Radiolaria Polycystina
Geographic patterns

Polycystines are typically open-ocean organisms, occurring throughout the World Ocean. However, distinct coastal associations, while uncommon or absent altogether in areas with an extended shelf, have been described in various studies. For example, Norwegian fjords host dense and diverse radiolarian assemblages, which differ from those of the open Norwegian Sea (Swanberg and Bjørklund, 1986, 1987, 1992). General differences between presumably neritic vs. oceanic radiolarian assemblages have been described occasionally in the literature (Kruglikova, 1984), and even used for paleoenvironmental reconstructions (e.g., Palmer, 1986). In contrast with these records, all of which are associated with salinity values above 30ä, Boltovskoy et al. (Ms) recently reported the first record of very dense monospecific living radiolarian populations in the estuary of the Río de la Plata (Atlantic coast of South America at approx. 35°S), at salinities as low as 15.4ä. Nevertheless, with the probable exception of specific diversities, which indeed seem lower in neritic assemblages (Nishimura et al., 1997), and the fact that a few selected polycystines are probably less intolerant to near-shore conditions than the bulk, most other traits (such as proportions of Spumellaria/Nassellaria, percentages of Spongodiscidae, percentages of "spiny Porodiscidae", percentages of small "Cyrtoidea"; cf. Kruglikova, 1984) need further confirmation.

Polycystine densities in the plankton are typically around 0.3-1 cells per liter, but values exceeding 50 ind. per liter have been recorded in some productive areas (Caron and Swanberg, 1990; Dennett et al., 2002). However, since reported densities are chiefly based on plankton net tow samples, most are probably underestimating of actual concentrations in the water. Shelled species are undersampled because their juveniles are missed by the 40-60 µm meshes used, whereas naked forms are destroyed beyond recognition in net samples. Dennet et al. (2002) found that for colonial skeletonless polycystines underwater video recordings yield density estimates several orders of magnitude higher that plankton nets. Interestingly, the highest radiolarian concentrations ever reported from the water column, almost 400 ind. per liter, were found in the Río de la Plata estuary, in subsurface layers at bottom depths below 10 m (Boltovskoy et al., Ms).

The quantitative distribution of polycystines in surface sediments of the South Atlantic is illustrated in the figure # quantitative radiolarian distribution. This pattern is probably an approximate representation of their concentrations in the water-column as well, and it also roughly reflects the overall distribution of primary production (e.g., Koblentz-Mishke and Vedernikov, 1977), and of phytoplanktonic (Semina, 1977) and zooplanktonic (Bogorov et al., 1968) biomasses. Highest numbers of polycystines would thus be expected along the upwelling areas off Africa (Abelmann and Gowing, 1997), where the highest radiolarian fluxes have been recorded to day (Boltovskoy et al., 1996), and in the equatorial current system. In the southern part of the ocean high densities are probably associated with the subantarctic belt and its northern extensions, the Malvinas (=Falkland) and the Benguela Currents. In a transect between the Antarctic and approximately 30°S, 10°E (off Namibia), Abelmann and Gowing (1997) recorded highest polycystine densities at 100-300 m in Antarctic waters, and at 0-150 m in subantarctic waters (up to 0.3 ind. l-1; these values, however, may be somewhat underestimating, see figure vertical distribution patterns, and Boltovskoy and Alder, 1992) . In the Southwestern Atlantic (30-60°S, along 55°W), surface (5-15 m) layers were found to host 0.5 polycystines per liter on the average, with maximum concentrations of 3 shells per liter (Alder et al., 1997). Lowest numbers are those present in Central Gyre and Tropical/Subtropical waters (see figure quantitative radiolarian distribution).

Flux rates of radiolarian shells at depths between 50 and ca. 5000 m vary from 0-4 to over 100,000 ind. per square meter per day (Boltovskoy et al., 1993a), with highest numbers having so far been recorded in the north-eastern tropical Atlantic (201,064 shells per square meter per day; cf. Boltovskoy et al., 1996).

The numbers of species that inhabit the different climatic zones of the World Ocean are difficult to estimate because most authors restrict their scopes to some 20-40 more or less well-defined morphotypes, ignoring the rest of the species. The few surveys that (presumably) did attempt to identify all the skeletons recorded indicate that these numbers oscillate around 100-200 for the tropics and subtropics, dropping to some 50-60 at the poles (see figure numbers of polycystine species). This decrease, however, is often punctuated by an isolated peak in the transitional areas which usually host both cold water and warm water taxa, especially in the sediments (see figure fluctuations in polycystine numbers) (Boltovskoy, 1981d, 1982, 1986).

