Heliozoa, commonly known as sun-animalcules, are microbial
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
(
protists
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
) 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
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
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
and
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military ...
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)
*
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
Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major euk ...
)
*
Taxopodida/''
Sticholonche'' (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
nucleariid
Nucleariida is a group of amoebae with filose pseudopods, known mostly from soils and freshwater. They are distinguished from the superficially similar vampyrellids mainly by having mitochondria with discoid cristae, in the absence of superfic ...
s 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