Dicladia

Li, Yang, Boonprakob, Atchaneey, Gaonkar, Chetan C., Kooistra, Wiebe H. C. F., Lange, Carina B., Hernández-Becerril, David, Chen, Zuoyi, Moestrup, Øjvind & Lundholm, Nina, 2017, Diversity in the Globally Distributed Diatom Genus Chaetoceros (Bacillariophyceae): Three New Species from Warm-Temperate Waters, PLoS ONE (e 0168887) 12 (1), pp. 1-38 : 34-35

Additional morphological characters in section Dicladia ?

A feature of C. laevisporus which occurs also in C. decipiens and C. elegans ( Figs 1G and 4D , arrowheads) is the presence of a non-silicified, V-shaped protrusion on the terminal valves ( Fig 8A , arrowhead). Chaetoceros cf. lorenzianus from the Gulf of Panama shows the same feature [ 35]. This, together with the reduced external tube of rimoportula on the terminal valve (loc. cit. figs 77–80 in [ 35]) clearly places this material as belonging to C. laevisporus , The Vstructure was observed in C. decipiens by Rines & Hargraves [ 10] and Jensen & Moestrup [ 9], and may be present in more species of the section. Because of its non-silicified nature, the structure disappears in acid-cleaned material.

The ear-like structures at the marginal border of the apertures have been observed in all the studied strains, often in different shapes on the intercalary valves and terminal valves. In C. decipiens and C. mitra , the structures are very distinct, and they interconnect the edges of sibling valves on each side of the aperture ( Figs 3B, 3C and 14F ). In C. elegans , C. laevisporus and C. mannaii they are smaller, sometimes with overlapping parts ( Figs 6A, 6B , 9A, 9B , 11C and 11D). On the terminal valves, these structures vary in shape, as fringes in C. decipiens , C. laevisporus and C. mitra ( Figs 2F , 9C and 14G ) and as slice-shaped structures in C. elegans and C. mannaii ( Figs 5D and 11E ). Based on the morphological variation observed, and the potential influence of environmental factors on the development of the structures, it is somewhat uncertain whether they can be used as a species-specific character.

Although our study was based on established strains, with the risk of culture condition artefacts affecting the morphology, we regard the present species delineation sound, due to the following: 1) strains were fixed soon after establishment and morphology is thus not very likely affected by artefacts, 2) several of the species are based on strains from separate locations showing the same morphology, 3) observations of field samples made it possible for us to identify the taxa based on the descriptions provided, 4) evaluations with previously published records of species belonging to Dicladia , for which electron micrographs are available (S3 Table) support our observations and 5) the study was performed over a long period, and subsequently established strains confirmed the species descriptions.

Figures

Fig 1. LM of Chaetoceros decipiens. A, C, E, F: Chain views of lectotype material MIC5366 (A) and strains P10E5 (C), P14B3B (E), D12 (F). B and D: Detail of chains showing constrictions (arrows) between the mantle and the girdle bands,and the fused extensions of sibling setae, which are short, longer or even absent (arrowheads); lectotype material MIC5366 (B) and strain P10E5 (D). G: Chain view of strain D10, note the V-shaped protrusion located centrally on the terminal valve (arrowhead). A and C: scale bars, 50 μm. B, D, E, F: scale bars, 20 μm. G: scale bar, 10 μm. doi:10.1371/journal.pone.0168887.g001

Fig 2. Chaetoceros decipiens. LM (A and B), TEM (C, D, H-J) and SEM (E-G). A-E: Seta structure of lectotype MIC5366 (A), strains P14B3B (B) and D10 (C–E), showing the 4–6 sided seta with poroids and small spines. F: Terminal valve with fringes (arrowheads); strain D10. G: Silica warts on the basal ring of the mantle; strain D10. H: Annulus, costae and poroid pattern on intercalary valve; strain P10E5. I: Terminal valve showing rimoportula without external process (arrowhead); strain D10. J: Girdle bands; strain D10. A and B scale bars, 10 μm. C–J scale bars, 2 μm. doi:10.1371/journal.pone.0168887.g002

Fig 3. Chaetoceros decipiens. Strain D10, SEM (A–C) and TEM (D). A: Solitary cell with silica fringes.B: Intercalary cells with overlapping silica membrane (arrow). C: Detail of fused seta bases, silica membrane and fringes on the mantle (arrowhead). D: Rows of poroids on the mantle. A and B scale bars, 10 μm. C and D scale bars, 2 μm. doi:10.1371/journal.pone.0168887.g003

