Radiolaria
From Freepedia
| Radiolaria Fossil range: Cambrian - Recent | ||||
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| Image:Radiolaria3434.JPG Radiolarian skeletons (artificially coloured) | ||||
| Scientific classification | ||||
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| Possible classes | ||||
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. These include the majority of fossils. They also include the Acantharea, which produce skeletons of strontium sulfate. Despite some initial suggestions to the contrary, genetic studies place these two groups close together. They also include the peculiar genus Sticholonche, which lacks an internal skeleton and so is usually considered a heliozoan.
Traditionally the radiolarians also include the Phaeodarea, which produce siliceous skeletons but differ from the polycystines in several other respects. However, on molecular trees they branch with the Cercozoa, a group including various flagellate and amoeboid protists. The other radiolarians appear near, but outside, the Cercozoa, so the similarity is due to convergent evolution. The radiolarians and Cercozoa are included within a supergroup called the Rhizaria.
External links
- Radiolarians
- Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests
- Radiolaria.org
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 Evolution 19 (1): 118-121.
- Art Forms from the Ocean: The Radiolarian Atlas of 1862, by Ernst Haeckel, Prestel Verlag, 2005 ISBN 3791333275
