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.^[2][3][4] The axopodia are microtubule-supported projections from the amoeboid cell body, and are variously used for capturing food, sensation, movement, and attachment. They are similar to Radiolaria, but they are distinguished from them by lacking central capsules and other complex skeletal elements, although some produce simple scales and spines.^[5] They may be found in both freshwater and marine environments.

Heliozoa
Heliozoan depicted by Louis Joblot, 1718
Scientific classification (obsolete)
Domain:	Eukaryota
Kingdom:	Chromista
Subkingdom:	Hacrobia
Phylum:	Heliozoa Haeckel, 1866
Classes[1]
Centrohelea

Classification

Originally the heliozoa were treated together as a formal taxon Heliozoa or Heliozoea, with the rank of class or phylum, but it has been realised that they are polyphyletic, as the various orders show notable differences and are no longer believed to be descended from a single common ancestor. Instead, "heliozoa" is regarded as a descriptive term applying to various lines of protists.

The primary groups include:^[6]

• Actinophryida, Pedinellida and Ciliophryida (currently in Stramenopiles)
• Centrohelida (currentily in Haptista)
• Desmothoracida, Heliomonadida/Dimorphida and Gymnosphaerida (currently in Rhizaria > Cercozoa)
• Taxopodida/Sticholonche (currently in Rhizaria > Radiolaria)
• Rotosphaerida (currently in Opisthokonta > Nucleariida and in Rhizaria)^[7]

Several nucleariids were once considered heliozoa, but they do not have microtubule-supported axopods and so are now considered filose amoeboids instead.

Freshwater heliozoan feeding on small flagellated algae

In 2012, Cavalier-Smith proposed maintaining the name Heliozoa as a formal taxon by reducing it to only centrohelids, which are the most species-abundant group of heliozoa to date. He established a new phylum Heliozoa emended to include only two classes: Centrohelea, the centrohelids; and Endohelea, a smaller group containing only the genera Heliomorpha and Microheliella.^[8] This was short-lived, however, because later the same author transferred Centrohelea to the phylum Haptista and Endohelea to the phylum Cryptista, making his phylum Heliozoa polyphyletic once more. From 2015 onwards, he proposed yet another definition for Heliozoa, exclusively as a monotypic subphylum of Haptista containing only the class Centrohelea.^[9] This renewed circumscription never reached consensus, and Heliozoa continues to be used in the same regard as the traditional, polyphyletic category including amoebae other than centrohelids.^[10][11][12]

Phylogeny

The heliozoa are a polyphyletic grouping of various protists that have independently evolved axopodial arms. Some of the heliozoan groups are intermingled in the supergroup Rhizaria with radiolarians, their mostly marine counterpart.^[6]

Eukaryotes	Diaphoretickes SAR Stramenopiles Raphidomonadea Actinophryida Dictyochophyceae Pedinellales Rhizaria Cercozoa Thecofilosea Phaeodaria (classical radiolarians) Granofilosea Desmothoracida ? Gymnosphaerida Radiolaria Acantharea (classical radiolarians) Taxopodida Polycystinea (classical radiolarians) Haptista Centroplasthelida Pancryptista Microhelida Plants Opisthokonta Nucleariida