Chaetoceros rostratus Ralfs

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 11-14

Chaetoceros rostratus Ralfs in Lauder (1864a: 79) ( Figs 33–41 )

References:— Hustedt (1930), Giuffrè & Ragusa (1988), Rines & Hargraves (1988), Hernández-Becerril (1996), Hernández-Becerril & Flores Granados (1998), Shevchenko et al. (2006), Sunesen et al. (2008), Lee et al. (2014b).

Synonym: — Chaetoceros glandazi Mangin.

Morphometry: —a.a.: 10–22 μm; p.a.: 16–48 μm.

LM: —The chains are robust, straight and short (usually 2–5 cells) ( Fig. 33 ). Cells in girdle view rectangular with more or less rounded corners ( Fig. 34 ). Cells and setae contain numerous plastids ( Fig. 35 ). Valve face slightly to strongly convex; the mantle is high with a visible constriction near the edge, girdle small and equidimensional with the mantle ( Fig. 34 ). Cells are held in a chain by the fusion of a central linkage process (linking spine) and not by the joining of setae. Very long, straight and thick setae originate close to the valve margin and diverge perpendicular to the chain axis ( Fig. 33 ). In the intercalary cells the sibling valves are twisted about 60° between them so the setae arise without touching the setae emerging from the neighbouring valve ( Figs 34–35 ). In valve view, the sibling setae diverge at an angle of ca. 30–45° from the apical plane.

EM: —The conically shaped linking spines are of variable length, and can become very short and wide in culture but in field material they are usually long and narrow ( Fig. 36 ). Their surface is completely smooth and may be perforated with a few poroids at the base. The terminal valve lacks the linking spine. A rimoportula is present in each valve and it is positioned slightly eccentric to one side of the spine ( Figs 36, 37 ). The rimoportula appears as a simple slit on the internal side of the valve ( Fig. 37 ) and brings a small protuberance on the external side ( Figs 36, 39 ). Valves are perforated by round poroids irregularly scattered across the valve face, mantle and setae bases ( Fig. 39 ). The valve face is ornamented with a pattern of dense branching costae which radiate from the eccentrically positioned annulus ( Fig. 39 ). The setae are four-sided in cross section with rows of arrowhead-shaped spines along each edge ( Figs 38, 41 ). The ornamentation on the setae wall consists of five transverse rows of poroids arranged in a cross pattern between transverse thickened costae. After each 4–9 costae there is often one larger round pore located near the side edge and close to the costae ( Fig. 40 ).

Distinctive features: —Sibling cells connected with central linking spine of variable length. Four-sided setae having a wall ornamented with thick transverse costae interspaced with five poroid rows.

Subgenus Hyalochaete Gran 1897

Figures

FIGURES 33–41. Chaetoceros rostratus. Figs 33, 38–41: culture material, strain PMFR4. Figs 34–37: field material. Figs 33–35: LM. Figs 36–38: SEM. Figs 39–41: TEM. 33) A chain. 34) Terminal part of a chain. Note the absence of the linking spine in the terminal valve (arrow). 35) Two intercalary cells showing the presence of chloroplasts in cell body and setae. 36) Intercalary sibling valves connected with the linking spine and visible tube-like processes of rimoportulae (arrows). 37) Internal view of the intercalary valve showing the conical hole of the linking spine and the slit-like eccentric rimoportula (arrow). 38) Detail of a seta. 39) External view of the terminal valve showing the small tube-like eccentric process (arrow) and the fine structure of the valve face, mantle and bases of the setae. 40) Detail of a seta showing the ornamentation of the side wall with larger poroids on either side of the poroid rows (arrows). 41) Tetragonal cross section of the seta. Scale bars: 33, 34=20 μm; 35=10 μm; 36, 37=5 μm; 39 = 2 μm; 38, 40, 41=1 μm.