Radiolarian
Encyclopedia
Radiolarians are amoeboid
protozoa
that produce intricate mineral
skeleton
s, 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 their skeletal remains cover large portions of the ocean bottom as radiolarian ooze. Due to their rapid turn-over of species, they represent an important diagnostic fossil
found from the Cambrian
onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.
Radiolarians have many needle-like pseudopod
ia supported by bundles of microtubule
s, called axopods, which aid in the Radiolarian's buoyancy. The nuclei
and most other organelle
s are in the endoplasm, while the ectoplasm is filled with frothy vacuole
s and lipid
droplets, keeping them buoyant. Often it also contains symbiotic
algae, especially zooxanthella
e, 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 with the icosahedron
-shaped Circogonia icosahedra pictured to the left.
.
Traditionally the radiolarians have been divided into four groups - Acantharia
, Nassellaria, Phaeodaria and Polycystina - on morphological grounds.
The main class
of radiolarians are the Polycystine
a, 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
protist
s. Other radiolarians appear near, but outside, the Cercozoa, so the similarity is probably due in part to shared ancestry and in part to convergent evolution
.
The heliozoan group Taxopodida have been shown by genetic studies to lie within this group.
Molecular studies have shown that the Radiolaria is divided into two main lineages: Polycystina (Spumellaria
+ Nassellaria) and Spasmaria (Acantharia
+ Taxopodida). They are also closely related to the Foraminifera groups supporting the Retaria hypothesis.
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
. 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
.
produced finely detailed drawings of radiolaria, helping to popularize these protists among Victorian parlor
microscopists alongside foraminifera
and diatom
s.
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
that produce intricate mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s, typically with a central capsule dividing the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
into inner and outer portions, called endoplasm
Endoplasm
Endoplasm generally refers to the inner part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer layer of the cytoplasm....
and ectoplasm
Ectoplasm
Ectoplasm may refer to:* Ectoplasm , the outer part of the cytoplasm* Ectoplasm , supposed physical substance that manifests as a result of spiritual energy or psychic phenomenon...
. They are found as zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
throughout the ocean, and their skeletal remains cover large portions of the ocean bottom as radiolarian ooze. Due to their rapid turn-over of species, they represent an important diagnostic fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
found from the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.
Description
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
ia supported by bundles of microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s, called axopods, which aid in the Radiolarian's buoyancy. The nuclei
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and most other organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s are in the endoplasm, while the ectoplasm is filled with frothy vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all 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 molecules including enzymes in solution, though in certain...
s and lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
droplets, keeping them buoyant. Often it also contains symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
algae, especially zooxanthella
Zooxanthella
Zooxanthellae are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia....
e, which provide most of the cell's energy. Some of this organization is found among the heliozoa
Heliozoa
Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
, but those lack central capsules and only produce simple scales and spines.
Some radiolarians are known for their resemblance to regular polyhedra, such as with the icosahedron
Icosahedron
In geometry, an icosahedron is a regular polyhedron with 20 identical equilateral triangular faces, 30 edges and 12 vertices. It is one of the five Platonic solids....
-shaped Circogonia icosahedra pictured to the left.
Taxonomy
The radiolarians and Cercozoa are included within a supergroup called the RhizariaRhizaria
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. This supergroup was proposed by Cavalier-Smith in 2002. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods...
.
Traditionally the radiolarians have been divided into four groups - Acantharia
Acantharia
Acantharia is a genus of fungi in the Venturiaceae family.-Species:* Acantharia aterrima Arx * Acantharia chaetomoides W.H. Hsieh, Chi Y. Chen & Sivan....
, Nassellaria, Phaeodaria and Polycystina - on morphological grounds.
The main class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of radiolarians are the Polycystine
Polycystine
The polycystines are a group of radiolarian protists. They include the vast majority of the fossil radiolaria, as their skeletons are abundant in marine sediments, making them one of the most common groups of microfossils. These skeletons are composed of opaline silica...
a, which produce siliceous skeletons. These include the majority of fossils. They also include the Acantharea
Acantharea
The Acantharea are a group of radiolarian protozoa, distinguished mainly by their skeletons.-Structure:These are composed of strontium sulfate crystals, which do not fossilize, and take the form of either ten diametric or twenty radial spines...
, which produce skeletons of strontium sulfate
Strontium sulfate
Strontium sulfate is the sulfate salt of strontium. It is a white crystalline powder and occurs in nature as the mineral celestine. It is poorly soluble in water to the extent of 1 part in 8,800. It is more soluble in dilute HCl and nitric acid and appreciably soluble in alkali chloride solutions...
. Despite some initial suggestions to the contrary, genetic studies place these two groups close together. They also include the peculiar genus Sticholonche
Sticholonche
Sticholonche is a genus of protozoan with a single species, S. zanclea, found in open oceans at depths of 99-510 metres. It is generally considered a heliozoan, placed in its own order, called the Taxopodida...
, which lacks an internal skeleton and so is usually considered a heliozoan.
Traditionally the radiolarians have also included the Phaeodarea
Phaeodarea
The Phaeodarea are a group of amoeboid protists. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa...
, which produce siliceous skeletons but differ from the polycystines in several other respects. However, on molecular trees they branch with the Cercozoa
Cercozoa
The Cercozoa are a group of protists. They are sometimes described as a kingdom.-Characteristics:The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in...
, a group including various flagellate
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
and amoeboid
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. Other radiolarians appear near, but outside, the Cercozoa, so the similarity is probably due in part to shared ancestry and in part to convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
The heliozoan group Taxopodida have been shown by genetic studies to lie within this group.
Molecular studies have shown that the Radiolaria is divided into two main lineages: Polycystina (Spumellaria
Spumellaria
Spumellaria is a radiolarian protists order in the class polycystinea....
+ Nassellaria) and Spasmaria (Acantharia
Acantharia
Acantharia is a genus of fungi in the Venturiaceae family.-Species:* Acantharia aterrima Arx * Acantharia chaetomoides W.H. Hsieh, Chi Y. Chen & Sivan....
+ Taxopodida). They are also closely related to the Foraminifera groups supporting the Retaria hypothesis.
Fossil record
The earliest known radiolaria date to the very start of the CambrianCambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, appearing in the same beds as the first small shelly fauna
Small shelly fauna
The small shelly fauna or small shelly fossils, abbreviated to SSF, are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian period. They are very diverse, and there is no formal definition of...
- 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 (biology)
A test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae....
. Ninety percent of radiolarian species are extinct. The skeletons, or tests, of ancient radiolarians are used in geological dating
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
, including for oil exploration
Oil exploration
Hydrocarbon exploration is the search by petroleum geologists and geophysicists for hydrocarbon deposits beneath the Earth's surface, such as oil and natural gas...
and determination of ancient climates
Paleoclimatology
Paleoclimatology is the study of changes in climate taken on the scale of the entire history of Earth. It uses a variety of proxy methods from the Earth and life sciences to obtain data previously preserved within rocks, sediments, ice sheets, tree rings, corals, shells and microfossils; it then...
.
Haeckel's radiolarians
German biologist Ernst HaeckelErnst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...
produced finely detailed drawings of radiolaria, helping to popularize these protists among Victorian parlor
Citizen science
Citizen science is a term used for the systematic collection and analysis of data; development of technology; testing of natural phenomena; and the dissemination of these activities by researchers on a primarily avocational basis...
microscopists alongside foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
and diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s.