Chaetoceros tortissimus Gran (1900: 122)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 34-35

Chaetoceros tortissimus Gran (1900: 122) ( Figs 173–182 )

References:— Ikari (1926), Hustedt (1930), Cupp (1943), Hoppenrath et al. (2009), Shevchenko et al. (2006).

Morphometry: —a.a.: 14–21 μm; p.a.: 5–14 μm.

LM: —Cells are united in usually long chains that are very strongly twisted around the chain axis in such a way that some sibling valves position themselves perpendicular to each other ( Figs 173–175 ). In valve view cells are broadly elliptical and compressed in the transapical direction, in girdle view they are rectangular with rounded valve corners. Each cell contains a single parietal plate-like chloroplast positioned around the girdle ( Fig. 176 ). The valve face is flat or slightly convex; the mantle is low and sometimes constricted near the margin, girdle equidimensional or higher than the mantle. Long and thin setae originate within the valve margin and slightly direct outwards forming a short basal part, and then cross slightly inside or at the chain margin ( Fig. 175 ). Apertures are hexagonally shaped. Intercalary setae are generally oriented more or less perpendicularly to the chain axis or appear slightly curved ( Fig. 173 ). In valve view they diverge at an equal angle (30–45°) from the apical plane. Terminal setae are thick like the intercalary ones and extend almost parallel to the colony axis in a U-shaped curve ( Fig. 173 ).

EM: —The valve has a central hyaline annulus from which extends a complex pattern of densely distributed costae, branching dichotomously towards the valve margin, sometimes forming small dots ( Figs 177, 178 ). The marginal ridge does not have a prominent rim; it is only observable as a thickened edge between the valve face and the mantle. Girdle bands are adorned with transverse fine costae and a slightly thickened longitudinal rib near one edge ( Fig. 181 ). A conspicuous character found in this species is the presence of a large elongated pore on the proximal part of the setae, right after the crossing point ( Figs 178–180 ). Setae are circular in cross-section, ornamented with very small spines and poroids spirally positioned along the seta length ( Fig. 182 ).

Distinctive features: —Very strong torsion of the chain. Large elongated pore on the proximal part of the seta.

Figures

FIGURES 173–182. Chaetoceros tortissimus. Figs 174–175, 178: field material. Figs 173, 176–177, 179–182: culture material, strain PMFT1. Figs 173–176: LM. Figs 177–178, 181–182: TEM. Figs: 179–180: SEM. 173) Complete chain with a slight torsion showing orientation of the setae. 174) Complete chain composed of cells with short pervalvar size strongly twisted around the chain axis. 175) Middle part of a chain showing hexagonal apertures. 176) Intercalary cells with one parietal plate-like chloroplast positioned around the girdle. 177) Detail of the valve face with network of costae some ending in round spots (arrow). 178) Two overlapping sibling valves showing the valve ornamentation and the elongated pores near the bases of the setae (arrows). 179) Sibling valves with the elongated pores (arrows). 180) Detail of the proximal part of the seta with the large elongated pore. 181) Detail of the girdle band. 182) Detail of a seta. Scale bars: 173–175=20 μm; 176=10 μm; 178, 179=5 μm; 177=2 μm; 180–182=0.5 μm.