Acantharia are common in surface waters of tropical and subtropical oceans worldwide; in temperate and polar seas they occur in much lower numbers. In very nearshore environments and coastal upwelling zones they are usually scarcer. Reported acantharian abundances vary widely, from <1 to 50 ind. lö-1 (Beers et al., 1975, 1982; Bishop et al., 1977, 1978, 1980; Michaels, 1988a, 1988b, 1991; Michaels et al., 1995; summarized in Caron and Swanberg, 1990).
They are generally restricted to the illuminated upper layer of the ocean, although some individuals have been collected in sediment traps thousands of meters below the surface (Bernstein et al., 1987). On calm days larger Acantharia usually aggregate in the upper few meters of the water column, and may be concentrated in Langmuir cells (windrows). This surface affinity is likely related to the photosynthetic physiology of their symbiotic algae. However, upon mechanical (wind) or chemical (freshening of the surface layer by heavy rain) disturbance, their active buoyancy control allows them to swiftly descend to lower strata (50-200 m), returning to the surface once adequate conditions there have reestablished (Schewiakoff, 1926). During periods of strong vertical mixing, their abundances become uniform with depth, but they tend to return to the surface within a few days of restratification.
There are few time-series studies of Acantharia. Schewiakoff (1926) described moderate seasonal changes in acantharian abundance and specific composition in the Mediterranean. During the winter some species were observed to avoid the upper layer, sinking to depths of 50-200 m. In the South China Sea, Reshetnjak (1981) recorded 23 species, 21 of which were present during the summer only. At the VERTEX time-series station in the northeast Pacific (33°N, 139°W) they tended to be more abundant in the summer and fall, although their biomass did not show the same pattern (Michaels, 1991). Integrated abundances were variable at the Bermuda Atlantic Time-series Study site (32°N, 64°W) in the Sargasso Sea (Michaels et al., 1995). Samples collected with plankton nets at a variety of locations throughout the Atlantic suggest higher abundances in spring and lower abundances in summer, although the data were not collected as an explicit time-series (Massera Bottazzi and Andreoli, 1982a). In coastal areas, seasonal patterns are sometimes observed if oceanic advection displaces eutrophic waters near the coast for a defined portion of the year. In the oligotrophic tropical and subtropical seas, Acantharia are a regular, common and conspicuous component of the microplankton year-round.
Because of difficulties associated with their collection, preservation, and identification, data on acantharian geographic ranges are scarce. Available information, however, suggests that they are geographically (but not ecologically) cosmopolitan. With the exception of Coleaspis obscura, which so far has been recorded only in high-latitude areas (Arctic and Antarctic), all other species were found in tropical and/or subtropical waters (Reshetnjak, 1981. The number of eurythermic species is comparatively low, probably around 10-20, and most of their records in cold waters are associated with the influences of warm-water currents, like the Gulf Stream in the Atlantic, and the Kuroshio in the Pacific (Reshetnjak, 1981).
Generalized distribution patterns of acantharian species (chiefly from Reshetnjak, 1981).
Table Ac1
Table Ac2
Table Ac3
Table Ac4
Despite low coverage of the ca. 150 known species (especially in the Indian and Pacific Oceans), 57 are circumtropical and have been recorded in the 3 major oceans (Reshetnjak, 1981). Although the Atlantic (in particular the Mediterranean) has been more intensively sampled than the Indo-Pacific, few materials come from the South Atlantic, which precludes drawing conclusions about their abundance and distribution in this basin. However, as noted above, judging from the widespread geographic distribution of acantharian species it is reasonable to assume that most of them could occur in the South Atlantic. Accordingly, we include all the species so far described and currently considered valid by Schewiakoff (1926) and/or Reshetnjak (1981).