Abstract
Abstract. The secretion process of the siliceous skeleton in polycystine radiolarians has drawn a great deal of interest during the last century; however, little is known about the actual physiological process of silica deposition. Recently, the PDMPO (2-(4-pyridyl)-5-[(4-(2-dimethylaminoethylaminocarbamoyl) methoxy)-phenyl] oxazole) method for staining silica deposition sites in polycystines was developed. In the present study we examined over 30 polycystine cells with PDMPO and found that both the skeletons and pseudopodia of three species (Lithelius sp., Rhizosphaera trigonacantha and Arachnosphaera hexasphaera) were stained and emitted green fluorescent light. Staining of the skeleton was probably the result of skeletal thickening growth, whereas staining of the pseudopodia may indicate that siliceous matter is assimilated within pseudopodia. We refer to this hypothesis as the ‘pseudopodial silica absorption hypothesis’ (PSA hypothesis). If this hypothesis is correct, PSA is an intermittent process, and the absorbed silica within pseudopodia is quickly transferred to the cytokalymma where it is deposited on the skeleton. To date, the PSA process has been observed in only the three species cited above; therefore we are unable to evaluate whether the PSA process is unique to these species or a common process that occurs in all polycystines; further investigation is necessary.
Highlights
The process of silica secretion in polycystine radiolarians has long attracted interest among scientists (e.g. Thompson, 1942)
Our team regularly examines many polycystine cells using PDMPO staining and we recently discovered that pseudopodia emit the green fluorescent light of PDMPO, in addition to the skeleton
Green fluorescence from pseudopodia was observed in three spherical spumellarian taxa: one Lithelius sp., two cells of Rhizosphaera trigonacantha Haeckel, 1860, and three cells of Arachnosphaera hexasphaera Popofsky, 1912, whereas we found no cells of nassellarian species with pseudopodia that emitted green fluorescent light
Summary
The process of silica secretion in polycystine radiolarians has long attracted interest among scientists (e.g. Thompson, 1942). Anderson (1981) found a granular matrix in the cytokalymma around the growing portion of the siliceous skeleton, and he believed that this granular matrix served as a nucleation site for silica deposition Independent of these cytological examinations of silicification, great efforts have been made to identify the signature of silicification and to reconstruct growth patterns in the siliceous skeleton of polycystines. Nishimura (1986, 1990) observed that, after etching with sodium hydrate, some broken Cenozoic fossils displayed concentric or overlapping siliceous lamellae on the corroded section of the skeleton She considered these lamellar structures to be growth lines and suggested that the variable lamellae thicknesses accounted for variation in the thickness of the skeleton and the size of pore frames (Nishimura, 1986, 1990). Flat polycystine species grow their skeleton by adding a new skeletal framework around their existing shells (Anderson & Bennett, 1985; Matsuoka, 1992)
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