Chaetoceros costatus Pavillard (1911: 24)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 22

Chaetoceros costatus Pavillard (1911: 24) ( Figs 95–104 )

References:— Hustedt (1930), Cupp (1943), Rines & Hargraves (1988), Jensen & Moestrup (1998), Hernández-Becerril & Flores Granados (1998), Hernández-Becerril & Aké-Castillo (2001), Kooistra et al. (2010), Ishii et al. (2011), Lee et al. (2014a).

Synonym: — Chaetoceros adhaerens Mangin.

Morphometry: —a.a.: 11–15 μm; p.a.: 21–32 μm.

LM: —Cells are arranged in straight, sometimes very long chains. The cells are rectangular in girdle view. There is a single, plate-like chloroplast per cell ( Fig. 95 ). Valve face is flat to slightly concave and possesses four submarginal flattened protuberances, two on each side of the valve positioned close to the valve apices. Sibling cells join by fusion of these protuberances, but also by fusion of sibling setae ( Fig. 96 ). The aperture is very narrow, shaped as a linear thin slit, extending between the protuberances, shorter than the apical axis ( Fig. 96 ). The valve mantle is very low and the girdle often very wide. The edges of the numerous girdle bands are readily visible in LM ( Fig. 96 ). The thin and long intercalary setae originate at the valve margin, cross immediately without a basal part and extend mostly perpendicular to the chain axis or slightly curve towards the end of the chain, diverging variously from the apical plane. The terminal setae are morphologically similar to intercalary ones and curved obliquely towards the chain axis.

EM: —The lightly silicified valve face has a distinct pattern of costae radiating from a central or a slightly eccentric large hyaline annulus ( Figs 97, 98 ). In the central part of the valve face, costae branch dichotomously towards the valve edge, whilst in part near the apices costae remain for a certain distance parallel with the apical axis converging towards the setae bases ( Fig. 98 ). Each valve, including the terminal ones, has four protuberances not hyaline, but adorned with parallel costae oriented perpendicularly to the apical axis ( Fig. 98 ). An eccentric, slit-shaped rimoportula bearing an external flattened tube is present only on terminal valves ( Figs 99, 100 ). The marginal ridge is ornamented with a low siliceous hyaline rim ( Fig. 97 ). Setae are circular in cross-section, ornamented with small shaped poroids and shark fin-spines arranged in a spiral pattern ( Fig. 101 ). The girdle bands are ornamented with transverse costae interspaced with hyaline areas. The advalvar bands have a distinct thickened longitudinal rib at one edge ( Fig. 102 ).

Resting spores are oval or rounded in shape. The primary valve is dome-shaped and covered with numerous hooked conical spines sometimes joined at their basal part by siliceous vela ( Fig. 103 ). The secondary valve has fewer conical spines at the surface ( Fig. 104 ).

Distinctive features: —Cells usually united in long chains. Girdle bands visible in LM. Sibling valves joined by setae and by four submarginal protuberances. Aperture shorter than the apical axis. Valve ornamentation pattern showing parallel costae in area proximal to the valve apices.

Figures

FIGURES 95–104. Chaetoceros costatus. Fig. 96: field material. Figs 95, 97–104: culture material. Figs 95, 99–100, 102: strain PMFB7. Figs 97–98: strain PMFB8. Figs 101, 103–104: strain PMFB6. Figs 95–96: LM. Figs 97–100, 102: TEM. Figs 101, 103–104: SEM. 95) Complete chain showing orientation of the setae. 96) Middle part of a chain showing the visible edges of the girdle bands (arrow). 97) Sibling valves, note the fusion of protuberances and sibling setae joining the cells. Arrow indicates hyaline rim on the marginal ridge. 98) Valve face of the terminal valve showing eccentric annulus and costae pattern. 99) External flattened tube of the rimoportula. 100) Slit-shaped aperture of the rimoportula. 101) Detail of a seta. 102) Girdle composed of half-bands with the thickened edges on advalvar bands. 103) Primary valve with hooked spines. The edge of the valve and the mantle are covered with silica membranes which appear to be remains of the girdle left from the parent cell. 104) Secondary valve with conical spines and single marginal ring of poroids. 95–96=10 μm; 97–98, 102, 103–104=2 μm; 99, 101= 0.5 μm.