Chaetoceros circinalis (Meunier) Jensen & Moestrup (1998: 28)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 17-20

Chaetoceros circinalis (Meunier) Jensen & Moestrup (1998: 28) ( Figs 72–78 )

References:— Hustedt (1930) (as Chaetoceros affinis var. circinalis ), Cupp (1943) (as Chaetoceros affinis var. circinalis ), Jensen & Moestrup (1998).

Synonyms:— Chaetoceros schüttii var. circinalis Meunier , Chaetoceros affinis var. circinalis (Meunier) Hustedt.

Morphometry: —a.a.: 5–16 μm; p.a.: 10–28 μm.

LM: —The cells are usually united in short, straight chains but single cells were commonly observed in both culture and field material ( Fig. 72 ). Each cell contains a single chloroplast. The valve face is flat or slightly concave with sharp valve corners slightly drawn up to touch the sibling valve. Valve mantle is high, girdle low or equidimensional with the mantle. Intercalary setae originate from the valve corners and immediately fuse at the chain margin. Apertures are very narrow and slit-shaped. Intercalary setae characteristically curve backwards around the cell away from the apical axis ( Fig. 73 ). In girdle view, setae curve strongly towards either side of the chain ( Fig. 72 ). Terminal setae diverge in a broad U-shaped curve often lying in the apical plane.

EM: —The valve face and the advalvar part of the mantle are densely perforated by numerous irregularly shaped and relatively large poroids ( Figs 74–76 ). The annulus and costae pattern are hardly observable in TEM due to the height of the mantle ( Fig. 76 ). The marginal ridge is ornamented with a distinct hyaline rim, close to the valve apices, that extends into projections that fuse with corresponding structures of the sibling valve ( Fig. 74 ). Terminal valves are often ornamented with short capilli ( Fig. 75 ) and possess a central rimoportula bearing an external, more or less flattened, wide tube ( Figs 75, 76 ). Intercalary setae appear completely smooth in the proximal portion and further on ornamented with longitudinal straight rows of small poroids and few larger elongated pores ( Fig. 77 ). On the central part of the setae and towards the distal part only small poroids are present together with spirally arranged sharkfin-shaped spines ( Fig. 78 ).

Distinctive features: —Intercalary setae characteristically curved backwards around the cell away from the apical axis. Valve face and advalvar part of the mantle densely perforated by many irregular and relatively large poroids.

Figures

FIGURES 72–82. Chaetoceros circinalis, Chaetoceros constrictus. Figs 72–73, 79, 81: LM. Figs 74–75, 77–78, 80, 82: SEM. Fig 76: TEM. Figs 72–78 Chaetoceros circinalis culture material, strain PMFG4. 72) A complete chain and a single cell showing the curvature of the setae. 73) Valve view of a cell showing the characteristic curvature of the setae. 74) Two sibling valves with partially fused hyaline rim at the aperture edge (arrows). 75) Terminal valve with wide flattened tube-like rimoportula process and short capilli on the valve surface. 76) Terminal valve with wide flattened tube-like rimoportula process. 77) Detail of proximal part of intercalary setae showing elongated poroids (arrow). 78) Detail of a seta. Figs 79–82 Chaetoceros constrictus field material. 79) Short chain showing orientation of the setae. 80) Detail of a seta. 81) Intercalary cells in girdle view showing the distinct constrictions near the mantle margin (arrows). 82) Terminal cell showing external flattened tube of the rimoportula, the small spines covering the valve surface and longitudinal groove near the mantle edge. Scale bars: 79=20 μm; 72–73, 81–82=10 μm; 74–78, 80=1 μm.