Radiolaria

Radiolaria are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as plankton throughout the ocean, and their shells are important fossils found from the Cambrian onwards.

Radiolarians have many needle-like pseudopods supported by microtubules, called axopods, which aid in flotation. The nuclei and most other organelles are in the endoplasm, while the ectoplasm is filled with frothy vacuoles and lipid droplets, keeping them buoyant. Often it also contains symbiotic algae, especially zooxanthellae, that 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.

There are three classes of radiolarians: the Polycystinea and Phaeodarea, which produce siliceous skeletons, and the Acantharea, a small group that produce strontium sulfate skeletons. The majority of fossils are from polycystines. Despite initial suggestions to the contrary, genetic studies support a close relationship between the Polycystinea and Acantharea, and they share certain characters such as the presence of strontium sulfate. However, the Phaeodarea instead appear to belong among the Cercozoa, making the radiolarians polyphyletic. All three classes are included in the Rhizaria.

References

• Zettler, Linda A. (1997). Phylogenetic relationships between the Acantharea and the Polycystinea: A molecular perspective on Haeckel's Radiolaria. Proceedings of the National Academy of Sciences 94: 11411-11416.
• P. Lopez-Garcia et al. (2002). Toward the Monophyly of Haeckel's Radiolaria: 18S rRNA Environmental Data Support the Sisterhood of Polycystinea and Acantharea. Molecular Biology and Evololution 19(1): 118 - 121.

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