Chaetoceros pseudodichaeta Ikari (1926: 517)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 11

Chaetoceros pseudodichaeta Ikari (1926: 517) ( Figs 27–32 )

References:— Ikari (1926), Hernández-Becerril (2000), Lee et al. (2014b).

Morphometry:—a.a.: 4.1–13.2 μm; p.a.: 8.6–28.4 μm.

LM: — Cells form short , straight and robust chains, with setae oriented in a three-dimensional space. ( Fig. 27 ). Cells in girdle view appear rectangular with round corners, usually with the pervalvar axis longer than the apical axis. Numerous spherical chloroplasts are present in the cell body as well as inside the long and thick setae. Valves are slightly concave or flat, mantle is high with no distinct constriction near the edge, the girdle is often broader than the mantle. The setae originate inside the valve margin with a prominent basal part extending parallel with the pervalvar axis. The sibling setae fuse at a single point, often crossing each other at the chain margin, and form a wide and rectangular aperture ( Fig. 27 ). Terminal setae originate within the valve margin and they first extend parallel to the pervalvar axis, then slightly bend outward but soon recurve back and become again almost parallel to the chain axis ( Fig. 27 ). A central rimoportula with a long external tubular process is present only on terminal valves ( Fig. 28 ).

EM: —The valves are heavily silicified ( Figs 29, 30 ), the valve face is ornamented with thick dichotomously branching costae radiating from a round central annulus ( Fig. 31 ). The sibling setae fuse at a single point and after the junction diverge almost equally at an angle of ca. 30–45° from the apical plane ( Fig. 29 ). Several short siliceous capilli are irregularly distributed across the surface ( Fig. 31 ). The marginal ridge, between valve face and mantle, is adorned with a strongly silicified low hyaline rim which often extends, in the pervalvar direction, into one or several projections of variable shape and length ( Fig. 30 ). The mantle is high and without constriction near the edge. The four-sided setae are ornamented with curved saw blade-shaped spines, which sometimes extend into capilli ( Figs 29, 32 ). Setae sides are ornamented with transverse thickened costae interspaced by two rows of poroids. Sometimes, one round and slightly larger pore is found, situated at either end of the areola row near the seta ridge.

Distinctive features: —Orientation of setae in a three-dimensional space. Setae ornamented with long curved saw blade- shaped spines. Long tubular process present only on terminal valves.

Figures

FIGURES 24–32. Chaetoceros peruvianus, Chaetoceros pseudodichaeta. Figs 24–28: LM.Figs 29–30: SEM.Figs 31–32: TEM. Figs 24– 26: Chaetoceros peruvianus field material. 24) Single cell showing setae orientation. 25) Cell in girdle view. Note the tooth-like structure between the setae on the anterior valve (arrow). 26) Detail of seta with spines and chloroplasts. Figs 27–29: Chaetoceros pseudodichaeta field material. 27) Complete chain showing the typical three-dimensional setae orientation. Note the orientation of the terminal setae. 28) Terminal cell of the chain with the long tubular process. 29) Two sibling valves showing the fusion point of the setae which afterwards diverge in various directions. Note the hyaline rim on the valve edge (arrow). Figs 30–32: Chaetoceros pseudodichaeta, culture material strain PMFED1. 30) Valve with silica projections and rimoportula (arrow). 31) Valve face with the annulus, branching costae and few capilli (arrow). 32) Detail of a seta with large poroid marked with an arrow. Scale bars: 24=50 μm; 25, 27=20 μm; 28=10 μm; 29–31=2 μm; 26, 32= 1 μm.