Diversity in the section Compressa of the genus Chaetoceros (Bacillariophyceae), with description of two new species from Chinese warm waters.

Monoclonal strains of Chaetoceros species were collected from Chinese warm waters. Vegetative cells and resting spores were examined using light microscopy as well as scanning and transmission electron microscopy. Combined morphological and DNA sequence data from the hypervariable D1-D3 region of the nuclear ribosomal large subunit showed the presence of two new species within the section Compressa, herein described as Chaetoceros bifurcatus sp. nov. and C. millipedarius sp. nov. Both species possessed features typical of the section Compressa, but C. bifurcatus was characterized by the heavy intercalary setae fusing and extending together for a distance before diverging. The heavy setae were not visually contorted, lacking poroids and spines, in contrast with how the section Compressa has been defined. Chaetoceros millipedarius was characterized by a horizontal stagger of ordinary intercalary setae when the chain was seen in broad girdle view, making the chain look like a millipede. The two new species were supported by molecular phylogenetic analyses with C. bifurcatus sister to C. contortus var. ornatus, while C. millipedarius was sister to C. contortus var. contortus. Based on the morphological features exhibited in C. bifurcatus, the diagnosis of the section Compressa was emended to exclude "contorted" heavy setae and compressed valve faces.