Chaetoceros elegans, Li & Boonprakob & Gaonkar & Kooistra & Lange & Hernández-Becerril & Chen & Moestrup & Lundholm, 2017

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 : 33

C haetoceros elegans

The species C. elegans possesses intermediate-sized poroids on the setae, 0.5 ± 0.2 μm, visible in LM. Besides the size, which is statistically different from the other examined taxa, the mostly tear-shaped setae poroids are unique ( Fig 6E and 6G ; Table 1 ). Poroid density also differentiates this species from all other taxa except C. decipiens ( Table 1 ).

Chaetoceros elegans is characterized by large rounded quadrangular-rectangular apertures ( Figs 4A, 4C , 5B and 5C ), and differs in this respect from the other species ( Table 1 ) except C. lorenzianus , which has been illustrated with quadrangular-hexagonal apertures ( Figs 16A, 16B and 20D ) [ 8]. A large aperture/pervalvar index also characterizes C. elegans ( Table 1 ), and differentiates it from the other taxa except C. decipiens and C. lorenzianus . Distinct basal parts of the setae are present in C. elegans ( Fig 5B and 5C ). A very short basal part is present also in C. mannaii ( Fig 11C and 11D ) while basal parts are absent in C. laevisporus , C. decipiens and C. mitra , and apparently also in C. lorenzianus ( Fig 16A and 16B ). Chaetoceros elegans seems to be a widely distributed species. In addition to our findings in China, Thailand and Chile, sequences of specimens from Canada (previously reported as C. cf. decipiens ) have been identified ( Fig 19 ), and material which we refer to this species has been illustrated from Japan (as C. decipiens in [ 36]) and Gulf of California (as C. lorenzianus in [ 12]).

The resting spores of C. elegans and C. mitra both possess two elevations and branching processes on the primary valve and 1–2 bulges on the secondary valve. Using the terminology of Suto [ 24], the resting spores are distinguished from each other based on 1) the length of the apical axis 2) the length of the pervalvar axis of the primary valve, 3) the relationship between the length of the pervalvar axis of the primary valve and the length of the branching processes, 4) the slope of the elevations, and 5) the position where the elevations join ( Table 2 ).

Figures

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 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 16. Lectotype material of C. lorenzianus from the Grunow Collection. A: Intercalary valves with oval-hexagonal aperture and some fusion of the basal parts of the setae. B: Intercalary seta poroids visible under the LM. Scale bars are 10 μm. doi:10.1371/journal.pone.0168887.g016

Fig 19. Molecular phylogenetic tree. Based on analyses of the D1-D3 region of the LSU rDNA sequences. Numbers indicated on the branches are posterior probability of Bayesian analyses (MrB) and boot strap support of neighbor joining (NJ), maximum parsimony (MP) and maximum likelihood (ML) analyses. doi:10.1371/journal.pone.0168887.g019

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