Chaetoceros laevisporus

This is the only species in the C. lorenzianus complex with smooth resting spores without processes, and in the phylogenetic tree the strains form a separate branch. The smooth resting spore (Fig 10A and 10B) differs markedly from the spores of C. lorenzianus, C. mitra and C. elegans with their dichotomously-branching extensions. Furthermore the apertures are oval-peanut shaped in C. laevisporus (Figs 8A, 8B, 8D and 10A), thus differing from most other taxa in the complex. Seta poroid density is intermediate, 13.8± 1.9 in 10 μm as in C. mannaii (Table 1), but different from all the other taxa. Poroid size on the setae was assessed to be c. 0.6 μm, slightly less than in C. mannaii, but the poroids were still visible in LM (Fig 8C).