Chaetoceros densus (Cleve) Cleve (1901: 15

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 9

Chaetoceros densus (Cleve) Cleve (1901: 15 , 16, 24, 28) ( Figs 13–15 )

References:— Hustedt (1930), Rines & Hargraves (1988), Hernández-Becerril (1996), Jensen & Moestrup (1998), Lee et al. (2014b).

Synonyms: — Chaetoceros borealis var. densa Cleve , Chaetoceros borealis var. brigthwellii Cleve , Chaetoceros brightwellii (Cleve) Mills.

Morphometry: —a.a.: 17–38 μm; p.a.: 13–63 μm

LM: —Cells are united into robust, straight and long chains. Cells are rectangular in girdle view with diagonally cut valve corners ( Fig. 13 ). Cells and setae contain numerous small chloroplasts. The valve face is flat or slightly concave; the mantle is high and shows a visible constriction at its edge ( Fig. 13 ). A simple hole-shaped rimoportula is present in the centre of each valve ( Fig. 14 ). The long, thick and straight setae originate inside the valve margin. Apertures are narrow and slit- or hexagonally shaped ( Fig. 13 ). In valve view, the intercalary setae diverge from the apical plane at equal angles (ca. 30–45°) ( Fig. 14 ). Terminal setae are almost parallel to the chain axis.

EM: —The setae are four-sided (not shown) with the rows of sharp stipule-shaped spines on the ridges. Each side is perforated by small poroids arranged in a grid pattern, comprising 12–15 longitudinal rows and a single row of poroids between transverse costae ( Fig. 15 ).

Distinctive features: —Very narrow apertures. Four-sided setae with each side ornamented with small poroids arranged in a grid pattern, with 12–15 longitudinal rows of poroids.

Figures

FIGURES 13–23. Chaetoceros densus, Chaetoceros eibenii. Figs 13, 14, 16: LM. Figs 15, 20, 21: TEM. Figs 17–19, 22–23: SEM. Figs 13–15: Chaetoceros densus field material. 13) Terminal part of a chain. 14) Sibling valves in valve view. 15) Detail of a seta. Figs 16, 18–22: Chaetoceros eibenii culture material, strain PMFH1. 16) Complete chain of two cells. Fig. 17: Chaetoceros eibenii field material. 17) Terminal part of a chain showing wide hexagonal apertures and a rimoportula visible on every valve. 18) Valve view of sibling valves showing setae divergence. 19) External view of the valve with rimoportula and hyaline rim on the marginal ridge (arrows). 20) Detail of the seta. 21) Detail of a broken seta showing the ornamentation pattern of the side wall. The larger round poroid is indicated by an arrow. 22) Hexagonal cross-section of the seta. 23) Seta tip. Scale bars: 16= 50 μm; 13, 14, 17=10 μm; 18, 19=5 μm; 15, 20–23=1 μm.