Radiolaria

Radiolarians have many needle-like pseudopods supported by bundles of microtubules, which aid in the radiolarian's buoyancy. The cell nucleus and most other organelles are in the endoplasm, while the ectoplasm is filled with frothy vacuoles and lipid droplets, keeping them buoyant. The radiolarian can often contain 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 the icosahedron-shaped Circogonia icosahedra pictured.

The radiolarians belong to the supergroup Rhizaria together with (amoeboid or flagellate) Cercozoa and (shelled amoeboid) Foraminifera.[2] Traditionally the radiolarians have been divided into four groups—Acantharea, Nassellaria, Spumellaria and Phaeodarea. Phaeodaria is however now considered to be a Cercozoan.[3][4] 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).[5][6]

There are several higher-order groups that have been detected in molecular analyses of environmental data. Particularly, groups related to Acantharia[7] and Spumellaria.[8] 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.[9] But whether they are sister lineages or if the Foraminifera should be included within the Radiolaria is not known.

Class	Order	Image	Families	Genera	Species	Description
Polycystinea	Nassellaria					...
	Spumellaria					...
	Collodaria					...
Acantharea						...
Sticholonchea	Taxopodida		1	1	1	...

Radiolarian biogeography with observed and predicted responses to temperature change

The color polygons in all three panels represent generalized radiolarian biogeographic provinces, as well as their relative water mass temperatures (cooler colors indicate cooler temperatures, and vice versa). Globe image adapted from NASA Blue Marble: Next Generation imagery. Ocean floor bathymetry from Google Earth seafloor elevation profile (5°N–74°S, at 120°W).

In the diagram on the right, a Illustrates generalized radiolarian provinces [10][11] and their relationship to water mass temperature (warm versus cool color shading) and circulation (gray arrows). Due to high-latitude water mass submergence under warm, stratified waters in lower latitudes, radiolarian species occupy habitats at multiple latitudes, and depths throughout the world oceans. Thus, marine sediments from the tropics reflect a composite of several vertically stacked faunal assemblages, some of which are contiguous with higher latitude surface assemblages. Sediments beneath polar waters include cosmopolitan deep-water radiolarians, as well as high-latitude endemic surface water species. Stars in (a) indicate the latitudes sampled, and the gray bars highlight the radiolarian assemblages included in each sedimentary composite. The horizontal purple bars indicate latitudes known for good radiolarian (silica) preservation, based on surface sediment composition.[12][13]

Data show that some species were extirpated from high latitudes but persisted in the tropics during the late Neogene, either by migration or range restriction (b). With predicted global warming, modern Southern Ocean species will not be able to use migration or range contraction to escape environmental stressors, because their preferred cold-water habitats are disappearing from the globe (c). However, tropical endemic species may expand their ranges toward midlatitudes. The color polygons in all three panels represent generalized radiolarian biogeographic provinces, as well as their relative water mass temperatures (cooler colors indicate cooler temperatures, and vice versa).[13]

• 
  Circogonia icosahedra, radiolarian species shaped like a regular icosahedron
• 
  Anthocyrtium hispidum Haeckel

Radiolarian shapes

          Drawings by Haeckel 1904 (click for details)

Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.[14]

• 
  Like diatoms, radiolarians come in many shapes
• 
  Also like diatoms, radiolarian shells are usually made of silicate
• 
  However acantharian radiolarians have shells made from strontium sulfate crystals

Animation of radiolarian diversity [15]

• 
  Cutaway schematic diagram of a spherical radiolarian shell
• 
  Cladococcus abietinus

Turing and radiolarian morphology

Shell of a spherical radiolarian

Shell micrographs

Computer simulations of Turing patterns on a sphere
closely replicate some radiolarian shell patterns[16]

External video
Radiolarian geometry
Ernst Haeckel's radiolarian engravings

Fossil radiolarian

X-ray microtomography of Triplococcus acanthicus

This is a microfossil from the Middle Ordovician with four nested spheres. The innermost sphere is highlighted red. Each segment is shown at the same scale.[17]

The earliest known radiolaria date to the very start of the Cambrian period,[18][19][20][21] 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.[20] 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.[22]

Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium

Wikimedia Commons has media related to Radiolaria.

Wikispecies has information related to Radiolaria.

• [1]Radiolarians
• Brodie, C. (February 2005). "Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests". Micscape (112). ISSN 1365-070X.
• Radiolaria.org
• Haeckel, Ernst (1862). Die Radiolarien (Rhizopoda radiaria). Berlin. Archived from the original on 2009-06-19. Retrieved 2007-09-07.
• Radiolaria—Droplet
• Tree Of Life—Radiolaria