|
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.
The main class of radiolarians are the Polycystinea, which produce siliceous skeletons and include the majority of fossils. The other traditional classes are the Phaeodarea, which also produce siliceous skeletons, and Acantharea, which produce skeletons of strontium sulfate. Despite some initial suggestions to the contrary, genetic studies group the Polycystinea and Acantharea together, along with the peculiar genus Sticholonche. However, the Phaeodarea are instead included among the Cercozoa. All of these classes are part of 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.
|
