The Radiolaria, also called Radiozoa, are
protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
of diameter 0.1–0.2 mm that produce intricate
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
s, typically with a central capsule dividing the
cell into the inner and outer portions of
endoplasm and
ectoplasm. The elaborate mineral skeleton is usually made of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
. They are found as
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
throughout the global ocean. As zooplankton, radiolarians are primarily
heterotroph
A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
ic, but many have photosynthetic
endosymbiont
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
s and are, therefore, considered
mixotrophs. The skeletal remains of some types of radiolarians make up a large part of the cover of the ocean floor as
siliceous ooze. Due to their rapid change as species and intricate skeletons, radiolarians represent an important diagnostic
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
found from the
Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
onwards.
Description
Radiolarians have many needle-like
pseudopods supported by bundles of
microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 1 ...
s, which aid in the radiolarian's buoyancy. The
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
and most other
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...
s are in the endoplasm, while the ectoplasm is filled with frothy
vacuole
A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...
s and
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
droplets, keeping them buoyant. The radiolarian can often contain
symbiotic
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or para ...
algae, especially
zooxanthellae, which provide most of the cell's energy. Some of this organization is found among the
heliozoa
Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes ( protists) with stiff arms (axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections fr ...
, 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
In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons".
There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
-shaped ''Circogonia icosahedra'' pictured below.
Taxonomy
The radiolarians belong to the supergroup
Rhizaria together with (
amoeboid or
flagellate
A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...
)
Cercozoa
Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead defined by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major euk ...
and (shelled amoeboid)
Foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
.
Traditionally the radiolarians have been divided into four groups—
Acantharea,
Nassellaria,
Spumellaria and
Phaeodarea. 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
heliozoa
Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes ( protists) with stiff arms (axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections fr ...
n. 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.
Biogeography
In the diagram on the right, a Illustrates generalized radiolarian provinces 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.
[ Material was copied from this source, which is available under ]
Creative Commons Attribution 4.0 International License
Data show that some species were extirpated from high latitudes but persisted in the tropics during the late
Neogene
The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...
, either by migration or range restriction (b). With predicted global warming, modern
Southern Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-smal ...
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).
[
File:Circogoniaicosahedra ekw.jpg, ''Circogonia icosahedra'', radiolarian species shaped like a regular icosahedron
File:Anthocyrtium hispidum Haeckel - Radiolarian (34986365113).jpg, ''Anthocyrtium hispidum Haeckel''
]
Radiolarian shells
Radiolarians are unicellular predatory protists
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
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.[Wassilieff, Maggy (2006]
"Plankton - Animal plankton"
''Te Ara - the Encyclopedia of New Zealand''. Accessed: 2 November 2019.
File:Mikrofoto.de-Radiolarien 6.jpg, Like diatoms, radiolarians come in many shapes
File:Theocotylissa ficus Ehrenberg - Radiolarian (34638920262).jpg, Also like diatoms, radiolarian shells are usually made of silicate
File:Acantharian radiolarian Xiphacantha (Haeckel).jpg, However acantharian radiolarians have shells made from strontium sulfate crystals
File:Spherical radiolarian 2.jpg, Cutaway schematic diagram of a spherical radiolarian shell
File:Cladococcus abietinus.jpg, ''Cladococcus abietinus''
Fossil record
The earliest known radiolaria date to the very start of the Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
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
Test(s), testing, or TEST may refer to:
* Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities
Arts and entertainment
* ''Test'' (2013 film), an American film
* ''Test'' (2014 film), ...
. About ninety percent of known radiolarian species are extinct. The skeletons, or tests, of ancient radiolarians are used in geological dating
Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is pr ...
, including for oil exploration and determination of ancient climates.
Some common radiolarian fossils include '' Actinomma'', '' Heliosphaera'' and '' Hexadoridium''.
See also
* Radiolarite
References
*
*
*
*
External links
*[Radiolarians]
*
Radiolaria.org
*
Tree Of Life—Radiolaria
{{Authority control
*
Amoeboids
Extant Cambrian first appearances
Bikont subphyla
Taxa named by Thomas Cavalier-Smith