color = lavender
name = Radiolaria

image_width = 225px
image_caption = Radiolaria illustration from the Challenger Expedition 1873-76.
fossil_range = Cambrian - Recent
domain = Eukaryota
regnum = Rhizaria
superphylum = Retaria
phylum = Radiolaria
phylum_authority = Müller 1858 "emend."
subdivision_ranks = Classes
subdivision = Polycystinea

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".


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. [the Rise and Fall of the Ediacaran Biota|Braun, Chen, Waloszek & Maas|First Early Cambrian Radiolaria|143|149|10]

Haeckel's radiolarians

German biologist Ernst Haeckel produced exquisite (and perhaps somewhat exaggerated) Fact|date=June 2008 drawings of radiolaria, helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.

Illustrations from Haeckel's "Kunstformen der Natur" (1904)


* cite journal | author = Zettler, Linda A.
title = Phylogenetic relationships between the Acantharea and the Polycystinea: A molecular perspective on Haeckel's Radiolaria
journal = Proceedings of the National Academy of Sciences
year = 1997 | volume = 94 | pages = 11411–11416 | doi = 10.1073/pnas.94.21.11411
pmid = 9326623

* cite journal | author = P. Lopez-Garcia "et al."
title = Toward the Monophyly of Haeckel's Radiolaria: 18S rRNA Environmental Data Support the Sisterhood of Polycystinea and Acantharea
journal = Molecular Biology and Evolution
year = 2002 | volume = 19 | issue = 1 | pages = 118–121

* cite journal | author = Sina M. Adl "et al."
title = The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists
journal = Journal of Eukaryotic Microbiology
year = 2005 | volume = 52 | issue = 5 | pages = 399–451
doi = 10.1111/j.1550-7408.2005.00053.x

*cite book
last= Haeckel
first= Ernst
title= Art Forms from the Ocean: The Radiolarian Atlas of 1862
year= 2005
publisher= Prestel Verlag
location= Munich; London
isbn= 3-7913-3327-5

External links

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

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Look at other dictionaries:

  • Radiolarian — Ra di*o*la ri*an (r[=a] d[i^]*[ o]*l[=a] r[i^]*an), a. (Zo[ o]l.) Of or pertaining to the Radiolaria. n. One of the Radiolaria. [1913 Webster] …   The Collaborative International Dictionary of English

  • radiolarian — (n.) 1862, from Radiolaria, Modern Latin class name, from L. radiolus, dim. of RADIUS (Cf. radius) (q.v.) …   Etymology dictionary

  • radiolarian — [rā΄dē ōler′ē ən] n. [< ModL Radiolaria < radiolus, dim. of L radius, ray (see RADIUS): from the radiating pseudopodia] any of several classes of one celled deep sea protozoans with long, slender pseudopodia and a spiny, or solid but… …   English World dictionary

  • radiolarian — /ray dee oh lair ee euhn/, n. any minute, marine protozoan of the class Radiolaria, or, in some classification schemes, the superclass Actinopoda, having an amebalike body with radiating, filamentous pseudopodia and a usually elaborate outer… …   Universalium

  • radiolarian — noun Date: 1875 any of three classes (Acantharia, Polycystina, and Phaeodaria) of usually spherical chiefly planktonic marine protozoans having radiating threadlike pseudopodia and often a siliceous skeleton of spicules • radiolarian adjective …   New Collegiate Dictionary

  • radiolarian — noun Any of many marine amoeboid protozoa, of subclass Radiolaria, having filamentous pseudopodia; they have intricate silica skeletons …   Wiktionary

  • radiolarian — [ˌreɪdɪə(ʊ) lɛ:rɪən] noun Zoology a single celled aquatic animal having a spherical amoeba like body with a spiny skeleton of silica. [Many species in the phylum Actinopoda.] Origin C19: from mod. L. radiolaria, from late L. radiolus faint ray ,… …   English new terms dictionary

  • radiolarian — ra·di·o·lar·i·an …   English syllables

  • radiolarian — ra•di•o•lar•i•an [[t]ˌreɪ di oʊˈlɛər i ən[/t]] n. mcr any of various very small marine protozoans of the class Radiolaria (or superclass Actinopoda), having slender radiating pseudopods and usu. bearing an elaborate outer skeleton • Etymology:… …   From formal English to slang

  • radiolarian — /ˌreɪdioʊˈlɛəriən/ (say .raydeeoh lairreeuhn) noun any of the Radiolaria, an extensive group or order of minute marine protozoans, having amoeba like bodies with fine radiating pseudopodia, and usually elaborate skeletons. {New Latin Radiolāria,… …   Australian English dictionary