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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 An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...
cell body, and are variously used for capturing food, sensation, movement, and attachment. They are similar to
Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...
, 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 environments.


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 A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
, 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 Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...
) * Centrohelida (some support for
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 ...
) * Desmothoracida, Heliomonadida/Dimorphida and Gymnosphaerida (currently in
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 forami ...
> Cercozoa) *
Taxopodida ''Sticholonche'' is a genus of radiolarians with a single species, ''Sticholonche zanclea'', found in open oceans at depths of 99–510 metres. It is generally considered a heliozoan, placed in its own order, called the Taxopodida. However it ...
/''
Sticholonche ''Sticholonche'' is a genus of radiolarians with a single species, ''Sticholonche zanclea'', found in open oceans at depths of 99–510 metres. It is generally considered a heliozoan, placed in its own order, called the Taxopodida. However it ...
'' (currently in
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 forami ...
>
Radiolaria The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The el ...
) * Rotosphaerida (currently in Opisthokonta > Nucleariida and in
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 forami ...
) Several nucleariids were once considered heliozoa, but they do not have microtubule-supported axopods and so are now considered filose amoeboids instead.


References


External links

* ''Actinosphaerium'' (single-celled) is an example of a heliozoa. For an image see http://www.microscopy-uk.org.uk/index.html?http://www.microscopy-uk.org.uk/ponddip/ * Obsolete eukaryote taxa Eukaryote orders {{Eukaryote-stub