Chaetoceros curvisetus Cleve (1889: 55)

Bosak, Sunčica & Sarno, Diana, 2017, The planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta) from the Adriatic Sea, Phytotaxa 314 (1), pp. 1-44 : 22-25

Chaetoceros curvisetus Cleve (1889: 55) ( Figs 105–116 )

References:— Hustedt (1930), Cupp (1943), Evensen & Hasle (1975), Hargraves (1979), Rines & Hargraves (1988), Hernández-Becerril

(1996), Jensen & Moestrup (1998), Hernández-Becerril & Flores Granados (1998), Shevchenko et al. (2006), Sunesen et al. (2008),

Kooistra et al. (2010), Lee et al. (2014a). Synonyms: — Chaetoceros secundus Schütt , Chaetoceros cochlea Schütt.

Morphometry:—a.a.: 11–34 μm; p.a.: 8–30 μm.

LM:—Cells united in usually long chains; in pervalvar plane curved or spiral ( Fig. 105 ). Cells are broadly elliptical in valve view and rectangular in girdle view with drawn up valve corners. In each cell there is a single large plate-like chloroplast ( Fig. 106 ). The valve face is markedly concave and the setae emerge from the valve corners immediately crossing each other at their point of origin with a short basal part. The apertures are large and elliptical to circular in shape ( Fig. 106 ). The girdle is usually wide and the mantle is narrow. The setae are long and relatively thin. In girdle view they appear all curved towards the same convex side of the chain ( Fig. 105 ), while in the valve view they are all bent to one side of the apical axis. The intercalary and terminal setae are similar in morphology and orientation.

EM: —The valve has a hyaline central annulus with lighter centre encircled with a slightly darker marginal area from which extends a regular pattern of relatively thick and rarely dichotomously branched costae ( Fig. 107 ). The pattern appears more complex towards the valve margin where, in some valves, a number of costae form small spirals. The marginal ridge possesses a low hyaline rim ( Fig. 108 ). A central or slightly eccentric rimoportula is present only in terminal valves. It is internally shaped as a simple slit and externally as a wide flattened tube ( Figs 108–110 ). Girdle bands are adorned with transverse fine ribs alternating with rows of small round poroids ( Figs 111, 112 ). Setae are circular in cross-section, ornamented with small poroids and shark fin shaped spines arranged in a spiral pattern ( Fig. 113 ). The resting spore appears to be perforated with small poroids on the valve surface. Resting spores have unequally vaulted valves with the dome-shaped primary valve and less rounded to almost flat secondary valve. The surface of both valves is smooth ( Fig. 114 ). Both valves of the resting spore bear a silica collar that is in the primary valve perforated by a series of elongated simple fissures ( Figs 115, 116 ).

Distinctive features: —Chains curved in pervalvar plane, large oval apertures.All setae curved towards the convex side of the chain. Smooth resting spores with a collar. Valve ultrastructure showing a regular pattern of relatively thick and rarely dichotomously branched costae.

Chaetoceros decipiens Cleve (1873: 11) [emend. Li, Boonprakob, Moestrup & Lundholm (2017: 5) ] ( Figs 117–127 ) References:— Hustedt (1930), Cupp (1943), Evensen & Hasle (1975), Rines & Hargraves (1988), Hernández-Becerril (1996), Jensen &

Moestrup (1998), Berard-Therriault et al. (1999), Hernández-Becerril & Flores Granados (1998), Shevchenko et al. (2006), Sunesen et al. (2008), Li et al. (2017). Synonyms: — Chaetoceros grunowii Schütt.

Morphometry: —a.a.: 7–51 μm; p.a.: 8–43 μm.

LM: —Cells are united in straight chains, sometimes slightly curved in the apical plane ( Fig. 117 ). There are several ((2)4–10) small discoid chloroplasts present in each cell ( Figs 118, 119 ). The valve face is weakly to strongly concave; the valve corners are sharp and drawn up, touching those of the sibling cell. Apertures are wide and elliptical. The mantle is high, usually with a distinct constriction near the margin. The girdle is usually lower than or equidimensional with the mantle. In cells from field samples there were often observed organic threads projecting from the girdle region ( Fig. 118 ). The setae originate from the valve corners, fuse immediately at the chain margin and then diverge at an angle of ca 30° equally from the chain axis.All the setae are lying in the apical plane. The length of the fused part of the setae appears to be a very variable character with the extent of fusion varying even within a single chain and generally reduced in culture material. In most cases and especially in field samples setae can fuse for up to several times their diameter ( Fig. 118 ), however, they can be also fused only at their point of cross over ( Fig. 117 ). The intercalary setae are very stiff, straight and variable in length. Terminal setae appear slightly thicker than intercalary ones, diverging from cell corners in a wide V-shaped curve. Resting spores were not observed.

