="'Rainierophrys'' (sic); ''Raineria">'Rainierophrys'' (sic); ''Raineria'' Mikrjukov 1999 non Osswald 1928 non de Notaris 1838; ''Echinocystis'' Mikrjukov1997 non Haeckel 1896 non Bhatia & Chatterjee 1925 non Torrey & Gray 1840 non Gregory 1897] **** Heterophryidae Poche 1913 ***** ''

'' Archer 1869 ***** '' Parasphaerastrum'' Mikrjukov 1996 ***** '' Sphaerastrum'' Greeff 1873 ** Panacanthocystida ***

Chthonida ''Yogsothoth'' is a genus of centrohelid protists, distinguished by the shape and arrangement of their external scales as well as their colonial life strategy. It was described in November 2018 by Shɨshkin and Zlatogursky, and is part of a newly ...

**** Yogsothothina ***** Yogsothothidae ****** '' Yogsothoth'' *** Acanthocystida Cavalier-Smith 2011 **** Marophryina Cavalier-Smith 2011 ***** Marophryidae Cavalier-Smith & von der Heyden 2007 ****** '' Marophrys'' **** Chalarothoracina Hertwig & Lesser 1874 stat n. Cavalier-Smith 2011 ***** Raphidocystidae ****** '' Raphidocystis'' Penard 1904 'Raphidiocystis'' (sic) Doflein 1928 ; ''Rhaphidocystis'' (sic)***** Acanthocystidae Claus 1874 emend. Cavalier-Smith & von der Heyden 2007 ****** '' Acanthocystis'' Carter 1863 non Kuehner 1926 non Bather 1889 non Haeckel 1896 nomen nudum ** Incertae sedis Centroplasthelida: *** '' Parasphaerastrum'' Mikrjukov 1996 *** '' Heteroraphidiophrys'' Mikrjukov & Patterson 2000 *** Spiculophryidae **** '' Spiculophrys'' Zlatogursky 2015

Characteristics

Taxonomy

History

Classification

Notes

References

Further reading