Further studies on the species diversity of the marine planktonic diatom genus Skeletonema (Bacillariophyta) in the Mexican Pacific Author Hernández-Becerril, David U. text Phytotaxa 2023 2023-08-09 607 3 182 196 http://dx.doi.org/10.11646/phytotaxa.607.3.2 journal article 10.11646/phytotaxa.607.3.2 1179-3163 8243269 Skeletonema tropicum Cleve Figs 23–28 References: Sarno et al . 2005 , p. 166, figs 9 A–H; Jung et al . 2009 , p. 202, figs 4 A–H; Naik et al . 2010 , p. 291, figs 7–9; Gu et al . 2012 , p. 256, figs 24–26; Hernández–Becerril et al . 2013, p. 81, figs 26–31. Description: Chains relatively robust, long and straight were encountered (although some chains and cells were found collapsed or broken), with 6–19 cells in the chains ( Figs 23, 25 ). Cells showed more than two chloroplasts per cell, usually 4–6 ( Fig. 23 ). The valves are flat to slightly convex and have a relatively low mantle ( Figs 24–27 ). Mantle has pseudoloculate areolae ( Fig. 26 ), and in some terminal valves very delicate siliceous ridges were apparent ( Fig. 27 ). Areolae are quadrangular to rectangular at the center in the valve, but become pseudoloculate at the margins ( Fig. 27 ). Slightly collapsed valvocopula, with large pores, and copulae, with longitudinal rows of small pores, are seen in some frustules ( Figs 26, 27 ). FIGURES 12–17. Skeletonema grevillei , LM (12), SEM (13–16) and TEM (17). Fig. 12. A rather long chain of 8 cells, some of them dividing, containing two chloroplasts per cell. Fig. 13. Terminal part of a chain showing terminal and intercalary valves, siliceous ridges (small arrows) between face and mantle at the terminal valve, TF’s, one TR (arrow), and broken IF’s. Fig. 14. Intercalary valves with IF’s fusing to those of sibling valve in 1:1 junction type, a short IR (arrow), and siliceous ridges (small arrows) around bases of IF’s. Fig. 15. Terminal valve with fultoportulae (TF’s) irregularly truncated, a marginal rimoportula (arrow), and siliceous ridges (small arrows) dividing valve face and mantle. Fig. 16. Another terminal valve showing details of the areolae, siliceous ridges (small arrows) around bases of the TF’s, and a marginal rimoportula with slightly inflated tip (arrow). Fig. 17. Details of the three to four satellite pores surrounding each fultoportula (arrows). 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). FIGURES 23–28. Skeletonema tropicum , LM (23) and SEM (24–28). Fig. 23. A long and robust chain with 12 cells, some cells dividing and each cell with 4–6 chloroplasts. Fig. 24. Terminal part of a chain, with developing or collapsed TF’s and TR, and very short IR’s (small arrows). Fig. 25. A complete chain (6 cells), most probably developing, with overlaid copulae between various intercalary valves. Fig. 26. Intercalary cells and valves with IF’s fusing in 1:1 and 1:2 types; delicate and thin threads of unknown nature are coming out from some IF’s (small arrows). Fig. 27. Terminal cell and valve showing the areolae on the face and mantle, mantle also shows very delicate siliceous ridges, the TF’s have clawlike tips or tips with fine spines, and a single, long TR (arrow). Fig. 28. Internal view of a valve with details of the areolation, the fultoportulae (with three satellite pores), and a single labiate structure of the rimoportula (arrow). The IF’s are relatively long and open, and each may connect with only one or two of the other IF of the sibling valve ( Figs 24–26 ), whereas TF’s appeared shorter, although probably broken ( Figs 24, 27 ); TF’s have clawlike tips or tips with fine spines similar to those in S . pseudocostatum ( Fig. 27 ). All fultoportulae are surrounded by three satellite pores ( Fig. 28 ). Few very delicate and thin threads of unknown nature were detected coming out from some IF’s ( Fig. 26 , small arrows). TR was located close to the valve margin ( Fig. 27 ) and showed an internal labiate structure ( Fig. 28 , small arrow), whereas the IR’s were marginal and thin, and with a very short tubular structure ( Fig. 24 , small arrows). Measurements: cell diameter 10.8–17 μm, areolae density 24–26 in 10 μm ( Table 1 ). Table 1 shows the great morphological and morphometric differences in the four species studied here, especially the relative length of the chains (cells per chain) and the cell diameter, whereas the violin plot ( Fig. 29 ) additionally shows differences in cell diameter of the four Skeletonema species. The species Skeletonema ardens appears as the smallest species and S . tropicum the largest one.