Polycystine radiolarians have enchanted scientists with their astonishing beauty ever since they were first discovered in the marine plankton in the nineteenth century. Their tiny skeletons – with often fascinating intricate shapes – have been interesting not only to natural scientists, but also to architects (like A. Gaudi and R. Binet), painters, jewelers, quilters, sculpturers and musicians who have used them as inspiration for their work. With fossil representatives extending continuously back to the Cambrian, i.e. more than 500 million years, few other microorganisms have existed longer and left behind a more detailed evolutionary record than radiolarians.
Many fossil radiolarian species have robust skeletons that preserve well in the sediments and distinct morphologies that are easily recognized under the microscope. Some of these also have a rapid evolutionary record and a widespread distribution making them excellent index fossils (biostratigraphic markers). They are therefore extensively used to date and correlate sediments and rocks in the Integrated Ocean Drilling Program and the petroleum industry. Fossil radiolarian assemblages are frequently used to reconstruct ocean temperatures throughout the Quaternary (Pisias et al. 1997; Dolven et al. 2002). These paleoclimatic reconstructions are based on the assumption that the living assemblages are modern analogs to the fossil ones, and that their current correlation with temperature or other environmental parameters has been consistent through time. Moreover, radiolarians have been used to prove plate tectonics and the formation of the Himalayas due to their presence in uplifted marine rocks now located at more than 5500 m above sea level.
So, who are these interesting and useful small creatures?
Radiolarians are marine planktonic unicellular eukaryotes. They are present in all oceans at all depths. Most species occur solitarily, ranging in size between 30 and 300 µm, although some colonial species are also known. The latter consist of numerous individuals enclosed in a gelatinous matrix and may reach up to a couple of meters in length (Anderson 1983).
Fig. 1. The test of Hexacontium pachydermum Jørgensen 1900. © Jane K. Dolven
Although it is the possession of a central capsulea that traditionally has been used to distinguish radiolarians from other protists, they are perhaps more commonly known for their siliceous skeleton (test). This test, which is present in most species, can be found in a large variety of shapes, from simple spicules to extremely intricate architectural features – more complex than what is found in any other protist organism.
Based on similarities and differences in skeletal elements and structures (e.g. shell geometry, number of segments or concentric shells, presence and number of spines, etc.) several thousand species (ca. 8000 according to De Wever et al. 2001) have been described, including both fossil and living forms, and new species are discovered every year. Of these, about 500 morphospecies live and thrive in the oceans today. However, the DNA sequencing of marine samples indicates a larger diversity (Not et al. 2007).
Radiolarians have been an important marine plankton group since the early Phanerozoic (about 500 million years before present), and are still a significant component of the pelagic environment today. As with most planktonic organisms, their diversity and abundance in a geographical region is related to variables such as temperature, salinity, productivity, and availability of nutrients. Highest abundance and diversity of radiolarians is found in the tropical and subtropical regions, especially in water masses rich in nutrients (e.g. upwelling areas). They are also usually more abundant in the upper 100 m than below, possibly due to their frequent association with symbiotic algae.
a The presence of a central capsule has traditionally been used as one of the main criteria for placing the polycystines, acantharians and phaeodarians together in the group of Radiolaria (Haeckel 1887). Acantharia was later removed from the group due to differences in chemical composition and skeletal morphology (Deflandre 1952). However, recent DNA analysis suggests a close relationship between acantharians and solitary spumellarians (Polet et al, 2004; Yuasa et al. 2006), suggesting that acantharians perhaps again should be placed within Radiolaria. Molecular analysis of phaeodarians, on the other hand, shows that these are cercozoans and do not belong with the radiolarians.
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