Despite these rather high numbers, very few of the species are abundant in any given sample. In terms of their relative contribution to the overall polycystine assemblage, usually only 1-3 species exceed 10%, and up to 5 represent over 5%; radiolarians whose average percentage abundances are below 1% of the fauna usually comprise 70-90% of all the species recorded (changes in species richness). Of the 164 polycystines included in this review, around 10 can attain average proportions in excess of 10% in any given area, 12-15 morphotypes can reach 5-7%, and ca. 50-70 are normally around 1-3% (see Table RaPo, at the bottom of this page). The remaining half of the polycystine species are present at levels below 1%. Highest dominances are associated with polar environments, where a single species or species group can account for 25-40% of the assemblage (e.g., Antarctissa spp. in the Antarctic, cf. Boltovskoy, 1987; Amphimelissa setosa in the Greenland Sea, cf. Swanberg and Eide, 1992; Phormacantha hystrix / Plectacantha oikiskos and Rhizoplegma boreale in coastal Antarctic sediments, cf. Nishimura et al., 1997; see figure changes in species richness).

Species-specific distributional data for the South Atlantic are scarce and fragmentary. Boltovskoy (1981e) produced a detailed listing of all known Southwestern Atlantic records up to that date, which basically represented 7 reports (Haeckel, 1887; Hays, 1965; Nigrini, 1967; Goll and Bjørklund, 1974; Lozano and Hays, 1976; Morley, 1977; and Boltovskoy and Riedel, 1980), chiefly based on sedimentary materials. This objective compilation produced a spotty picture with no discernible patterns. In the 15 years elapsed since that review several contributions based on South Atlantic materials appeared, but they mostly focused on downcore analyses (e.g., Pisias and Moore, 1978; Coco, 1982; Weaver, 1983; Bjørklund and Jansen, 1984; Grinstead, 1984; Charles and Morley, 1988; Alperín, 1987), or were restricted geographically to rather small areas (Robson, 1983; Dworetzky and Morley, 1987; Boltovskoy et al., 1993a, 1993b, 1995, 1996a, 1996b; Abelmann and Gowing, 1997). Thus, in order to furnish a more comprehensive insight into polycystine biogeography in the South Atlantic, distributional species-specific data are referred to the 7 distinct areas illustrated in the figuremain biogeographic areas. These divisions take into account the distribution of general planktonic biogeographic provinces (e.g., E. Boltovskoy, 1970; Koblentz-Mishke and Vedernikov, 1977; Boltovskoy, 1979, 1981d, 1982, 1986; Dadon and Boltovskoy, 1982; Longhurst, 1995), as well as radiolarian-based biogeographic patterns (Goll and Bjørklund, 1974; Morley, 1977; see insets A and B in figuremain biogeographic areas). For some of the especially abundant and better defined taxa relative (percentage) contributions to all polycystines can be predicted with reasonable accuracy. For most others, however, only a very rough indication of their numbers (abundant, present) can be offered for the time being.

It should be stressed that specific distribution ranges are hardly ever actually restricted to any of the biogeographic domains indicated inmain biogeographic areas. Rather, as illustrated with examples of polycystine distrib. patterns 1 andpolycystine distrib. patterns 2, these areas define the "core" ranges of the species, i.e., the sector where the taxon reaches it’s highest abundance. Total ranges are, as a rule, much larger, and usually cover several climatic zones. Data presented in latter figures are based on surface sedimentary materials, and therefore most probably offer a somewhat blurred image of actual distributions in the plankton. This is particularly noticeable in the case of the cold water patterns, where typically subantarctic and Antarctic species are recorded as far north as 30°S. As described on page RaPo. 4 Sedimentary versus water-column materials, submersion with cold currents and subsurface or deep water northward displacement of these shells is presumably responsible for these biases.

The information used to compile Table RaPo (see at bottom of this page) was not restricted to data from the South Atlantic Ocean, but was extracted from many reports on various oceanic areas, putting special emphasis on water column-based surveys (see page RaPo. 4 once more). Although very subtle differences between oceanic basins probably do exist (Nigrini, 1967; Goll and Bjørklund, 1974), polycystine species are chiefly restricted in their distribution by climatic and productivity fields, rather than by ocean basins, as are most other pelagic planktonic organisms. Thus, with very few exceptions, similar assemblages characterize the equatorial circumglobal belt, the subtropical zones of the two hemispheres, and the polar waters (Petrushevskaya, 1971a). Geographic endemics are rare, probably accounting for less than 5% of all the species (one outstanding example is Antarctissa spp. [Antarctissa spp. group? Antarctissa spp. group? 2] which is absent in the Arctic, but dominates both the plankton and the sediments of the Antarctic zone).

It should be born in mind that the degree of mixture between most of the areas shown inmain biogeographic areas is extremely large. For example, in the western South Atlantic the Transition Zone stretches up to almost 15 degrees in latitude (ca. 34-35°S to 47-48°S; see the chapter General biological features of the South Atlantic); Subantarctic species are regularly found here in the same tows as the Subtropical representatives (E. Boltovskoy, 1970, 1981a,1981b; E. Boltovskoy et al., 1996). Because the Brazil current is a southwest flowing branch of the South Equatorial Current, tropical assemblages differ little from the subtropical ones. Central Gyre fauna is also very similar to the Tropical and Subtropical one, yet these oligotrophic waters, characterized by very low overall plankton abundances, host enhanced proportions of several colonial radiolarians.

Geographic and vertical distributions of polycystines are gathered in Table RaPo —Êalphabetically devided 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)].