Radiolaria
| Radiolaria Temporal range: Cambrian – Recent |
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|---|---|
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| Radiolaria illustration from the Challenger Expedition 1873–76. | |
| Scientific classification | |
| Domain: | Eukaryota |
| (unranked): | SAR |
| (unranked): | Rhizaria |
| Phylum: | Retaria |
| Subphylum: |
Radiozoa Cavalier-Smith, 1987 |
| Classes | |
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Polycystinea |
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Polycystinea
Acantharea
Sticholonchea (formerly in Heliozoa)
Excluded: Phaeodaria (now in Cercozoa)
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. They are found as zooplankton throughout the ocean, and their skeletal remains make up a large part of the cover of the ocean floor as siliceous ooze. Due to their rapid turn-over of species, they represent an important diagnostic fossil found from the Cambrian onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.
Radiolarians have many needle-like pseudopodia supported by bundles of microtubules, which aid in the radiolarian's buoyancy. The cell nucleus and most other organelles are in the endoplasm, while the ectoplasm is filled with frothy vacuoles and lipid droplets, keeping them buoyant. The radiolarian can often contain symbiotic algae, especially zooxanthellae, which provide most of the cell's energy. Some of this organization is found among the heliozoa, but those lack central capsules and only produce simple scales and spines.
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