Everything about Centrohelid totally explained
| name = Centrohelids
| image = Raphidiophrys contractilis.jpg
| image_width = 220px
| image_caption =
Raphidiophrys contractilis
| domain =
Eukaryota
| ordo =
Centrohelida
| ordo_authority =
Kühn 1926
| subdivision_ranks = Families
| subdivision =
Raphidiophryidae
Acanthocystidae
Heterophryidae
}}
The
centrohelids or
centroheliozoa are a large group of
heliozoan protists. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth. Individuals are unicellular and spherical, usually around 30-80 μm in diameter, and covered with long radial axopods, narrow cellular projections that capture food and allow mobile forms to move about.
A few genera have no cell covering, but most have a gelatinous coat holding scales and spines, produced in special deposition vesicles. These may be organic or siliceous and come in various shapes and sizes. For instance, in
Raphidiophrys the coat extends along the bases of the axopods, covering them with curved spicules that give tham a pine-treeish look, and in
Raphidiocystis there are both short cup-shaped spicules and long tubular spicules that are only a little shorter than the axopods. Some other common genera include
Heterophrys,
Actinocystis, and
Oxnerella.
The axopods of centrohelids are supported by
microtubules in a triangular-hexagonal array, which arise from a tripartite granule called the
centroplast at the center of the cell. Axopods with a similar array occur in
gymnosphaerids, which have traditionally been considered centrohelids (though sometimes in a separate order from the others). This was questioned when it was found they've
mitochondria with tubular
cristae, as do other heliozoa, while in centrohelids the cristae are flat. Although this is no longer considered a very reliable character, on balance gymnosphaerids seem to be a separate group.
The evolutionary position of the centrohelids isn't clear. Structural comparisons with other groups are difficult, in part because no
flagella occur among centrohelids, and genetic studies have been more or less inconclusive. Cavalier-Smith has suggested they may be related to the
Rhizaria, but for the most part they're left with uncertain relations to other groups.
Further Information
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