The centrohelids or centroheliozoa are a large group of

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

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 ancestor (the last eukaryotic common ancestor), the exc ...

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 nm and have an inner diameter between 11 an ...

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 fro ...

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 Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

with tubular

crista A crista (; plural cristae) is a fold in the 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 amount of surface area fo ...

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.

Classification

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

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

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 an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramini ...

, 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

haptophyte The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for ''Prymnesium''), are a clade of algae. The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at t ...

s (see

Cryptomonads-haptophytes assemblage The cryptomonads-haptophytes assemblage is a proposed 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 from "ha-" refe ...

). They are currently classified as

Hacrobia The cryptomonads-haptophytes assemblage is a proposed 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 from "ha-" refe ...

, under the

Plants+HC clade Plants+HC clade is a group of eukaryotes proposed by Burki ''et al.'' (2008). It includes: * Plants * Hacrobia (the "HC" refers to Cryptophyta and Haptophyta, the two major members of this group) See also * photosynthesis Photosynthesis i ...

, although some research studies have found evidence against the monophyly of this group. Centrohelids are currently divided into two orders with contrasting scale morphology and ultrastructure: Pterocystida and Acanthocystida. * Class Centrohelea Kuhn 1926 stat. n. Cavalier-Smith 1993 ** Genus '' Spiculophrys'' Zlatogursky 2015 ** Order Pterocystida Cavalier-Smith 2011 *** Family Choanocystidae Cavalier-Smith & von der Heyden 2007 **** Genus '' Choanocystis'' Penard 1904 non Cognetti 1918 *** Family Oxnerellidae Cavalier-Smith & Chao 2012 **** Genus ''

'' Dobell 1917 *** Family Heterophryidae Poche 1913 **** Genus ''

'' Archer 1869 **** Genus '' Parasphaerastrum'' Mikrjukov 1996 **** Genus '' Sphaerastrum'' Greeff 1873 *** Family Pterocystidae Cavalier-Smith & von der Heyden 2007 **** Genus '' Chlamydaster'' Rainer 1968 **** Genus '' Pterocystis'' Siemensma & Roijackers 1988 non Lohmann 1904 **** Genus '' 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] ** Order Acanthocystida Cavalier-Smith 2011 *** Suborder Marophryina Cavalier-Smith 2011 **** Family Marophryidae Cavalier-Smith & von der Heyden 2007 ***** Genus '' Marophrys'' *** Suborder Chalarothoracina Hertwig & Lesser 1874 stat n. Cavalier-Smith 2011 **** Family

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 ***** Genus '' Heteroraphidiophrys'' Mikrjukov & Patterson 2000 ***** Genus '' Polyplacocystis'' Mikrjukov 1996 ***** Genus '' Pseudoraphidiophrys'' Mikrjukov 1997 ***** Genus ''

'' Archer 1867 'Raphidiaphrys'' (sic) Greeff 1869**** Family Acanthocystidae Claus 1874 emend. Cavalier-Smith & von der Heyden 2007 ***** Genus '' Raphidocystis'' Penard 1904 'Raphidiocystis'' (sic) Doflein 1928 ; ''Rhaphidocystis'' (sic)***** Genus '' Pseudoraphidocystis'' Mikrjukov 1997 'Pseudoraphidiocystis'' (sic)***** Genus '' Acanthocystis'' Carter 1863 non Kuehner 1926 non Bather 1889 non Haeckel 1896 nomen nudum

Characteristics

Classification

References

Further reading