Radiolarian
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| Radiolaria Fossil range: Cambrian - Recent |
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 Radiolaria illustration from the Challenger Expedition 1873-76.
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Radiolarians (also 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 zooplankton throughout the ocean, and because of their rapid turn-over of species, their tests are important diagnostic fossils found from the Cambrian onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.
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[edit] Description
Radiolarians have many needle-like pseudopodia supported by bundles of microtubules, called axopods, which aid in the Radiolarian's buoyancy. 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, 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.
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 have also included 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.
Some radiolarians are known for their resemblance to regular polyhedra, such as with the icosohedron-shaped Circogonia icosahedra pictured to the left.
[edit] Haeckel's radiolarians
German biologist Ernst Haeckel produced exquisite (and perhaps somewhat exaggerated) drawings of radiolaria, helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.
[edit] Illustrations from Kunstformen der Natur (1904)
 1. Phaeodaria |
 11. Discoidea |
 21. Acanthometra |
 22. Spyroidea |
 31. Cyrtoidea |
 41. Acanthophracta |
 51. Polycyttaria |
 61. Phaeodaria |
 71. Stephoidea |
 91. Spumellaria |
[edit] 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. doi:.
- 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.
- Sina M. Adl et al. (2005). "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists". Journal of Eukaryotic Microbiology 52 (5): 399-451. doi:.
- Haeckel, Ernst (2005). Art Forms from the Ocean: The Radiolarian Atlas of 1862. Munich; London: Prestel Verlag. ISBN 3-7913-3327-5.
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