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Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, 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 fresh water and marine environments.
Originally the heliozoa were treated together as a formal class Heliozoa or Heliozoea, but it has been realised that they are polyphyletic, as the various orders show notable differences and are no longer believed to be related. Instead, heliozoa is regarded as a descriptive term applying to various lines of protists.
The primary groups include:
- Actinophryida (currently in Chromalveolata)
- Centrohelida (some support for Hacrobia)
- Desmothoracida, Heliomonadida and Gymnosphaerida (currently in Rhizaria > Cercozoa)
- Sticholonche (currently in Rhizaria > Radiozoa)
Several nucleariids were once considered heliozoa, but they do not have microtubule-supported axopods and so are now considered filose amoeboids instead.
- Cavalier-Smith T, von der Heyden S (September 2007). "Molecular phylogeny, scale evolution and taxonomy of centrohelid heliozoa". Mol. Phylogenet. Evol. 44 (3): 1186–203. doi:10.1016/j.ympev.2007.04.019. PMID 17588778.
- Nikolaev SI, Berney C, Fahrni JF, et al (May 2004). "The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes". Proc. Natl. Acad. Sci. U.S.A. 101 (21): 8066–71. doi:10.1073/pnas.0308602101. PMC 419558. PMID 15148395.
Actinosphaerium (single-celled) is an example of a heliozoa. For an image see