References:— Hustedt (1930), Cupp (1943), Rines & Hargraves (1988) (as Chaetoceros cf. brevis), Hernández-Becerril (1996), Jensen & Moestrup (1998), Bérard-Therriault et al. (1999), Sunesen et al. (2008).

Synonyms: — Chaetoceros hiemalis (Cleve) Cleve, Chaetoceros didymus var. hiemalis Cleve, Chaetoceros pseudobrevis Pavillard.

Morphometry: —a.a.: 14–37 μm; p.a.: 9–40 μm.

LM: —Cells are united in straight chains (Fig. 63). Cells are cylindrical, rectangular in girdle view with sharp and drawn up corners. Each cell has a single chloroplast often located close to one valve (Fig. 64). The mantle is low with a slight constriction near the margin. Setae originate from the drawn up valve corners and cross immediately at the chain margin. Apertures are wide and peanut-shaped due to the presence of a central inflation on the sibling valves (Fig. 64). Intercalary setae extend almost parallel to the valvar plane and symmetrically diverge from the apical plane. Setae of the intercalary cells positioned in the centre of the chain are mostly oriented perpendicular to the chain axis while those near the chain end have setae curved towards that end. The terminal setae form a U-shape. Setae very often contain some unknown plastid-like material, which did not fluoresce under UV light, indicating they do not contain chlorophyll (Fig. 65).

EM: —Resting spores have the primary valve ornamented with numerous thin capilli and a distinct mantle region (Fig. 66). The valve has a central annulus from which radiate widely spaced parallel and scarcely dichotomously branched costae (Fig. 67). A central slit-shaped rimoportula bearing an external wide flattened tube is present only in terminal valves (Figs 67, 68). The area between costae is hyaline; however, in the centre of the intercalary valve face there is a slightly more silicified area, which in TEM observations appears as a large and irregularly shaped darker patch (Fig. 69). The marginal ridge usually possesses a very low hyaline rim from which, sometimes in terminal valves, extend irregular silica projections (Fig. 68). The girdle bands are ornamented with alternating transverse parallel costae and hyaline striae that are perforated by small and irregularly distributed poroids (Fig. 70). The setae are circular in cross-section, adorned with densely distributed sharkfin-shaped spines and small poroids (Fig. 71).

Distinctive features: —Valve face with a central inflation. Thin setae cointaining plastid-like material. Dark patch in the central area of the valve surface. Primary valve of resting spore with numerous thin capilli.