Cleve (1894: 13, pl. 1, fig. 2)
VanLandingham (1968: 714), Garrison (1991b: 22), Hasle & Syvertsen (1997: 211, pl. 44, tab. 51)
Cells chain-forming in distinct spirals, rectangular in girdle view, with valves of adjacent cells not touching and apertures rectangular, sometimes slightly constricted in the centre. Valves flat to slightly convex; apical axis 8–40 µm. Setae long, thin, smooth, crossing slightly after emergence from the valves, extending outwards from the spiral. Chloroplast single.
Off southern California (Cupp, 1943); Antarctic (Garrison, 1991b); cosmopolitan, mainly cooler waters (Hasle & Syvertsen, 1997).
Chaetoceros debilis appears to be rare in the Antarctic. It is similar to C. curvisetus, but the valves of C. debilis are flat to slightly convex and the cells do not touch at the corners as they do in C. curvisetus. The cell chains of C. debilis are markedly spiralled, not simply curved.