Radiolarian

Radiolaria Temporal range: Cambrian - Recent PreЄ Є O S D C P T J K Pg N
Radiolaria illustration from the Challenger Expedition 1873-76.
Scientific classification
Domain:	Eukaryota
Kingdom:	Rhizaria
Superphylum:	Retaria
Phylum:	Radiolaria Müller 1858 emend.
Classes
Polycystinea Acantharea Sticholonchea

Radiolarians (also radiolaria) are amoeboid protozoa (diameter 0.1-0.2 mm) 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 their skeletal remains cover large portions of the ocean bottom as radiolarian 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.

Description

Circogonia icosahedra, a species of Radiolaria, shaped like a regular icosahedron

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.

Some radiolarians are known for their resemblance to regular polyhedra, such as with the icosahedron-shaped Circogonia icosahedra pictured to the left.

Taxonomy

The radiolarians and Cercozoa are included within a supergroup called the Rhizaria.

Traditionally the radiolarians have been divided into four groups - Acantharia, Nassellaria, Phaeodaria and Polycystina - on morphological grounds.

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. Other radiolarians appear near, but outside, the Cercozoa, so the similarity is probably due in part to shared ancestry and in part to convergent evolution.

The heliozoan group Taxopodida have been shown by genetic studies to lie within this group.

Molecular studies have shown that the Radiolaria is divided into two main lineages: Polycystina (Spumellaria + Nassellaria) and Spasmaria (Acantharia + Taxopodida). They are also closely related to the Foraminifera groups supporting the Retaria hypothesis.

Fossil record

The earliest known radiolaria date to the very start of the Cambrian period, appearing in the same beds as the first small shelly fauna - they may even be terminal Precambrian in age. They have significant differences from later radiolaria, with a different silica lattice structure and few, if any, spikes on the test. Ninety percent of radiolarian species are extinct. The skeletons, or tests, of ancient radiolarians are used in geological dating, including for oil exploration and determination of ancient climates.

Haeckel's radiolarians

German biologist Ernst Haeckel produced finely detailed drawings of radiolaria, helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.

Illustrations from Haeckel's Kunstformen der Natur (1904)

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  1. Phaeodaria

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  11. Discoidea

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  21. Acanthometra

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  22. Spyroidea

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  31. Cyrtoidea

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  41. Acanthophracta

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  51. Polycyttaria

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  61. Phaeodaria

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  71. Stephoidea

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  91. Spumellaria

References

• Zettler, Linda A.; Sogin, ML; Caron, DA (1997). "Phylogenetic relationships between the Acantharea and the Polycystinea: A molecular perspective on Haeckel's Radiolaria". Proceedings of the National Academy of Sciences 94 (21): 11411–11416. doi:10.1073/pnas.94.21.11411. PMC 23483. PMID 9326623. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=23483. 
• 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. PMID 11752197. 
• 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:10.1111/j.1550-7408.2005.00053.x. PMID 16248873. 
• Haeckel, Ernst (2005). Art Forms from the Ocean: The Radiolarian Atlas of 1862. Munich; London: Prestel Verlag. ISBN 3-7913-3327-5. 

External links

• Radiolarians
• Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests
• Radiolaria.org
• Ernst Haeckel: Die Radiolarien (RHIZOPODA RADIARIA) Berlin, 1862
• Radiolaria - Droplet