The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineralskeletons, 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.[1] They are found as zooplankton throughout the global ocean. As zooplankton, radiolarians are primarily heterotrophic, but many have photosynthetic endosymbionts 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 found from the Cambrian onwards.
Description
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 below.
Taxonomy
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 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.
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
Radiolarians are unicellular predatory protists 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, 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
Cutaway schematic diagram of a spherical radiolarian shell
Cladococcus abietinus
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 normalised Legendre 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 by HMS Challenger 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.
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.[23]
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.[24][25][26][27] They have significant differences from later radiolaria, with a different silica lattice structure and few, if any, spikes on the test.[26] About ninety percent of known 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.[28]
^Yuasa T, Takahashi O, Honda D, Mayama S (2005). "Phylogenetic analyses of the polycystine Radiolaria based on the 18s rDNA sequences of the Spumellarida and the Nassellarida". European Journal of Protistology. 41 (4): 287–298. doi:10.1016/j.ejop.2005.06.001.
^Decelle J, Suzuki N, Mahé F, de Vargas C, Not F (May 2012). "Molecular phylogeny and morphological evolution of the Acantharia (Radiolaria)". Protist. 163 (3): 435–50. doi:10.1016/j.protis.2011.10.002. PMID22154393.
^Cavalier-Smith T (July 1999). "Principles of protein and lipid targeting in secondary symbiogenesis: euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree". J. Eukaryot. Microbiol. 46 (4): 347–66. doi:10.1111/j.1550-7408.1999.tb04614.x. PMID18092388. S2CID22759799.
^Boltovskoy, D., Kling, S. A., Takahashi, K. & BjØrklund, K. (2010) "World atlas of distribution of recent Polycystina (Radiolaria)". Palaeontologia Electronica, 13: 1–230.
^Casey, R. E., Spaw, J. M., & Kunze, F. R. (1982) "Polycystine radiolarian distribution and enhancements related to oceanographic conditions in a hypothetical ocean". Am. Assoc. Pet. Geol. Bull., 66: 319–332.
^Lazarus, David B. (2011). "The deep-sea microfossil record of macroevolutionary change in plankton and its study". Geological Society, London, Special Publications. 358 (1): 141–166. Bibcode:2011GSLSP.358..141L. doi:10.1144/SP358.10. S2CID128826639.
^ abRichards, Bernard (2017). "Chapter 35 – Radiolaria: Validating the Turing theory". In Copeland, Jack; et al. (eds.). The Turing Guide. pp. 383–388.
^Chang, Shan; Feng, Qinglai; Zhang, Lei (14 August 2018). "New Siliceous Microfossils from the Terreneuvian Yanjiahe Formation, South China: The Possible Earliest Radiolarian Fossil Record". Journal of Earth Science. 29 (4): 912–919. Bibcode:2018JEaSc..29..912C. doi:10.1007/s12583-017-0960-0. S2CID134890245.
^Zhang, Ke; Feng, Qing-Lai (September 2019). "Early Cambrian radiolarians and sponge spicules from the Niujiaohe Formation in South China". Palaeoworld. 28 (3): 234–242. doi:10.1016/j.palwor.2019.04.001. S2CID146452469.
^ abA. Braun; J. Chen; D. Waloszek; A. Maas (2007), "First Early Cambrian Radiolaria", in Vickers-Rich, Patricia; Komarower, Patricia (eds.), The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 143–149, doi:10.1144/SP286.10, ISBN978-1-86239-233-5, OCLC156823511
López-García P, Rodríguez-Valera F, Moreira D (January 2002). "Toward the monophyly of Haeckel's radiolaria: 18S rRNA environmental data support the sisterhood of polycystinea and acantharea". Mol. Biol. Evol. 19 (1): 118–121. doi:10.1093/oxfordjournals.molbev.a003976. PMID11752197.
^Boltovskoy, Demetrio; Anderson, O. Roger; Correa, Nancy M. (2016). "Radiolaria and Phaeodaria". In Archibald, John M.; Simpson, Alastair G. B.; Slamovits, Claudio H.; Margulis, Lynn; Melkonian, Michael; Chapman, David J.; Corliss, John O. (eds.). Handbook of the Protists. Springer International Publishing. pp. 1–33. doi:10.1007/978-3-319-32669-6_19-1. ISBN978-3-319-32669-6.
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