Morphology and molecular phylogeny of Chaetoceros dayaensis sp. nov. (Bacillariophyceae), characterized by two 90° rotations of the resting spore during maturation.

Recent studies suggest that a high species diversity of Chaetoceros exists in tropical waters. Based on plankton samples from Chinese tropical waters, Daya Bay of the South China Sea, a new species, Chaetoceros dayaensis sp. nov., is described. Vegetative cells and resting spores were examined by light microscopy as well as transmission and scanning electron microscopy. The nuclear rDNA molecular markers SSU, ITS and D1-D3 of LSU, and the chloroplast rbcL gene were sequenced for information on phylogenetic relationships. The species is characterized by the presence of two 90° rotations of the resting spore during maturation. First a rotation in the apical plane of the mother cell resulting in the valvar plane of the resting spore turning from parallel to perpendicular to that of the mother cell, and a second 90° rotation in the valvar plane of the mother cell resulting in the valve faces of the resting spore turning from facing narrow girdle view to broad girdle view of the mother cell. It is the first report of two 90° rotations of resting spores during maturation in Chaetoceros. Based on this, the maturation rotation of the resting spore of C. rotosporus was reevaluated. C. dayaensis belongs to the subgenus Hyalochaete Gran and we suggest placing it in the section Laciniosa, although a new subdivision of Chaetoceros is needed. The description of C. dayaensis was based on analyses of the phylogenetic relationships combined with morphological comparisons with other similar species.