EM: —The valve has a central annulus from which extends a pattern of radiating costae branching dichotomously towards the valve margin and becoming parallel on the valve mantle ( Fig. 120 ). The hyaline areas between costae are densely perforated with poroids ( Figs 120, 122, 124 ). A small rimoportula, with an internal distinct labiate structure ( Figs 123, 124 ) and a very small external protrusion ( Fig. 121 ), is present only on terminal valves. The marginal ridge is ornamented with a hyaline rim and siliceous projections which extend as fringes from the valve mantle ( Fig 120 ). Projections may become large and slice-shaped in proximity of the valve apices ( Fig. 121 ). These projections, in the intercalary valve, fuse with the corresponding structures of the sibling valve ( Fig. 120 ) to form a hyaline structure that can partially occlude the aperture. Girdle bands are ornamented with transverse ribs alternating with less silicified rows of irregularly scattered poroids ( Fig. 125 ). The setae are polygonal, four- to five-sided ( Figs 126, 127 ). Each side is ornamented with a single straight longitudinal row of large round-oval poroids. The size of poroids varies between 0.3 and 0.7 μm with a density of 14–22 poroids in 10 μm in the proximal and central part of the setae ( Fig. 126 ). Larger poroids (up to 1.1 μm) have been rarely observed near the seta tip. Spines are shark fin-shaped and positioned along the setae ridges ( Figs 126, 127 ).

Distinctive features: —Corners of sibling valves touching and forming a large aperture; intercalary setae usually fused for a variable distance. Setae stiff, extended in the apical plane, polygonal in cross section. Valve surface densely perforated with poroids. Rimoportula very small with a labiate structure.

Figures

FIGURES 105–116. Chaetoceros curvisetus. Figs 105, 114: field material. Figs 106–113, 115–116: culture material. Figs 107, 109–112: strain PMFD2. Figs 108, 113, 115–116: strain PMFD6. Fig. 106: strain PMFD8. Figs 105–106, 114: LM. Figs 107, 109–112: TEM. Figs 108, 113, 115–116: SEM. 105)A chain in narrow girdle view showing orientation of the setae.106) Intercalary cells with a single plate-like chloroplast. 107) Detail of the valve showing the valve ornamentation pattern. The arrow shows a dot-like costa end. 108) Outside view of the terminal valve with the hyaline rim on the marginal ridge and external flattened tube of the rimoportula. 109) Detail of the valve with external flattened tube of the rimoportula. 110) Central annulus with the slit-like aperture of the rimoportula. 111) Detail of the girdle. 112) Detail of a girdle band with transverse ribs and the inter-rib areas perforated with poroids. 113) Detail of a seta. 114) A cell with the resting spore. 115) Internal view of the resting spore primary valve with the fissured collar. 116) External view of the resting spore perforated by poroids. Scale bars: 105=20 μm; 106, 114=10 μm; 108, 115–116=5 μm; 107, 109–113=1 μm.

FIGURES 117–127. Chaetoceros decipiens. Fig.118: field material.Figs 119–127: cultured material. Figs 119, 123, 126: strain PMFDE1. Figs 120, 122: strain PMFDE3. Figs 117, 121, 124–125, 127: strain PMFE1. Figs 117–119: LM. Figs 120, 123–126: TEM. Figs 121, 122, 127: SEM. 118) Complete chain showing orientation of the setae. 118) Intercalary cells with the organic structures projecting from the girdle (arrows). 119) Cells with several chloroplasts. 120) Sibling valves joined by setae fusion and slice-shaped hyaline silica projections (black arrowheads) from the rim on the marginal ridge (arrow). Siliceous fringes extend from the mantle (white arrowheads). 121) Terminal valve with slice-shaped silica projection (arrowheads) and small external protrusion of the rimoportula (arrow). 122) Terminal valve with no visible external part of the rimoportula. 123) Detail of the terminal valve with rimoportula. 124) Detail of the terminal valve with rimoportula within the central annulus. 125) Detail of broken girdle bands. 126) Detail of a seta. 127) Seta tip. Scale bars: 117, 118=20 μm; 119=10 μm; 122=5 μm; 120=2 μm; 121, 125–127=1 μm; 123–124=0.5 μm.