| Radiolaria | |
|---|---|
 | |
| Radiolaria illustration from theChallenger expedition 1873–76 | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | SAR |
| Clade: | Rhizaria |
| Phylum: | Retaria |
| Subphylum: | Radiolaria Cavalier-Smith, 1987 |
| Classes | |
| Part of a series on |
| Plankton |
|---|
 |
TheRadiolaria, also calledRadiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricatemineralskeletons, typically with a central capsule dividing thecell into the inner and outer portions ofendoplasm andectoplasm. The elaborate mineral skeleton is usually made ofsilica.[1] They are found aszooplankton throughout the global ocean. As zooplankton, radiolarians are primarilyheterotrophic, but many have photosyntheticendosymbionts and are, therefore, consideredmixotrophs. The skeletal remains of some types of radiolarians make up a large part of the cover of the ocean floor assiliceous ooze. Due to their rapid change as species and intricate skeletons, radiolarians represent an important diagnosticfossil found from theCambrian onwards.
Radiolarians have many needle-likepseudopods supported by bundles ofmicrotubules, which aid in the radiolarian's buoyancy. Thecell nucleus and most otherorganelles are in the endoplasm, while the ectoplasm is filled with frothyvacuoles andlipid droplets, keeping them buoyant. The radiolarian can often containsymbiotic algae, especiallyzooxanthellae, which provide most of the cell's energy. Some of this organization is found among theheliozoa, but those lack central capsules and only produce simple scales and spines.
Some radiolarians are known for their resemblance toregular polyhedra, such as theicosahedron-shapedCircogonia icosahedra pictured below.
The radiolarians belong to the supergroupRhizaria together with (amoeboid orflagellate)Cercozoa and (shelled amoeboid)Foraminifera.[2] Traditionally the radiolarians have been divided into four groups—Acantharea,Nassellaria,Spumellaria andPhaeodarea. 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 groupPolycystina. Despite some initial suggestions to the contrary, this is also supported by molecular phylogenies. The Acantharea produce skeletons ofstrontium sulfate and is closely related to a peculiar genus,Sticholonche (Taxopodida), which lacks an internal skeleton and was for long time considered aheliozoan. 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 support their close relationship—a grouping termed Retaria.[9] But whether they are sister lineages or whether the Foraminifera should be included within the Radiolaria is not known.
| Class | Order | Image | Families | Genera | Species | Description |
|---|---|---|---|---|---|---|
| Polycystinea | Nassellaria |  | ... | |||
| Spumellaria |  | ... | ||||
| Collodaria |  | ... | ||||
| Acantharea | .jpg) | ... | ||||
| Sticholonchea | Taxopodida |  | 1 | 1 | 1 | ... |
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 lateNeogene, either by migration or range restriction (b). With predicted global warming, modernSouthern 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]
.jpg)
Radiolarians are unicellular predatoryprotists encased in elaborate globular shells (or "capsules"), 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, asmicrofossils, provide valuable information about past oceanic conditions.[14]
.jpg)
So I set to work on seeking a solution to the Morphogenesis Equations on a sphere. The theory was that a spherical organism was subject to diffusion across its surface membrane by an alien substance, eg sea-water. The Equations were:
The function, taken to be the radius vector from the centre to any point on the surface of the membrane, was argued to be representable as a series of normalisedLegendre functions. The algebraic solution of the above equations ran to some 30 pages in my Thesis and are therefore not reproduced here. They are written in full in the book entitled “Morphogenesis” which is a tribute to Turing, edited by P. T. Saunders, published by North Holland, 1992.[16]
The algebraic solution of the equations revealed a family of solutions, corresponding to a parameter n, taking values 2, 4. 6.
When I had solved the algebraic equations, I then used the computer to plot the shape of the resulting organisms. Turing told me that there were real organisms corresponding to what I had produced. He said that they were described and depicted in the records of the voyages of HMS Challenger in the 19th Century.
I solved the equations and produced a set of solutions which corresponded to the actual species of Radiolaria discovered byHMSChallenger in the 19th century. That expedition to the Pacific Ocean found eight variations in the growth patterns. These are shown in the following figures. The essential feature of the growth is the emergence of elongated "spines" protruding from the sphere at regular positions. Thus the species comprised two, six, twelve, and twenty, spine variations.
Bernard Richards, worked under the supervision ofAlan Turing (1912–1954) at Manchester as one of Turing's last students, helping to validateTuring’s theory ofmorphogenesis.[18][19][20][21]
"Turing was keen to take forward the work thatD’Arcy Thompson had published inOn Growth and Form in 1917".[20]
.jpg)
| External videos | |
|---|---|
 Radiolarian geometry | |
| Ernst Haeckel's radiolarian engravings |
The gallery shows images of the radiolarians as extracted from drawings made by the German zoologist and polymathErnst Haeckel in 1887.
The earliest known radiolaria date to the very start of theCambrian period, appearing in the same beds as the firstsmall shelly fauna—they may even be terminal Precambrian in age.[24][25][26][27] They have significant differences from later radiolaria, with a different silica lattice structure and few, if any, spikes on thetest.[26] About ninety percent of known radiolarian species are extinct. The skeletons, or tests, of ancient radiolarians are used ingeological dating, including foroil exploration and determination ofancient climates.[28]
Some common radiolarian fossils includeActinomma,Heliosphaera andHexadoridium.
Material was copied from this source, which is available under aCreative Commons Attribution 4.0 International License. Material was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.
{{cite book}}: CS1 maint: location missing publisher (link)| International | |
|---|---|
| National | |
