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Approximately 400 to 500 relatively common polycystine radiolarian "species" live in the modern oceans. About 200 species live in shallow (0 to 200 m; 0 to 656 ft), temperate and tropical waters (the "upper warm water sphere"); 40 to 50 in high latitude (poleward of subtropical and polar convergences) shallow waters; 150 to 200 in deep (greater than 200 m; 656 ft) waters, most of which appear to be tropical submergent (shallow in high latitudes and diving as tropical submergent or tropical avoidant forms); and about 40 to 50 eurybathyal forms. In general, spumellarian "family groups" of the warm water sphere (surface to subsurface) include the beloids, collosphaerids, saturnulids, artiscids, phacodiscids, spongasterids, dictyocoryids, spongurids, pylonids, lithellids (loo ely coiled), and stylodictids; with the spongotrochids, spongopylids, tholonids, lithellids (tightly coiled), and orosphaerids being dominantly intermediate and deep water (cold water sphere) forms. In general, nassellarian family group of the warm water sphere and transition-central waters include the lophophaenids; sethoperids, sethophormids (complex), carpocanids, eucyrtidids, pterocoryids, cannobotryids, spyroids, and artostrobids (not robust). Plectopyramids, artostrobids (robust), sethoperids, sethophormids (simple), and theocalyptrids dominate in either the transition-central (lower warm water sphere) or cold water sphere.
Polycystine radiolarian species and family level groups appear to fit into four basic broad "nutrition niches:" nannoherbivore-carnivore; bacterivore; detritivore, in association with symbiotic algae; and, perhaps an osmotrophic niche. At the family level these groups dominate the following oceanic habitats: nannoherbivore-carnivore the broad shallow warm water sphere, with algal symbiotes dominating the subtropical anticyclonic gyres, eastern equatorial, and shelf subregions; bacterivores in subsurface and deeper (nutricline and deeper); and, detritivores and perhaps osmotrophs the deep ocean.
A positive relationship exists between the degree of eutrophism in the epipelagic region, and underlying mesopelagic and deeper radiolarian standing crops. Radiolarians are usually sparse in waters overlying the continental shelves. However, those radiolarians harboring symbiotic algae (collosphaerids, artiscids, spyroids, spongasterids, dictyocoryids, etc) are more tolerant of shelf waters. Radiolarians harboring symbiotic algae are also dominant in the oligotrophic subtropical anticyclonic gyres and the eutrophic eastern tropical regions. Acantharian and diatom bloom conditions exclude polycystine radiolarians from these bloom areas (many nearshore areas and the photic zones poleward of polar convergences).
These modern day dominant patterns of radiolarian distribution appear to have been initiated in the early Neogene and
related to the development of the Neogene (and modern) water mass regimes at that time. One of the major radiolarian zoogeographic anomalies, since the initiation of the Neogene water mass regimes and their contained radiolarian faunas, has been the isolation of relict radiolarian species in the warm water sphere of the North Atlantic. This oceanographic realm has been semi-isolated from the world ocean for about the last 3 to 3.5 m.y. and contains a relict (and expatriated) radiolarian fauna. Of special interest in the relict fauna are the presence of the species Lamprocyrtis heteroporos, Didymocyrtis penultimus, D. avitus, and Spongaster pentas--all, until recently, thought to be extinct.
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