The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, 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. They are found as zooplankton throughout the ocean, and their skeletal remains make up a large part of the cover of the ocean floor as siliceous ooze. Due to their rapid turn-over[clarification needed] 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 pseudopodia 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.
The radiolarians belong to the supergroup Rhizaria together with (amoeboid or flagellate) Cercozoa and (shelled amoeboid) Foraminifera. Traditionally the radiolarians have been divided into four groups—Acantharea, Nassellaria, Spumellaria and Phaeodaria. 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 heliozoan. 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.
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 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.
Higher concentrations of dissolved carbon dioxide (CO2) in sea water dissolves their fine skeletons made of silica, destroying their delicate structure, seen as fractured scattered pieces under a microscope. This is linked to periods of heightened volcanic activity.
German biologist Ernst Haeckel produced finely detailed drawings of radiolaria in Kunstformen der Natur (1904), helping to popularize these protists among parlor microscopists alongside foraminifera and diatoms, in addition to two lengthy illustrated monographs from field observation dedicated to Radiolaria, intended for academic study.
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