Temporal range: Cambrian – Recent
|Radiolaria illustration from the Challenger Expedition 1873–76.|
Müller 1858 emend.
The 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.
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 radiolarians belongs to the supergroup Rhizaria together with Cercozoa and Foraminifera. Traditionally the radiolarians have been divided into four groups—Acantharea, Nassellaria, Spumellaria and Phaeodaria. Phaeodaria is however now considered to be a Cercozoan. Nassellaria and Spumellaria both produce siliceous skeletons and were therefore grouped together in the group Polycystina. Despite some initial suggestions to the contrary, this is also supported by molecular phylogenies. The Acantharea produce skeletons of strontium sulfate and is closely related to a peculiar genus, Sticholonche (Taxopodida), which lacks an internal skeleton and was for long time considered a heliozoan. The Radiolaria can therefore be divided into two major lineages: Polycystina (Spumellaria + Nassellaria) and Spasmaria (Acantharia + Taxopodida).
There are several higher-order groups that have been detected in molecular analyses of environmental data. Particularly, groups related to Acantharia and Spumellaria. These groups are so far completely unknown in terms of morphology and physiology and the radiolarian diversity is therefore likely to be much higher than what is currently known.
The relationship between the Foraminifera and Radiolaria is also debated. Molecular trees supports their close relationship—a grouping termed Retaria. But whether they are sister lineages or if the Foraminifera should be included within the Radiolaria is not known.
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 in Kunstformen der Natur (1904), helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.
- Pawlowski J, Burki, F (2011) "Untangling the phylogeny of amoeboid protists". Journal of Eukaryotic Microbiology 56:16–25.
- Yuasa T, Takahashi O, Honda D, Mayama S (2005) "Phylogenetic analyses of the polycystine Radiolaria based on the 18s rDNA sequences of the Spumellarida and the Nassellarida". European Journal of Protistology 41:287–298.
- Nikolaev SI, Berney C, Fahrni J, Bolivar I, Polet S, Mylnikov AP, Aleshin VV, Petrov NB, Pawlowski J (2004) "The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes". Proceedings of the National Academy of Sciences 101:8066–8071.
- Krabberød AK, Bråte J, Dolven JK, Ose RF, Klaeness D, Kristensen T, Bjørklund KR, Shalchian-Tabrizi K (2011) "Radiolaria divided into Polycystina and Spasmaria in combined 18S and 28S rDNA phylogeny". PLoS One 6(8):e23526.
- Cavalier-Smith T (1993) "Kingdom protozoa and its 18 phyla". Microbiology and Molecular Biology Reviews 57:953–994.
- Decelle J, Suzuki N, Mahé F, de Vargas C, Not F (May 2012). "Molecular Phylogeny and Morphological Evolution of the Acantharia (Radiolaria)". Protist 163 (3): 435–450. doi:10.1016/j.protis.2011.10.002. PMID 22154393.
- Not F, Gausling R, Azam F, Heidelberg JF, Worden AZ (2007) "Vertical distribution of picoeukaryotic diversity in the Sargasso Sea". Environmental Microbiology 9:1233–1252.
- Cavalier-Smith T (1999) "Principles of protein and lipid targeting in secondary symbiogenesis: euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree". The Journal of Eukaryotic Microbiology 46:347–366.
- Braun, Chen, Waloszek & Maas (2007), "First Early Cambrian Radiolaria", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications 286, London: Geological Society, pp. 143–149, doi:10.1144/SP286.10, ISBN 9781862392335, OCLC 156823511 191881597
- 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.
- 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.
|Wikimedia Commons has media related to: Radiolaria|
- Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests
- Ernst Haeckel: Die Radiolarien (Rhizopoda radiaria) Berlin, 1862
- Tree Of Life—Radiolaria