(Balech) Balech (1974: 61)
Peridinium mediocre Balech (1958b: 389, pl. 2, figs 54–61)
Balech (1976: 65, fig. 46), Andreoli et al. (1995: 473)
Cells subspherical, 30–40 µm long (excluding apical horn and antapical spines), 30–40 µm diameter. Thecal surfaces smooth, with scattered pores; epitheca tapering to a short apical horn supported by septa along apical plate boundaries; hypotheca with 2 small close-set antapical spines that incline inwards. Cingulum c. 4 µm wide, non-incised, ornamented with pores, displaced by 1 cingular width; cingular lists wide, irregularly ribbed. Tabulation 1pa, 4', 3a, 7'', 3c, 6s?, 5''', 1p, 1''''. Plate configuration: pentagonal (metaform); 2a plate rhombic (quadra).
Widespread in oceanic and coastal areas of the Antarctic; type locality, Terre Adélie (Balech, 1958b); Southern Ocean, near South Georgia (Dodge & Priddle, 1987); Ross Sea (Andreoli et al., 1995).
This species can be distinguished from most others with a similar plate pattern by its almost spherical shape and small, close-set horns. It is most similar to P. concavum, but the horns of that species extend outwards, whereas those of P. mediocre slope inwards.
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).