Chaetoceros socialis Lauder (1864a: 77)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 32

Chaetoceros socialis Lauder (1864a: 77) [emend. Chamnansinp, Li, Lundholm & Moestrup (2013: 1133] ( Figs 162–

172)

References:— Hustedt (1930, as C. radians ), Evensen & Hasle (1975, as C. radians ), Hargraves (1979), Rines & Hargraves (1988), Jensen & Moestrup (1998), Sunesen et al. (2008), Shevchenko et al. (2008), Kooistra et al. (2010), Degerlund et al. (2012), Chamnansinp et al. (2013).

Synonyms: — Chaetoceros socialis f. autumnalis Proschkina-Lavrenko , Chaetoceros socialis f. socialis Proschkina-Lavrenko , Chaetoceros socialis f. radians Proschkina-Lavrenko , Chaetoceros radians Schütt.

Morphometry: —a.a.: 9–13 μm; p.a.: 5–13 μm.

LM: —Cells are joined in short chains curved in broad girdle view. Each cell usually possesses three curved setae and one long and straight seta which extends towards the concave part of the chain. By connection of the long setae tips short chains join to form large spherical colonies having in their centre the interconnected tips ( Figs 162, 163 ). The cells are elliptical in valve view and rectangular in girdle view with sharp corners. Each cell possesses a single large chloroplast ( Fig. 165 ). The delicate and thin setae originate inside the valve margins and direct outwards, with sibling setae crossing each other after a long basal part thus forming wide hexagonal apertures ( Figs 164, 165 ). In valve view, the orientation of the setae has one pair of sibling setae diverging 30–50° from the apical plane. One member of the second pair curves back around the cell and continuing in the direction of the first pair, and the other being especially elongated and growing toward the colony centre. Resting spores are round in shape, with both valves dome-shaped and spiny ( Fig. 164 ).

EM: —The frustule is weakly silicified. Valve face is flat to concave with an inflated central part ( Fig. 168 ); the mantle is low without a constriction near its edge, girdle usually equidimensional with the mantle. The valve is ornamented with a weak pattern of scarcely distributed costae radiating from the central annulus and converging towards the two insertion points of the setae ( Figs 167 ). The setae are circular in cross-section, bearing arrowhead shaped spines arranged in a helicoidal pattern ( Fig. 169 ). Setae are also ornamented with small poroids arranged in a spiral pattern, and with irregularly distributed larger elongated pores ( Fig. 169 ). Both valves of the resting spore are covered with spines over the whole surface and a ring of spines at the margin of the primary valve ( Figs 170, 171 ). The spines appear to be conically shaped and occasionally the longest spines have a minute bifurcation at the tip. The spines vary greatly in length with generally the primary valve spines being much longer than on the secondary valve ( Fig. 172 ).

Distinctive features: —Curved chains usually joined in large spherical colonies by connection of the tips of long setae. Delicate and thin setae having a distinct basal part and forming large hexagonal apertures. Resting spores with spines on both valves.

Figures

FIGURES 162–172. Chaetoceros socialis from culture material. Figs 162–163, 165: strain PMFA3. Figs 164, 166–169, 172: strain PMFA6. Figs 170–171 strain PMFA7. Figs 162–165: LM. Figs 166, 170–172: SEM. Figs 167–169: TEM. 162) A spherical colony. 163) Three curved chains connected with the long setae.164) Middle part of a chain with the resting spores within the parents cells. 165) Middle part of a chain showing the apertures. 166) Sibling valves in valve view showing the orientation of the setae. 167) Detail of the valve with the ornamentation pattern. 168) Intercalary valve in the girdle view. 169) Detail of a seta. Note the elongated poroid (arrow). 170) Complete resting spore. 171) Primary valve of the resting spore. 172) Secondary valve of the resting spore. Note the ring of puncta on the advalvar mantle margin (arrow). Scale bars: 162, 163=50 μm; 164=20 μm; 165=10 μm 166–168=2 μm; 170–172=1 μm; 169=0.5 μm.