radiolarian

Radiolaria Fossil 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 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.

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.

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 differ little from later radiolaria.^[1]

Haeckel's radiolarians

German biologist Ernst Haeckel produced exquisite (and perhaps somewhat exaggerated)^{[citation needed]} drawings of radiolaria, helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.

Illustrations from Haeckel's Kunstformen der Natur (1904)

1. Phaeodaria	11. Discoidea	21. Acanthometra	22. Spyroidea
31. Cyrtoidea	41. Acanthophracta	51. Polycyttaria	61. Phaeodaria
71. Stephoidea	91. Spumellaria

See also

• The Radiolarian Series - Album project by "avant-groove" jazz trio Medeski Martin & Wood

References

External links

Wikimedia Commons has media related to: Radiolaria

• 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