Chaetoceros bacteriastroides Karsten (1907: 390)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 35-37

Chaetoceros bacteriastroides Karsten (1907: 390) ( Figs 195–198 )

References: — Fryxell (1978), Hernández-Becerril (1993)

Morphometry: —a.a.: 9–10 μm; p.a.: 22–30 μm.

LM: —Cells are united in short, straight and robust chains ( Fig. 195 ). In valve view cells are nearly circular, in girdle view rectangular with pervalvar axis longer than apical axis. Each cell contains numerous small chloroplasts ( Fig. 196 ). The setae appear to contain some plastid-like material in few of the observed specimens, but it was not possible to determine if these granules indeed contain chlorophyll. The valve face is flat to slightly concave and the mantle is quite high without a constriction near the margin. Each intercalary valve possesses two common, long and robust setae, and four short, thin and strongly spirally undulated setae ( Figs 196 ). Common setae originate at the valve margin on two opposite sides of each valve, and join with sibling setae after a short basal part. They fuse for a rather long distance (ca. half of the valve diameter) and then diverge equally at an angle of ca. 30° from the apical axis, lying in the apical plane. Spiral setae also have a short basal part but they fuse at a single point and then extend perpendicular to the chain axis lying in the valvar plane ( Fig. 195 ). The aperture between adjacent cells is distinct, and appears to be elliptical in shape due to the most visible connection between two long sibling setae. Terminal valve possesses only two long common setae extended in the direction of the chain. The two terminal setae slightly differentiate in the curvature, one being more parallel to the chain axis while the other is first more widely curved and then becomes parallel with the same axis.

EM: —The valve face and the mantle are densely perforated with very small poroids in an irregular pattern ( Fig. 197 ). Two sibling intercalary valves were observed with shoehorn-shaped outgrowths arising from the valve margin between setae ( Fig. 197 ). Common setae are circular in cross-section. They are perforated with longitudinal rows of very small poroids running parallel with the seta axis from their basal part throughout the whole length of the seta. After the fused part small spines are found arranged in a spiral pattern around the seta ( Fig. 198 ). Short setae are straight and with few small spines in the proximal part and T-shaped spines in the distal spirally undulated part. The surface of the undulated part is not smooth, but adorned with small silicate nodules. The spiral coils are at first very narrow and become wider in the terminal part ( Fig. 198 ).

Distinctive features: —Cell circular in valve view. Shape and orientation of terminal, common and spirally undulated setae.

Figures

FIGURES 195–198. Chaetoceros bacteriastroides field material. Figs 195–196: LM. Figs 197–198: SEM. 195) Complete chain showing orientation of the setae. Arrow shows the reduced undulated setae. 196) Terminal part of the chain with cells containing numerous chloroplasts and coiled setae (arrow). 197) Sibling valves in girdle view showing two pairs of reduced coiled setae and two fused parts of common intercalary setae.Arrow points to the shoehorn shaped outgrowth on the valve edge. 198) Detail of the intercalary common setae with small spines (arrowhead) and spirally undulated setae with T-shaped spines (arrow). Scale bars: 195=20 μm; 196=10 μm; 197=5 μm; 198=1 μm.