Heliozoa
Group of protists with spherical bodies
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Through Protist, Amoeba and Fresh water
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:
- 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)
Several nucleariids 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.
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.