Centrohelids

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The Centrohelids, or Centroheliozoa, are a large group of heliozoan protists, including both mobile and sessile forms found in both freshwater and marine environments, especially at some depth. They exist as individual cells, spherical in shape and covered with long radial pseudopods which capture food and allow mobile forms to roll along the substrate. These are supported by microtubules in a hexagonal-triangular array, which arise from a tripartite granule located at the center of the cell, called the centroplast. A similar organization is found within the Gymnosphaerids, formerly included in this group, but the Centrohelids are distinguished from these and indeed most other heliozoans by having flat rather than tubular cristae within the mitochondria.

Centrohelids are usually around 30-80 μm in body size. A few are naked but most have a gelatinous coat holding organic or silicaceous scales and spines, produced in special deposition vesicles, which 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 them a sort of pine-treeish look, and in Raphidiocystis there are both short cup-shaped spicules as well as radiate tubular spicules that are only a little shorter than the pseudopods.

There are three families of Centrohelids: the Raphidiophryidae, including both the genera named above, the Acanthocystidae, and the Heterophryidae, which includes relatively simple marine forms.