Fig 4. LM of Chaetoceros elegans sp. nov. Strain YL7. A: Chain view displaying seta divergence in the apical plane. B: Solitary cell. C: Chain view demonstrating large apertures and chloroplasts. D: End of chain showing constrictions (arrows) between the mantle and the girdle, and V-shaped protrusion (arrowhead) located centrally on terminal valve. A and B scale bars, 50 μm. C and D scale bars, 20 μm. doi:10.1371/journal.pone.0168887.g004

Fig 5. Chaetoceros elegans sp. nov. SEM (A–E) and TEM (F and G). A: Terminal valve view with external process of rimoportula and poroids on valve face; strain MC1048. B and C: Intercalary cells demonstrating large apertures and valve faces; strain M1 (B), strain MC1048 (C). D and E: Terminal valves with central processes (arrowheads) and silica ear-like structures (arrows in D) in strain YL7. F: Annulus, costae and poroid pattern near intercalary valve centre; strain YL7. G: Parallel rows of poroids on the mantle; arrowhead indicates ring-shaped constriction; strain YL7. A–E scale bars, 5 μm. F and G scale bars, 2 μm. doi:10.1371/journal.pone.0168887.g005

Fig 6. Chaetoceros elegans sp. nov. LM (C), TEM (A, B, H) and SEM (D–G). A and B: Overlapping ear-like structures (arrows) and small gap between the crossing bases of sibling setae in strain YL7. C: Terminal seta with partly visible poroids in LM; strain YL7. D and E: Seta structure showing elongated poroids (D, strain Ch12A1) and tear-shaped poroids (E, strain M1) and F and G: Detail of setae poroids; strain Ch12A1 (F) and strains MC785 (G). H: Girdle band; strain YL7. All scale bars are 2μm, except 10 μm in C. doi:10.1371/journal.pone.0168887.g006

Fig 8. Chaetoceros laevisporus sp. nov. LM (A–C), SEM (D and E) and TEM (F). Fig A: Part of a chain showing constrictions (arrows) between mantle and girdle, and V- shaped protrusion (arrowhead) located centrally on the terminal valve; strain N7. B: Dividing chain showing seta divergence; strain DY1. C: Structure of terminal seta; strain DY1. D: Chain in girdle view showing cells and apertures; strain N7. E: Terminal valve with rimoportula without external process (arrowhead); strain N7. F: Valve views of intercalary (upper) and terminal valves (below); strain N7. A, C, D scale bars,20 μm. B scale bar, 100 μm. E and F scale bars, 5 μm. doi:10.1371/journal.pone.0168887.g008

Fig 9. TEM of Chaetoceros laevisporus sp. nov. Strain N7. A, B: Non-overlapping (arrows in A) and overlapping ears (arrow in B) between sibling cells. C: Terminal valve with silica fringe (arrows) and constriction (arrowhead). D and E: Open girdle bands.F: Detail of girdle band with pores. G: Seta with round-oval poroids. A, B scale bars, 2 μm. C–E, G scale bars, 5 μm. F scale bar, 1 μm. doi:10.1371/journal.pone.0168887.g009

Fig 11. Chaetoceros mannaii sp. nov. LM (A and B), SEM (C and E) and TEM (D and F); strain N1. A: Straight chain showing seta divergence and constrictions (arrows) between the mantle and the girdle. B: Oval valve face. C and D: Intercalary cells, with ear-like structures (arrowheads in D) at the bases of setae in heavily silicified frustule. E and F: Terminal valves, with ear-like structures at the seta bases (arrowheads in E) and distinct constriction above the ring (arrowheads in F). A scale bar, 20 μm. B–F scale bars, 5 μm. doi:10.1371/journal.pone.0168887.g011

Fig 14. Chaetoceros mitra. LM (A), SEM (B, C, F, G) and TEM (D, E, H–J); strain P10A1. A–E: Setae showing round-oval poroids and spines, using different microscopy techniques. F: Intercalary valves showing wing-like structures (arrowhead), and furrows above the basal ring of mantle (arrows). G: Terminal valve showing rimoportula without external tube (arrowhead), furrow above the basal ring of mantle (arrows) and fringe (curved arrow). H: Intercalary valve face. I and J: Girdle bands. A scale bar, 10 μm. B, C, F–H scale bars, 5 μm. D, E, I, J scale bars, 2 μm. doi:10.1371/journal.pone.0168887.g014