Chaetoceros mitra

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 : 30

Chaetoceros mitra

This was the first species of the C. lorenzianus complex to be described, as Dicladia mitra from the Sea of Kamtschatka ( Fig 20A ) [ 17] based only on a valve of a resting spore. When chains of cells were found, the taxon was transferred to Chaetoceros [ 18]. Cleve described the cells as forming straight chains with narrow peanut-shaped to narrow elliptical apertures, concave valves and strong terminal setae diverging c. 90˚, and diverging from the apical plane (Brunel Group II) with spirally arranged puncta and indistinct transverse striations ( Fig 20B and 20C ) [ 18]. This agrees with our observations, except for the spiral arrangement of puncta. Cleve’s drawings (loc. cit. Fig 1C ) show a pattern of spines or puncta on the spines similar to what we have seen ( Fig 14A–14D ), perhaps spirally inserted spines.

Chaetoceros mitra can distinguished from all the other species of the complex by the sibling setae diverging 30˚–80˚ from the apical plane, defined as Brunel Group II ( Fig 13A and 13B ) (Brunel 1972), as depicted by Cleve ([ 18], Fig 20C in the present paper). In the other species, setae are positioned more or less in the apical plane, i.e., Brunel Group I ( Figs 1C–1D , 4A–4C , 8A–8B and 11A ). Hasle & Syvertsen [ 6] stated that C. mitra , when seen in broad girdle view, has parallel or convergent terminal setae and in this way differs from C. lorenzianus , whose terminal setae are diverging. This does not agree with our findings, the terminal setae in our material of C. mitra were diverging, as illustrated by Cleve [ 18], Fig 20C .

The morphology of the resting spore as a distinguishing character for C. mitra needs evaluation as similar resting spores can be found in C. elegans ( Fig 7A–7F ) and were reported in material identified as C. lorenzianus [ 6, 9, 10, 39]. The differences with the C. elegans resting spore is discussed below and differentiation with the spore of C. lorenzianus must await “true” C. lorenzianus (from the type locality) being brought into culture. The illustration of a resting spore ([ 11], pl. 38, fig e) appears to disagree with C. mitra due to the length of the elevations, the relationship between the length of the elevation and the processes as well as the slope of the elevation. In an account of fossil Chaetoceros spores by [ 24, 40], three ‘species’ (form-species) of Dicladia are treated. Suto’s Dicladia mitra is different from our Fig 15B of Chaetoceros mitra . The relationship between the recent and the fossil taxa is unknown.

The size and density of the seta poroids ( Fig 14B–14E ) differentiates C. mitra from all the other species in the complex ( Table 1 ), as C. mitra has the smallest type of seta poroids ( Fig 17 ), measuring 0.2 ± 0.1 um, a size that can hardly be resolved in LM. The shape of the apertures is similar in C. decipiens and C. mitra , and poroid sizes overlap, but the two species Chaetoceros decipiens and C. mitra differ from each other in the density of setae poroids, presence/absence of fusion of the seta bases, the divergence of the intercalary setae from the apical plane (compare Figs 1D and 13B ) and the ornamentation of the valves. While C. decipiens has scattered poroids, C. mitra lacks poroids on the valve face (compare Figs 2H and 14H ).

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 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 7. Resting spores of Chaetoceros elegans sp. nov. LM (A and B) and SEM (C–F). A and B: Resting spores within mother cells in a chain; strain MC785. C and D: Released resting spores, showing two elongated elevations with dichotomous branching processes distally on the primary valve face and one (C) or two bulges (Fg D) on the secondary valve face; strain Ch12A1. E: Internal view of secondary valve with a ring of marginal punctae; strain Ch12A1. F: Hooks on the distal tips; strain MC785. A and B scale bars, 20 μm. C–E scale bars, 5 μm. F scale bar, 1 μm. doi:10.1371/journal.pone.0168887.g007

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 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 13. LM of Chaetoceros mitra. Strain P10A1. A: Straight chain showing seta divergence and apertures.B: Detail of chain showing constrictions (arrows) between mantle and girdle. C: Valve structure of terminal valve. A and B scale bars, 20 μm. C scale bar, 10 μm. doi:10.1371/journal.pone.0168887.g013

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

Fig 15. Chaetoceros mitra resting spores. LM (A–C), TEM (D) and SEM (E–G); strain P10A1. A: Early stage of resting spore formation. B and C: Mature resting spores within mother cells. D: Two elongated processes with dichotomous branches distally. E–G: Resting spores in different views,showing a row of silica warts along the secondary valve edge (arrowheads in E) and a ring of puncta at the secondary valve mantle (arrowheads in G). A–C scale bars, 10 μm. D–G scale bars, 5 μm. doi:10.1371/journal.pone.0168887.g015

Fig 17. Comparison of setae poroid sizes. Small letters on the x-axis indicate statistically significant differences among taxa. doi:10.1371/journal.pone.0168887.g017

Fig 20. The original drawings of species of the C. lorenzianus complex. A: Resting spore of C. mitra by Bailey [17]. B and C: C. mitra by Cleve [18]. Vegetative chain, intercalary valve,terminal seta and apical view of seta positions (C), and chain with resting spores (B, lectotype).D: Vegetative chain of C. lorenzianus by Grunow [8]. E and F: Chaetoceros decipiens by Cleve [7]. G and H: Resting spores of C. lorenzianus by Okamura [39]. doi:10.1371/journal.pone.0168887.g020