(A.Schimp. ex G.Karst.) Balech (1973: 21)
Peridinium antarcticum A.Schimp. ex G.Karst. (Karsten, 1905: 131, pl. 19, figs 1–4)
Peridinium depressum (Bailey) N.Peters (1928: 63, fig. 17d–f)
Balech (1976: 46, fig. 32), Andreoli et al. (1995: 473, fig. 19)
Cells large, expanded, 80–140 µm long (excluding horns), 80–120 µm diameter. Surface ornamentation spiculate. Apical horn to 30 µm long; antapical horns of equal length, to 25 µm long. Cingulum slightly helical, comparatively narrow, non-incised, bordered by narrow ribbed ridges. Narrow growth bands usually present between plates. Tabulation 1pa, 4', 3a, 6'', 3c, 6s?, 5''', 1p, 1''''; 1' plate large and rhombic (orthoform); 2a plate trapezoidal (quadra).
Southern Ocean, south of Australia; type locality, Antarctic Ocean, “Challenger” Expedition (Karsten, 1905); Bransfield Strait (Priddle, 1985); Southern Ocean, near South Georgia (Dodge & Priddle, 1987); Terra Nova Bay, Ross Sea (Andreoli et al., 1995); Weddell Sea (Estrada & Delgado, 1990); offshore from Davis Station, East Antarctica (Archer et al., 1996a); southern Indian Ocean, 46–69°S, 75°E (Okamoto et al., 1986); KERFIX, 50°40’S 68°25’E, Southern Ocean (Kopczyńska et al., 1998).
The largest of the Antarctic dinoflagellates, P. antarcticum is sometimes mistaken for P. pseudoantarcticum. However, while the latter has a similar tabulation pattern (orthoform with a quadra 2a), it is smaller, narrower, and has longer and more expanded antapical horns.
Protoperidinium is by far the most diverse dinoflagellate genus in Antarctic waters. Approx-imately 40 species have been recorded, most of which are endemic. While a number of taxa, e.g. P. antarcticum, are readily identifiable by their gross morphology, it is usually necessary to obtain good SEM images to determine the tabulation pattern essential for accurate identification. For most species, identification is based on the shape and configuration of the first apical plate (1') and the second intercalary plate (2a). Table 3.1 summarises the plate configuration of Antarctic Protoperidinium species.Balech (1974) recognised three subgenera, Minusculum, Archaeoperidinium and Protoperid-inium. Subgenus Minusculum, with only a single Antarctic species, P. defectum, is characterised by having 6 precingular plates of which 6'' is especially large. Only 2 anterior, intercalary plates are present.Subgenus Archaeoperidinium, represented by two Antarctic species, has 7 precingular plates but only 2 anterior, intercalary plates.Most Antarctic species are referrable to subgenus Protoperidinium which is characterised by 7 precingular plates and 3 anterior, intercalary plates. Primary taxonomic differentiation is based on the shape of the first apical plate, which can be surrounded by 4 (orthoform), 5 (metaform) or 6 (paraform) other plates (Fig. 3.18). The next most significant attribute is the shape of the second anterior, intercalary plate which, once again, can be in contact with 4 (quadra), 5 (penta) or 6 (hexa) surrounding plates (Fig. 3.18). For accurate identification, it is usually essential to determine the form of these two plates, this requiring both a dorsal and ventral view or an apical view. The most common plate configuration among the Antarctic species is metaform quadra (Table 3.1).