Radiolaria Polycystina
Sedimentary versus water-column materials

As opposed to other zooplanktonic groups, studies on the geographic distribution of extant polycystines have been chiefly based on sedimentary — rather than on planktonic — materials. As mentioned above, sediment samples present some advantages, but also several important shortcomings.

Whereas polycystine abundances seldom exceed 5 cells per liter in the plankton (e.g., Caron and Swanberg, 1990), one gram of (dry) surface sediments can contain thousands to hundreds of thousands of radiolarian skeletons (see quantitative radiolarian distribution). Plankton samples yield a snapshot-type image of the composition of the assemblages, which does not necessarily adequately reflect long-term trends. The daily, seasonal and interannual variability involved is smoothed out in the sedimentary record, which may be a welcome trait when general patterns are sought. Further, sedimentary materials are more readily available from the various repositories around the globe than plankton samples. In any case, plankton samples not collected for microplanktonic purposes may be useless for radiolarian studies due to inadequate net mesh-size.

On the other hand, interpretation of the geographic distribution of extant radiolarian assemblages on the basis of sediment samples presents several important drawbacks (Boltovskoy, 1988, 1994, 1995; Kling and Boltovskoy, 1995; distort of sedimentary imprint). On their way to the sea-floor and after settling, radiolarian remains are grazed upon by various consumers thus breaking their skeletons into unidentifiable fragments. Because more delicate shells are destroyed more readily than the more robust ones, specific makeups on the bottom and at mid- depths can differ significantly from the living assemblage in the upper water-column (Boltovskoy et al., 1993b, 1996). Selective dissolution of whole siliceous skeletons en route to the sea-floor and after deposition, although often advocated as an important source of plankton vs. sediments dissimilarities (e.g., Petrushevskaya, 1971b; Renz, 1976), is probably much less critical than fragmentation due to grazing (Boltovskoy and Alder, 1992; Morley et al., Ms). Bottom materials can be reworked after deposition (as a result of which non-Recent deposits, sometimes characteristic of quite dissimilar oceanographic settings, are brought up to the surface layer, or winnowed by bottom currents (dislodging settled skeletons and carrying them thousands of kilometers away;distort of sedimentary imprint). Sediments integrate the imprint of near-surface faunas (which are generally associated with surficial temperature, salinity and primary production fields, as well as with currents and water masses), with the meso- and bathypelagic species whose geographic distribution is uncoupled with upper-water oceanography (distort of sedimentary imprint). In general terms the sedimentary distributions of cold-water species tend to show conspicuous equatorward extensions as compared with their planktonic patterns. This distortion is most probably due to the fact that extended survival of the expatriated cold water taxa is facilitated by submersion (Boltovskoy, 1988, 1994;distort of sedimentary imprint); as a consequence, sediment-derived species-specific ranges may wrongly suggest an enhanced tolerance to gradients in the ecological factors.

Despite the limitations outlined, because radiolarian distributional studies based on water-column materials are extremely scarce, the biogeographic data summarized in Table RaPo [alphabetically arranged in seven parts:Table RaPo (A-B);Table RaPo (C);Table RaPo (D-H);Table RaPo (L-O);Table RaPo (P);Table RaPo (S-Spo);Table RaPo (Sti-Z)] had to draw heavily on reports that used surface sediments.