Chaetoceros vixvisibilis Schiller

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

Chaetoceros vixvisibilis Schiller in Hustedt (1930: 727) ( Figs 183–194 )

References:— Hustedt (1930), Hernández-Becerril (2010).

Morphometry: —a.a.: 8–17 μm; p.a.: 7–25 μm.

LM: —Cells are united in straight and usually very long chains, sometimes slightly twisted around the chain axis ( Fig. 183 ). The cells are elliptical in valve view and rectangular to square in girdle view with sharp, slightly drawn up valve corners. The number and shape of chloroplasts is not clear as the cell content often appears fragmented in several smaller parts in field material ( Figs 184, 185 ). The valve face is flat with a slight central inflation; the valve mantle is low with no observable constriction near the margin. The girdle is moderately high. The setae originate at the valve corners and immediately cross each other. Apertures are quite narrow and very slightly constricted in the middle due to the presence of the slight central inflation on the sibling valves ( Fig. 185 ). The setae are straight and usually very long and thin, however, they are very delicate and often found broken in the field samples ( Fig. 183 ). They are generally oriented perpendicular to the chain axis but can be bent more or less towards either chain end. Terminal setae do not morphologically differ from the intercalary ones and either are perpendicular to the chain axis or widely curved forming a U-shape. Resting spores are found very often within the cells displaying a wide range of morphological variation in shape and size (see the description below).

EM: —Valves have a very conspicuous pattern with a slightly eccentric hyaline annulus from which costae radiate and mostly bifurcate towards the valve margin ( Fig. 186 ). Some costae are twisted forming small spirals ( Fig. 186 ). Terminal valves show the same basic features, but additionally, have a rimoportula with a labiate structure ( Fig. 187 ). Girdle bands are ornamented with transverse ribs ( Fig. 188 ). Setae are circular in cross-section and ornamented solely with spirally arranged rows of small poroids, lacking any kind of spines or larger pores ( Figs 189, 190 ). The resting spores possess unequal valves. The primary valve is markedly dome-shaped, often with a variable number (1–4) of strong and long spines usually with many dichotomous branches at the tips ( Figs 191–194 ). Secondary valve is smooth, either flat or with a more or less expanded inflation in the centre. The mantle of both valves may be low to relatively high depending on the spore size ( Fig. 194 ).

Distinctive features: —Very long chains, setae delicate and straight, often broken. Narrow apertures. Valve ornamentation with costae ends twisted in small spirals. Setae without spines. Resting spores with one to four branching spines on primary valve.

Subgenus Bacteriastroidea Hernández-Becerril (1993)

Figures

FIGURES 183–194. Chaetoceros vixvisibilis field material. Figs 183–185, 191–192: LM. Figs 186–189, 193: TEM. Figs 190, 194: SEM. 183) Middle part of a chain with broken setae. 184, 185) Intercalary cells with the narrow aperture. 186) Intercalary valve showing valve ornamentation with central annulus and ends of costae twisted in small spirals. 187) Detail of terminal valve with central annulus and rimoportula. 188) Detail of the girdle. 189, 190) Details of setae, note the absence of spines. 191, 192) Resting spores within parent cells with different number of dichotomously branched spines. 193) Resting spore with two branched spines. 194) Open resting spore, secondary valve has a single ring of puncta on the advalvar margin (arrow). Scale bars: 183=20 μm, 184–185, 191–192=10 μm; 186, 193–194=2 μm; 187–190=1 μm.