Chaetoceros affinis Lauder (1864a: 78)

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

Chaetoceros affinis Lauder (1864a: 78) ( Figs 42–53 ).

References:— Hustedt (1930), Cupp (1943), Evensen & Hasle (1975), Rines & Hargraves (1988), Hernández-Becerril (1996), Jensen & Moestrup (1998), Berard-Therriault et al. (1999), Shevchenko et al. (2006), Sunesen et al. (2008), Kooistra et al. (2010), Ishii et al. (2011), Lee et al. (2014a).

Synonyms: — Chaetoceros javanicus Cleve , Chaetoceros ralfsii Cleve , Chaetoceros schüttii Cleve , Chaetoceros angulatus Schütt , Chaetoceros distichus Schütt , Chaetoceros procerus Schütt , Chaetoceros paradoxus var. schüttii Schütt , Chaetoceros clevei Peragallo , Chaetoceros raflsii Karsten , Chaetoceros schüttii var. genuina Meunier , Chaetoceros najadianus Schussnig , Chaetoceros adriaticus Schussnig.

Morphometry: —a.a.: 9–42 μm; p.a.: 8–16 μm.

LM: —The cells are united in straight chains ( Fig. 42 ). Each cell contains a single large chloroplast. The valve surface is concave and the valve corners are sharp and slightly drawn up. Intercalary setae are thin, originating from the valve apices and immediately cross each other at the chain margin. Apertures are rather narrow but usually distinct in LM, slit-shaped to elliptical. Setae are very straight and positioned in the apical plane. Terminal setae can be similar or can differ very strongly from the intercalary ones, in both orientation and morphology. When different, they are much thicker and taper towards the end, strongly diverging in a broad V- (not shown) or U-shaped ( Fig. 42 ) curve towards the end of the chain, lying in the apical plane. The resting spores have unevenly vaulted valves with a broad distinct mantle. Primary valve is dome-shaped, ornamented with numerous long spines whereas the secondary valve has a central inflated part and it is ornamented with few longer spines ( Fig. 50 ).

EM: —Valve mantle is high and often with a visible constriction near the margin ( Fig. 43 ). The aperture is very often completely occluded by a thin silica wall with no distinct pattern, except for the single row of pores on its edge where it fuses with the marginal ridge ( Fig. 43 ). The smooth silica membrane is very fragile and appears ruptured in some of the observed specimens. The valves have a pattern of dichotomously branching costae radiating from a round central annulus and are perforated with numerous irregularly shaped very small poroids ( Fig. 44 ). The marginal ridge is ornamented with a hyaline rim ( Fig. 45 ). Terminal valves possess a central slit-shaped rimoportula that forms an external process shaped as a wide, short, flattened tube ( Fig. 46 ). Terminal valves are generally ornamented with few short capilli covering the valve face in the central part ( Fig. 45 ). Intercalary setae are circular in cross-section ( Fig. 47 ) and ornamented with small poroids and shark fin-shaped spines arranged in a spiral pattern ( Fig. 48 ). Terminal setae are heavily silicified with stronger spines and few larger elongated pores irregularly distributed along the seta length ( Fig. 49 ). The surface of the resting spores is covered with knobs and spines, which can be simple or have dichotomously branching tips ( Figs 51, 52 ). The mantle of the secondary valve possesses one single row of puncta ( Figs 51, 53 ).

Distinctive features: —Narrow apertures, completely occluded by a silica membrane observable in EM.Intercalary setae straight, all setae lying in the apical plane. Terminal setae often thicker. Resting spores with numerous long spines.

Figures

FIGURES 42–53. Chaetoceros affinis. Figs 42–43, 50–53: field material. Figs 44–53: culture material. Figs 44, 47–48: strain PMFE1. Figs 45–46, 49–53: strain PMFC2. Figs 42, 50: LM. Figs 43, 45–49, 51–53: SEM. Fig 44: TEM. 42) A chain. 43) Sibling valves with silica membrane covering the aperture. Note the row of pores on the point of fusion with the marginal ridge (arrow). 44) Sibling valves showing the silica membrane ruptured on one side. The arrow indicates central annulus. 45) Detail of the terminal valve with short capilli and marginal rim (arrow). 46) Detail of the terminal valve with short capilli and beak-like process (arrow). 47) Circular cross-section of the intercalary seta. 48) Detail of intercalary seta. 49) Detail of thick terminal seta. 50) Resting spores within parental cells. 51) Secondary valve of the resting spore with both dichotomously branching and simple long spines. Arrow shows the single ring of puncta on the advalvar margin. 52) Valve view on the resting spore (not certain if primary or secondary valve) showing surface covered with knobs and dichotomously branching spines. 53) Secondary valves of the resting spores in formation inside two sibling valves joined by the siliceous membrane. Note the single ring of puncta on the advalvar margin of the mantle (arrow). Scale bars: 42=20 μm; 50=10 μm; 43, 53=5 μm; 44, 51–52=2 μm; 45–46, 48–49=1 μm. 47=0.5 μm.