G.Karst. (Karsten, 1905: 119, pl. 16, fig. 5)
Hendey (1937: 303), Hustedt (1958: 136), Manguin (1960: 286, pl. 11, figs 112, 113; pl. 28, fig. 336), Priddle & Fryxell (1985: 46, figs A–C), Hasle & Syversten (1996: 207, pl. 44)
Cells chain-forming, weakly silicified, rectangular in girdle view, isovalvate; pervalvar axis 5.5–10 µm. Valves flat, elliptical; apical axis 8.5–15 µm, no central spine visible. Apertures between cells large, hexagonal. Setae long, thin, arising from narrow ends of valve margins, diverging and lying perpendicular to the flexuous cell chain. Chloroplast single, parietal.
Southern Ocean, south of Australia; Weddell-Scotia Sea (Garrison et al., 1987); southern cold-water region (Hasle & Syvertsen, 1997); Ross Sea (Watanabe, 1982); Terre Adélie (Manguin, 1960); South Atlantic, Antarctic sector, between 51°32’S 0°08’W and 41°30’S 0°03’W (Hustedt, 1958); South Georgia, South Sandwich Group, South Shetlands, Weddell Sea, Bellingshausen Sea (Hendey, 1937).
Cells collected from the Southern Ocean were slightly smaller (apical axis 5.8–6.4 µm) than previous records. Chaetoceros neglectus and C. socialis are similar small species, the latter characterised by having 3 short and 1 long setae per cell. Chaetoceros neglectus is also morphologically similar to C. dichaeta, but the latter has a long central spine on each valve and numerous small chloroplasts distributed into the setae.