Radiolaria are marine unicellular zooplankton characterised by the presence of two types of pseudopodia called axopodia (pseudopodia having axial portion) and reticulopodia (pseudopodia make reticulated network). Class Phaeodarea, traditionally included in Radiolaria, were revised their taxonomic position (out of Radiolaria but in Cercozoa) on the basis of molecular studies. Also, a molecular perspective shows us that Radiolaria have five orders (Spumellaria, Nassellaria, Collodaria, Acantharia and Taxopodida) in two classes (former three in Polycystina, latter two in Spasmaria). Most of Radiolaria (excepting from some species of Collodaria) possess siliceous or celestite shells or spines, and both the fine structure and architectural diversity of the skeletons are used as taxonomic criteria. Good preservation of siliceous shells allows Radiolaria to be used as stratigraphic markers in Paleontology.

Key Concepts

  • Radiolaria are marine heterotrophic protists, and some harbour symbionts.
  • Most radiolarians, in particular class Polycystina, have good fossil record from the Cambrian era due to possessing siliceous shells.
  • Phylogenetic relationship of Radiolaria was revised through molecular phylogeny.
  • The closest lineage of Radiolaria is the other important microfossil group, Foraminifera.
  • Radiolaria are a member of eukaryotic supergroup Rhizaria, together with Cercozoa, Endomyxa and Foraminifera.

Keywords: Eukaryota; protozoa; Rhizaria; Retaria; silica‐based skeleton

Figure 1. Structure of a polycystid radiolarian. Skeleton (black), microtubule‐organising centre and axis of the axopodia (green), capsular wall (red), surface of the cross section through the cytoplasm (yellow), surface of the cell membrane outside the section plane (blue).
Figure 2. Scanning electron micrograph of a radiolarian skeleton: Polycystina Spumellarida, Hexacontium sp. The original resolution of this image is × 1000; it is reproduced here at × 1260.
Figure 3. Scanning electron micrograph of a radiolarian skeleton: Polycystina Nassellarida, Eucyrtidium hexagonatum. The original resolution of this image is × 1100; it is reproduced here at × 620.
Figure 4. Light micrograph of a living radiolarian: Spasmaria Acarharia. The original resolution of this image is × 200; it is reproduced here at × 630.
Figure 5. Schematics of cell structure of (a) Spumellaria, (b) Nassellaria, (c) Collodaria, (d) Acantharia and (e) Taxopodida. Nucleus (N) and microtubule‐organising centre (MTOC) are shown as grey and black, respectively. Axopod (Ax) and microfilament (MF) are shown as grey line. Capsular wall (CW) is shown as black line.


Anderson OG (1980) Radiolaria. In: Levandowsky M and Hutner SH (eds) Biochemistry and Physiology of Protozoa, pp. 1–42. New York: Academic Press.

Cachon J and Cachon M (1985) Superclass Actinopodea Calkins, 1902. In: Lee JJ, Hutner SH and Bovee EC (eds) Illustrated Guide to the Protozoa, pp. 274–302. Lawrence, KS: Allen Press.

Cachon J and Cachon M (1990) Phylum Actinopoda, classes Polycystina (= Radiolaria) and Phaeodaria. In: Margulis L, Corliss JO, Melkonian M and Chapman D (eds) Handbook of Protoctista, pp. 334–346. Boston: Jones and Bartlett.

Cavalier‐Smith T (1993) Kingdom protozoa and its 18 phyla. Microbiology and Molecular Biology Reviews 57: 953–994.

Decelle J, Martin P, Paborstava K, et al. (2012) Diversity, ecology, and biogeochemistry of cyst‐forming Acantharia (Radiolaria) in the oceans. PLoS One 8: e53598.

Grassé PP (1953) Traité de Zoologie. Paris: Masson.

Ishitani Y, Ishikawa SA, Inagaki Y, et al. (2011) Multigene phylogenetic analyses including diverse radiolarian species support the “Retaria” hypothesis – the sister relationship of Radiolaria and Foraminifera. Marine Micropaleontology 81: 32–42.

Krabberød AK, Bråte J, Dolven JK, et al. (2011) Radiolaria divided into Polycystina and Spasmaria in combined 18S and 28S rDNA phylogeny. PLoS One 6 (8): e23526.

Müller J (1859) Über die Thalassicollen, Polycystinen und Acanthometren des Mittelmeeres. Abhandlungen der Königlichen Akademie der Wissenschaften zu Berlin, 1858: 1–62.

Further Reading

Abelmann A and Gowing MM (1997) Spatial distribution pattern of living polycystine radiolarian taxa – baseline study for paleoenvironmental reconstructions in the Southern Ocean (Atlantic sector). Marine Micropaleontology 30: 3–28.

Anderson OR (1996) The physiological ecology of planktonic sarcodines with applications to paleoecology: patterns in space and time. Journal of Eukaryotic Microbiology 43: 261–274.

Balzano S, Corre E, Decelle J, et al. (2015) Transcriptome analyses to investigate symbiotic relationships between marine protists. Frontiers in Microbiology 6: 98.

Caron DA, Michaels AF, Swanberg NR and Howse FA (1995) Primary productivity by symbiont‐bearing planktonic sarcodines (Acantharia, Radiolaria, Foraminifera) in surface waters near Bermuda. Journal of Plankton Research 17: 103–129.

Chen M and Tan Z (1997) Radiolarian distribution in surface sediments of the northern and central South China Sea. Marine Micropaleontology 32: 173–194.

Gilg IC, Amaral‐Zettler LA, Countway PD, et al. (2010) Phylogenetic affiliations of mesopelagic Acantharia and acantharian‐like environmental 18S rRNA genes off the Southern California Coast. Protist 161: 197–211.

Ishitani Y, Ujiié Y, de Vargas C, Not F and Takishita K (2012) Phylogenetic relationships and evolutionary patterns of the order Collodaria (Radiolaria). PLoS One 7: e35775.

Ishitani Y, Ujiié Y and Takishita K (2014) Uncovering sibling species in Radiolaria: evidence for ecological partitioning in a marine planktonic protist. Molecular Phylogenetics and Evolution 78: 215–222.

Lazarus DB, Kotrc B, Wulf G and Schmidt DN (2009) Radiolarians decreased silicification as an evolutionary response to reduced Cenozoic ocean silica availability. Proceedings of the National Academy of Sciences of the United States of America 106: 9333–9338.

Welling LA and Pisias NG (1998) Radiolarian fluxes, stocks and population residence times in surface waters of the central equatorial Pacific. Deep‐Sea Research 45: 639–671.

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Ishitani, Yoshiyuki, Febvre‐Chevalier, Colette, and Febvre, Jean(Jan 2016) Radiolaria. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001985.pub2]