Chaetoceros mannaii

Cells of C. mannaii are ultrastructurally similar to field material identified as C. lorenzianus from Thailand, Mexico and Japan ([12, 36], own observations). Compared to the original description of C. lorenzianus, C. mannaii has large poroids on the setae, 0.7±0.2 μm long, the largest among strains of section Dicladia examined by us, and readily visible in LM. They are more closely spaced (12.3±1.6) than in the type material of C. lorenzianus (7.2±1.7), and smaller than those of the Mexican ([12], pl. 23, figs 3 and 4) and Japanese material ([36], pl. VI, fig 7) tentatively identified as C. lorenzianus, where poroids of 1 μm or more were illustrated. In addition, the apertures of C. mannaii are hexagonal, compared to the more quadrangular apertures of C. lorenzianus (compare Figs 11C, 11d, 16A and 16B) [8]. Chaetoceros mannaii also differs in having a smaller apical axis, 5.7–12.9 μm, compared to 20–43 μm in C. lorenzianus [8], 13–25 μm in the Mexican material [12] and around 30 μm (with a variation of 10–

80 μm) in the Japanese material [36].

Cells of C. mannaii are otherwise characterized by heavily silicified frustules and a distinct furrow above the basal ring of the mantle (Fig 11F, arrowheads). The terminal cell of C. mannaii carries a relatively long and distinct external tube of the rimoportula (Figs 11E, 11F and 12C). An external tube was not observed in C. decipiens, C. laevisporus and C. mitra (Figs 2F, 8E and 14G), and a very short one was present in C. elegans (Fig 5D and 5E). This character still needs to be explored in C. lorenzianus .

Resting spores were not observed in C. mannaii although several attempts were made to induce spore formation.