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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 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. They may be found in both freshwater and marine biology environments.
Several were once considered heliozoa, but they do not have microtubule-supported axopods and so are now considered filose amoeboids instead.
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. 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. 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.
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