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).