Skeletonema pseudocostatum Medlin emend. Zingone et Sarno

Hernández-Becerril, David U., 2023, Further studies on the species diversity of the marine planktonic diatom genus Skeletonema (Bacillariophyta) in the Mexican Pacific, Phytotaxa 607 (3), pp. 182-196 : 187

Skeletonema pseudocostatum Medlin emend. Zingone et Sarno

Figs 18–22

References: Medlin et al. 1991, p. 522; Sarno et al. 2005, p. 162, figs 7, A–I; Gu et al. 2012, p. 254, figs 21–23; Hernández-Becerril et al.

2013, p. 81, figs 16–25.

Description: Chains are relatively long and straight (slightly curved), with 4–9 cells per chain ( Figs 18, 19 ). Cells have two chloroplasts ( Fig. 18 ). The valves are flat to slightly convex, with a relatively high valve mantle ( Figs 19–21 ). The areolae are almost quadrangular to rectangular on the valve face and become pseudoloculate near the mantle ( Figs 20–22 ). Apart from the observations of the valvocopula, with longitudinal elongate poroids and ribs ( Figs 20–22 ), other copulae were not seen in detail.

All fultoportulae are open ( Figs 20, 21 ), although some IF’s and TF’s have their bases closed and are more tubular ( Figs 20, 22 ), and the tips of TF’s have a long, very characteristic spine ( Fig. 21 , small arrows). Each IF fuses to one or, less commonly, two of the sibling valves ( Figs 19, 20 ). Terminal rimoportula locates close to the margin, and has a long tube, with a slightly inflated tip ( Figs 21, 22 , arrows), whereas the IR is marginal, but very short.

Measurements: cell diameter 6.4–9.2 μm, areolae density 33–40 in 10 μm ( Table 1 ).

Figures

FIGURES 18–22. Skeletonema pseudocostatum, LM (18) and SEM (19–22). Fig. 18. A complete chain with 9 cells, one cell is dividing (arrow). Fig. 19. Broken chain with 4 cells in development. Fig. 20. Terminal part of a chain, with fusion of the IF’s, including 1:1 and 1:2 junctions. Fig. 21. Two terminal valves with relative high valve mantle and TF’s showing the characteristic spines of their tips (small arrows) and a TR (arrow). Fig. 22. A terminal valve showing the areolae on the face, TF’s and TR (arrow).