Radiolaria Phaeodaria
Geographic and vertical distribution

Phaeodaria inhabit all oceans at all water depths. The table of phaeodarian numbers, based on data from Reshetnjak (1966), illustrates the total number of species recorded in all major oceanic basins, as well as the percentages of forms endemic to each of them. Highest total numbers have been recorded for the Atlantic and Pacific Oceans (343 and 276 species, respectively), where also the highest degrees of endemism are observed. Indian Ocean inventories are largely shared with the Pacific. As expected, the polar seas host the lowest specific diversities. It should be noted, however, that to a large extent these estimates reflect the different degrees of coverage of the various areas. Relative differences between polar and extrapolar domains, however, probably reflect actual relationships more faithfully.

Unlike most other sarcodines, phaeodarians are typically deep-water animals usually peaking in both abundance and diversity below 200 m. A detailed depth zonation for the area of the Kurile-Kamchatka trench was produced by Reshetnjak (1955, 1966). Her data (table of phaeodarian numbers 2) indicate that only 2 (of the 103 species recorded) inhabit the upper 50 m; approximately 30 more phaeodarians have restricted vertical ranges at various depths; while over 50% of the taxa were retrieved from the broad depth interval of 50 to 2000-8000 m. It should be borne in mind, though, that although uppermost records for a given species are generally reliable, the lowermost ones often are not, since dead sinking shells can be counted together with in situ living organisms (see Kling and Boltovskoy, 1995). Interestingly, zoogeographic zonations based on the degrees of endemism of phaeodarian assemblages (biogeographic zonation map) are more similar to zonations based on bottom, deep-water organisms, than on those based on planktonic groups. This circumstance reflects the deep-living habitat of a large fraction of the phaeodarians described (Reshetnjak, 1966).

These vertical patterns at a given locale, however, may change significantly because of the dynamics of deep ocean circulation, with species exhibiting quite variable depth ranges over oceanic distances. For example, species tied to surface water masses in polar or subpolar regions, where dense waters sink to form the Intermediate Waters, are typically found at gradually deeper horizons toward equatorial waters, where they appear at depths of hundreds or thousands of meters. Thus, on regional scales, depth distributions of phaeodarians are not describable in terms of fixed ranges.

Vertical profiles in the North Pacific (Kling, 1966, 1976) illustrate these kinds of relationships (distribution table). Some species occupy the Subarctic Pacific Water Mass at depths as shallow as 25 meters, but sink to greater depths southward where a salinity minimum defines the Intermediate Water (distribution table, C. variabile). Above the southern ranges of these species are others occupying an unnamed salinity-maximum zone (distribution table, C. amphora). And these in turn underlie surface-dwelling Central Water and equatorial species (distribution table, C. longispinum).

Equatorward submergence may account for so-called bipolar distributional patterns characteristic of many phaeodarian species. Thus, high-latitude species could pass under equatorial waters via the Intermediate Water or the Deep Water, to reappear in the opposite polar or subpolar seas. In the Pacific Ocean, the Intermediate Water circulates in anti-cyclonic gyres that mimic the surface circulation (Reid, 1965), thus providing continuity for the water masses and their biological contents. Such bipolar patterns have been described for a number of zooplanktonic species in both the Atlantic (Pierrot-Bults, 1974) and the Pacific Ocean (Alvariño, 1965), whereby the north and south polar or subpolar near-surface populations are joined at depths of 800-1000 m across the equator.

In addition to such physical factors, phaeodarian distributions may also be influenced by biological variables. In the South Atlantic Meyer (1934) showed close coupling between the absolute abundances of Challengeridae and primary producers such as diatoms and peridinians, as well as between larger phaeodarians and total plankton and phosphate content. Dworetzky and Morley (1987) showed depth zones for the upper 200 meters of the eastern equatorial Atlantic related to various physical-chemical features and to a chlorophyll maximum.

Some authors (e.g., Haecker, 1908; Reshetnjak, 1966) suggested that deep-living species are generally larger in size than their shallower-dwelling relatives. Reshetnjak (1966), based on comparison of the size of 9 Challengeridae, noticed that the shells of 3 species which live at 50-400 m vary between 0.06 and 0.18 mm, for 3 others characteristic of the 400-1500 m layer the size spans 0.18-0.30 mm, while 3 more that inhabit the 1500-5000 stratum range from 0.30-0.65 mm.

It is difficult to specify precisely the distribution patterns of phaeodarian species in the South Atlantic because the data are sparsely distributed spatially and temporally and have been gathered with various techniques and equipment. Most of the phaeodarian data come from reports of the large oceanographic expeditions of the late nineteenth and early twentieth centuries: the Challenger Expedition (1873-1876), the Plankton-Expedition (1889), the Deutsche Tiefsee-Expedition (1898-1899), the Deutsche Südpolar-Expedition (1901-1903), and the Deutsche Atlantische Expedition (1925-1927). Stations were generally widely spaced and relatively few closing net samples were obtained. Nevertheless, many species were reported from restricted geographic provinces, and in some cases relatively narrow depth ranges are suggested. Because of better sample coverage in near-surface depths, ties to surface water masses and current systems are fairly well established by existing South Atlantic data. It is also likely that a number of species that appear at shallow depths in the Antarctic and at greater depths at the equator are occupying the Intermediate Water as has been shown in the North Pacific, and was suggested by earlier authors for the South Atlantic (e.g. Meyer, 1934). However, continuous data from great depths, that would be required to substantiate such anticipated deep distributions, have not yet been gathered.

Table Phaeodaria (Ph-caption[/textfile][/l]) summarizes the [l][m]Glossary[/m][r]distribution[/r]distribution of phaeodarian species reported from the South Atlantic Ocean. Data are entered according to major biogeographic provinces reported in the literature. In addition to expedition reports and other sources cited above, data were compiled from the following publications: Dworetzky and Morley (1987), Gowing (1986, 1993a) Morley and Stepien (1985), Nöthig and Gowing (1991), Peters (1929), Takahashi and Honjo (1981), Tibbs (1969, 1976), and Tibbs and Tibbs (1986). Where depth data suggest a reasonably restricted range for a given province, the depths are entered under the appropriate heading. Generalized depth ranges are entered in the column headed "Depth". When data are sufficient to suggest a concentration of individuals at a particular depth or depth-range, the depths are noted as maxima (abbreviated "x").